CHAPTER I.

OF THE PHENOMENA OF FORCE, AND OF THE CONSTITUTION OF NATURAL BODIES.

(232.) Natural History may be considered in two very different lights: either, 1st, as a collection of facts and objects presented by nature, from the examination, analysis, and combination of which we acquire whatever knowledge we are capable of attaining both of the order of nature, and of the agents she employs for producing her ends, and from which, therefore, all sciences arise; or, 2dly, as an assemblage of phenomena to be explained; of effects to be deduced from causes; and of materials prepared to our hands, for the application of our principles to useful purposes. Natural history, therefore, considered in the one or the other of these points of view, is either the beginning or the end of physical science. As it offers to us, in a confused and interwoven mass, the elements of all our knowledge, our business is to disentangle, to arrange, and to present them in a separate and distinct state: and to this end we are called upon to resolve the important but complicated problem,--Given the effect, or assemblage of effects, to find the causes. The principles on which this enquiry relies are those which constitute the relation of cause and effect, as it exists with reference to our minds; and their rules and mode of application have been attempted to be sketched out, (though in far less detail than the intrinsic interest of the subject, both in a logical and practical point of view, would demand,) in the foregoing pages. It remains now to bring together, in a summary statement, the results of the general examination of nature, so far as it has been prosecuted to the discovery of natural agents, and the mode in which they act.

(233.) The first great agent which the analysis of natural phenomena offers to our consideration, more frequently and prominently than any other, is force. Its effects are either, 1st, to counteract the exertion of opposing force, and thereby to maintain _equilibrium_; or, 2dly, to produce _motion_ in matter.

(234.) Matter, or that, whatever it be, of which all the objects in nature which manifest themselves directly to our senses consist, presents us with two general qualities, which at first sight appear to stand in contradiction to each other--activity and inertness. Its activity is proved by its power of spontaneously setting other matter in motion, and of itself obeying their mutual impulse, and moving under the influence of its own and other force; inertness, in refusing to move unless obliged to do so by a force impressed externally, or mutually exerted between itself and other matter, and by persisting in its state of motion or rest unless disturbed by some external cause. Yet in reality this contradiction is only apparent. Force being the cause, and motion the effect produced by it on matter, to say that matter is inert, or has _inertia_, as it is termed, is only to say that the cause is expended in producing its effect, and that the same cause cannot (without renewal) produce double or triple its own proper effect. In this point of view, equilibrium may be conceived as a continual production of two opposite effects, each undoing at every instant what the other has done.

(235.) However, if this should appear too metaphysical, at all events this difference of effects gives rise to two great divisions of the science of force, which are commonly known by the names of STATICS and DYNAMICS; the latter term, which is general, and has been used by us before in its general sense, being usually confined to the doctrine of motion, as produced and modified by force. Each of these great divisions again branches out into distinct subdivisions, according as we consider the equilibrium or motion of matter in the three distinct states in which it is presented to us in nature, the solid, liquid, and aëriform state, to which, perhaps, ought to be added the _viscous_, as a state intermediate between that of solidity and fluidity, the consideration of which, though very obscure and difficult, offers a high degree of interest on a variety of accounts.

_Statics and Dynamics._

(236.) The principles have been definitively fixed by Galileo and his successors, down to Newton, on a basis of sound induction; and as they are perfectly general, and apply to every case, they are competent, as we have already before observed, to the solution of every problem that can occur in the deductive processes, by which phenomena are to be explained, or effects calculated. Hence, they include every question that can arise respecting the motions and rest of the smallest particles of matter, as well as of the largest masses. But the mode of reasoning from these general principles differs materially, whether we consider them as applied to masses of matter of a sensible size, or to those excessively minute, and perhaps indivisible, molecules of which such masses are composed. The investigations which relate to the latter subject are extremely intricate, as they necessarily involve the consideration of the hypotheses we may form respecting the intimate constitution of the several sorts of bodies above enumerated.

(237.) On the other hand, those which respect the equilibrium and motions of sensible masses of matter are happily capable of being so managed as to render unnecessary the adoption of any particular hypothesis of structure. Thus, in reasoning respecting the application of forces to a solid mass, we suppose its parts indissolubly and unalterably connected; it matters not by what tie, provided this condition be satisfied, that one point of it cannot be moved without setting all the rest in motion, so that the relative situation of the parts one among another be not changed. This is the abstract notion of a solid which the mechanician employs in his reasonings. And their conclusions will apply to natural bodies, of course, only so far as they conform to such a definition. In strictness of speaking, however, there are no bodies which absolutely conform to it. No substance is known whose parts are absolutely incapable of yielding one among another; but the amount by which they do yield is so excessively small as to be demonstrably incapable, in most cases, of having any influence on the results: and in those where it has such influence, an especial investigation of its amount can always be made. This gives rise to two subdivisions of the application of mechanical reasonings to solid masses. Those which refer to the action of forces on flexible or elastic, and on inflexible or rigid, bodies, comprehending under the latter all such whose resistance to flexure or fracture is so very great as to permit our adoption of the language and ideas of the extreme case without fear of material error.

(238.) In like manner, when we reason respecting the action of forces on a fluid mass, all we have occasion to assume is, that its parts are freely moveable one among the other. If, besides this, we choose to regard a fluid as incompressible, and deduce conclusions on this supposition, they will hold good only so far as there may be found such fluids in nature. Now, in strictness, there are none such; but, practically speaking, in the greater number of cases their resistance to compression is so very great that the result of the reasoning so carried on is not sensibly vitiated; and, in the remaining cases, the same general principles enable us to enter on a special enquiry directed to this point: and hence the division of fluids, in mechanical language, into compressible and incompressible, the latter being only the extreme or limiting case of the former.

(239.) As we propose here, however, only to consider what is the actual constitution of nature, we shall regard all bodies, as they really are, more or less flexible and yielding. We know for certain, that the space which any material body appears to occupy is not entirely filled by it; because there is none which by the application of a sufficient force may not be _compressed_ or forced into a smaller space, and which, either wholly, as in air or liquids, or in part, as in the greater number of solids, will not recover its former dimensions when the force is taken off. In the case of air, this condensation may be urged to almost any extent; and not only does a mass of air so condensed completely recover its original bulk, when the applied pressure is removed, but if that ordinary pressure under which it exists at the earth’s surface (and which arises from the weight of the atmosphere) be also removed by an air-pump, it will still further dilate itself without limit so far as we have yet been able to try it. Hence we are led to the conclusion that the particles of air are mutually elastic, and have a _tendency to recede from one another_, which can only be counteracted by _force_, and therefore is itself a force of the repulsive kind. Nevertheless, as air is heavy, and as gravitation is a universal property of matter, there is no doubt that this repulsive tendency must have a limit, and that there is a distance to which, if the particles of the air could be removed from each other, their mutual repulsion would cease, and an attraction take its place. This limit is probably attained at some very great height above the earth’s surface, beyond which, of course, its atmosphere cannot extend.

(240.) What, however, we can only conclude by this or similar reasoning respecting air, we see distinctly in liquids. They are all, though in a small degree, compressible, and recover their former dimensions completely when the pressure is removed; but they cannot be dilated (by mechanical means), and have no tendency, while they remain liquids, to enlarge themselves beyond a certain limit, and therefore they assume a determinate _surface_ while at rest, and their parts actually resist further separation with a considerable force, thus giving rise to the phenomenon of the _cohesion of liquids_.

(241.) Both in air and in liquids, however, the most perfect freedom of motion of the parts among each other subsists, which could hardly be the case if they were not separate and independent of each other. And from this, combined with the foregoing considerations, it has been concluded that they do not actually touch, but are kept asunder at determinate distances from each other, by the constant action of the two forces of attraction and repulsion, which are supposed to balance and counteract each other at the ordinary distances of the particles, but to prevail, the one, or the other, according as they are forcibly urged together or pulled asunder.

(242.) In solids, however, the case is very different. The mutual free motion of their parts _inter se_ is powerfully impeded, and in some almost destroyed. In some, a slow and gradual change of figure may be produced to a great extent, by pressure or blows, as for instance in the metals, clay, butter, &c.; in others, fracture is the consequence of any attempt to change the figure by violence beyond a certain very small limit. In solids, then, it is evident, that the consideration of their intimate structure has a very great influence in modifying the general results of the action of such attractive and repulsive forces as may be assumed to account for the phenomena they present; yet the general facts that their parts _cohere_ with a certain energy, and that they resist displacement or intrusion on the part of other bodies, are sufficient to demonstrate at least the existence of such forces, whatever obscurity may subsist as to their mode of action.

(243.) This division of bodies into airs, liquids, and solids, gives rise, then, to three distinct branches of mechanical science, in each of which the general principles of equilibrium and motion have their peculiar mode of application; viz. pneumatics, hydrostatics, and what might, without impropriety, be termed stereostatics.

_Pneumatics._

(244.) Pneumatics relates to the equilibrium or movements of aërial fluids under all circumstances of pressure, density, and elasticity. The weight of the air, and its pressure on all the bodies on the earth’s surface, were quite unknown to the ancients, and only first perceived by Galileo, on the occasion of a sucking-pump refusing to draw water above a certain height. Before his time it had always been supposed that water rose by suction in a pipe, in consequence of a certain natural _abhorrence of a vacuum_ or empty space, which obliged the water to enter by way of supplying the place of the air sucked out. But if any such abhorrence existed, and had the force of an _acting cause_, which could urge water a single foot into a pipe, there is no reason why the same principle should not carry it up two, three, or any number of feet; none why it should suddenly stop short at a certain height, and refuse to rise higher, however violent the suction might be, nay, even fall back, if purposely forced up too high.

(245.) Galileo, however, at first contented himself with the conclusion, that the natural abhorrence of a vacuum was not strong enough to sustain the water more than about thirty-two feet above its level; and, although the true cause of the phenomenon at length occurred to him, in the pressure of the air on the general surface, it was not satisfactorily demonstrated till his pupil, Torricelli, conceived the happy idea of instituting an experiment on a small scale by the use of a much heavier liquid, mercury, instead of water, and, in place of sucking out the air from above, employing the much more effectual method of filling a long glass tube with mercury, and inverting it into a basin of the same metal. It was then at once seen, as by a _glaring instance_, that the maintenance of the mercury in the tube (which is nothing else than the common barometer) was the effect of a perfectly definite external cause, while its fluctuations from day to day, with the varying state of the atmosphere, strongly corroborated the notion of its being due to the pressure of the external air on the surface of the mercury in the reservoir.

(246.) The discovery of Torricelli was, however, at first much misconceived, and even disputed, till the question was finally decided by appeal to a _crucial instance_, one of the first, if not the very first on record in physics, and for which we are indebted to the celebrated Pascal. His acuteness perceived that if the weight of the incumbent air be the direct cause of the elevation of the mercury, it must be measured by the amount of that elevation, and therefore that, by carrying a barometer up a high mountain, and so ascending into the atmosphere _above_ a large portion of the incumbent air, the pressure, as well as the length of the column sustained by it, must be diminished; while, on the other hand, if the phenomenon were due to the cause originally assigned, no difference could be expected to take place, whether the observation were made on a mountain or on the plain. Perhaps the decisive effect of the experiment which he caused to be instituted for the purpose, on the Puy de Dôme, a high mountain in Auvergne, while it convinced every one of the truth of Torricelli’s views, tended more powerfully than any thing which had previously been done in science to confirm, in the minds of men, that disposition to experimental verification which had scarcely yet taken full and secure root.

(247.) Immediately on this discovery followed that of the air-pump, by Otto von Guericke of Magdeburgh, whose aim seems to have been to decide the question, whether a vacuum could or could not exist, by endeavouring to make one. The imperfection of his mechanism enabled him only to diminish the aërial contents of his receivers, not entirely to empty them; but the curious effects produced by even a partial exhaustion of air speedily excited attention, and induced our illustrious countryman, Robert Boyle, to the prosecution of those experiments which terminated in his hands, and in those of Hauksbee, Hooke, Mariotte, and others, in a satisfactory knowledge of the general law of the equilibrium of the air under the influence of greater or less pressures. These discoveries have since been extended to all the various descriptions of aërial fluids which chemistry has shown to exist, and to maintain their aëriform state under artificial pressure, and even to those which may be produced from liquids reduced to a state of vapour by heat, so long as they retain that state.

(248.) The manner in which the observed law of equilibrium of an elastic fluid, like air, may be considered to originate in the mutual repulsion of its particles, has been investigated by Newton, and the actual statement of the law itself, as announced by Mariotte, “that the density of the air, or the quantity of it contained in the same space, is, _cæteris paribus_, proportional to the pressure it supports,” has recently been verified within very extensive limits by direct experiment, by a committee of the Royal Academy of Paris. This law contains the principle of solution of every dynamical question that can occur relative to the equilibrium of elastic fluids, and is therefore to be regarded as one of the highest _axioms_ in the science of pneumatics.

_Hydrostatics._

(249.) The principles of the equilibrium of liquids, understanding by this word such fluids as do not, though quite at liberty, attempt to dilate themselves beyond a certain point, are at once few and simple. The first steps towards a knowledge of them were made by Archimedes, who established the general fact, that a solid immersed in a liquid loses a portion of its weight equal to that of the liquid it displaces. It seems very astonishing, after this, that it should not have been at once concluded that the weight thus said to be _lost_ is only _counteracted_ by the upward pressure of the liquid, and that, therefore, a portion of any liquid, surrounded on all sides by a liquid of the same kind, does really exert its weight in keeping its place. Yet the prejudice that “liquids do not gravitate in their natural place” kept its ground, and was only dispelled with the mass of error and absurdity which the introduction of a rational and experimental philosophy by Galileo swept away.

(250.) The hydrostatical law of _the equal pressure of liquids in all directions_, with its train of curious and important consequences, is an immediate conclusion from the perfect mobility of their parts among one another, in consequence of which each of them tends to recede from an excess of pressure on one side, and thus bears upon the rest, and distributes the pressure among its neighbours. In this form it was laid down by Newton, and has proved one of the most useful and fertile principles of physico-mathematical reasoning on the equilibrium of fluid masses, as affording a means of tracing the action of a force applied at any point of a liquid through its whole extent. It applies, too, without any modification, to expansible fluids as well as to liquids; and, in the applications of geometry to this subject, enables us to dispense with any minute and intricate enquiries as to the mode in which individual particles act on each other.

(251.) In a practical point of view, this law is remarkable for the directness of its application to useful purposes. The immediate and perfect distribution of a pressure applied on any one part, however small, of a fluid surface through the whole mass, enables us to communicate _at one instant_ the same pressure to any number of such parts by merely increasing the surface of the fluid, which may be done by enlarging the containing vessel; and if the vessel be so constructed that a large portion of its surface shall be moveable together, the pressures on all the similar parts of this portion will be united into one consentient force, which may thus be increased to any extent we please. The hydraulic press, invented by Bramah, (or rather applied by him after a much more ancient inventor, Stevin,) is constructed on this principle. A small quantity of water is driven by sufficient pressure into a vessel _already full_, and provided with a moveable surface or piston of great size. Under such circumstances something must give way; the great surface of the piston accumulates the pressure on it to such an extent that nothing can resist its violence. Thus, trees are torn up by the roots; piles extracted from the earth; woollen and cotton goods compressed into the most portable dimensions; and even hay, for military service, reduced to such a state of coercion as to be easily packed on board transports.

(252.) Liquids differ from aëriform fluids by their _cohesion_, which may be regarded as a kind of approach to a solid state, and was so regarded by Bacon (193.). Indeed, there can be little doubt that the solid, liquid, and aëriform states of bodies are merely stages in a progress of gradual transition from one extreme to the other; and that, however strongly marked the distinctions between them may appear, they will ultimately turn out to be separated by no sudden or violent line of demarcation, but shade into each other by insensible gradations. The late experiments of Baron Cagnard de la Tour may be regarded as a first step towards the full demonstration of this (199.). But the cohesion of liquids is not, like that of solids, so modified by their structure in other respects as to destroy the mobility of their parts one among another (unless in those cases of nearer approach to the solid state which obtain in viscid or gummy liquids). On the contrary, the two qualities co-exist, and give rise to a number of curious and intricate phenomena.

(253.) One of the most remarkable of these is capillary attraction, or capillarity as it is sometimes called. Every body has remarked the adhesion of water to glass. The elevation of the general surface of the liquid where it is in contact with the containing vessel; the form of a drop suspended at the under side of a solid: these are instances of capillary attraction. If a small glass tube with a bore as fine as a hair be immersed in water, the water will be observed to rise in it to a certain height, and to assume a concave surface at its upper extremity. The attraction of the glass on the water, and the cohesion of the parts of the water to each other, are no doubt the joint causes of this curious effect; but the mode of action is at once obscure and complex; and although the researches of Laplace and Young have thrown great light on it, further investigation seems necessary before we can be said distinctly to understand it.

(254.) As the capillarity and cohesion of the parts of liquids shows them to possess the power of mutual attraction, so their elasticity demonstrates that they also possess that of repulsion when forcibly brought nearer than their natural state. From the extremely small extent to which the compression of liquids can be carried by any force we can employ, compared with that of air, we must conclude that this repulsion is much more violent in the former than in the latter, but counteracted also by a more powerful force of attraction. So much more powerful, indeed, is the resistance of liquids to compression, that they were usually regarded as incompressible; an opinion corroborated by a celebrated experiment made at Florence, in which water was forced through the pores (as it was said) of a golden ball. More recent experiments by Canton, and since by Perkins, Oërsted, and others, have demonstrated however the contrary, and assigned the amount of compression.

(255.) The consideration of the motions of fluids, whether liquid or expansible, is infinitely more complicated than that of their equilibrium. When their motions are slow, it is reasonable to suppose that the law of the equable distribution of pressure obtains; but in very rapid displacements of their parts one among the other, it is not easy to see how such an equable distribution can be accomplished, and some phenomena exist which seem to indicate a contrary conclusion.

(256.) Independent of this, there are difficulties of an almost insuperable nature to the regular deductive application of the general principles of mechanics to this subject, which arise from the excessive intricacy of the pure mathematical enquiries to which its investigation leads. It was Newton who set the example of a first attempt to draw any conclusions respecting the motion of fluid masses by direct reasoning from dynamical principles, and thus laid the foundation of HYDRODYNAMICS; but it was not till the time of D’Alembert that the method of reducing any question respecting the motions of fluids under the action of forces to strict mathematical investigation could be said to be completely understood. But the cases even now in which this mode of treating such questions can be applied with full satisfaction are few in comparison of those in which the experimental method of enquiry as already observed (189.) is preferable. Such, for example, is that of the resistance of fluids to bodies moving through them; a knowledge of which is of great importance in naval architecture and in gunnery, where the resistance of the air acts to an enormous extent. Such, too, among the practical subjects which depend mainly on this branch of science, are the use of sails in navigation; the construction of windmills, and water-wheels; the transmission of water through pipes and channels; the construction of docks and harbours, &c.

_Nature of Solids in general._

(257.) The intimate constitution of solids is, in all probability, very complicated, and we cannot be said to know much of it. By some recent delicate experiments on the dimensions of wires violently strained, it has been shown that they are to a certain small extent capable of being dilated by tension, as they are also of being compressed by pressure, but within limits even narrower than those of liquids. Usually, when strained too far, they break, and refuse to re-unite; or, if compressed too forcibly, take a permanent contraction of dimension. Thus, wood may be indented by a blow, and metals rendered denser and heavier by hammering or rolling. There is a certain degree of confusion prevalent in ordinary language about the hardness, elasticity, and other similar qualities, of solids, which it may be well to remove. Hardness is that disposition of a solid which renders it difficult to displace its parts among themselves. Thus, steel is harder than iron; and diamond almost infinitely harder than any other substance in nature: but the compressibility of steel, or the extent to which it will yield to a given pressure and recover itself, is not much less than that of soft iron, and that of ice is very nearly the same with that of water.

(258.) Again, we call Indian rubber a very elastic body, and so it is; but in a different sense from steel. Its parts admit of great mutual displacement without permanent dislocation; however distorted, it recovers its figure readily, but with a small force. Yet, if Indian rubber were to be enclosed in a space that it just filled, so as not to permit its parts to yield laterally, doubtless it would resist actual compression with great violence. Here, then, we have an instance of two kinds of elasticity in one substance; a feebler effort of recovery from distorted figure, and a more violent one from a state of altered dimension. Both, however, originate in the same causes, and are referable to the same principles; the former being in fact only a modified case of the latter, as the effort of a steel spring, when bent, to recover its former shape, is referable to the same forces which give to steel its hardness and strength to resist actual compression and fracture.

(259.) The toughness of a solid, or that quality by which it will endure heavy blows without breaking, is again distinct from hardness though often confounded with it. It consists in a certain yielding of parts with a powerful general cohesion, and is compatible with various degrees of elasticity. Malleability is again another quality of solids, especially metals, quite distinct from toughness, and depends on their capability of being deprived of their figure without an effort to recover it and without fracture.

(260.) Tenacity, again, is a property of solids more directly depending on the cohesion of their parts than toughness. It consists in their power of resisting separation by a strain steadily applied, while the quality of toughness is materially influenced by their disposition to communicate through their substance the jarring effect of a blow. Accordingly, the tenacity of a solid is a direct measure of the cohesive attraction of its parts, and is the best proof of the existence of such a power.

_Crystallography._

(261.) It cannot be supposed that these and many other tangible qualities, as they may be called, should subsist in solids without a corresponding mechanism in their internal structure. That they have such a mechanism, and that a very curious and intricate one, the phenomena of crystallography sufficiently show. This interesting and beautiful department of natural science is of comparatively very modern date. That many natural substances affected certain forms must have been known from the earliest times. Pliny appears to have been acquainted with this fact, at least in some instances, as he describes the forms of quartz and diamond. But till the time of Linnæus no material attention seems to have been bestowed on the subject. He, however, observed, and described with care, the crystalline forms of a variety of substances, and even regarded them as so definite a character of the solids which assumed them, that he supposed every particular form to be generated by a particular salt. Romé de l’Isle pursued the study of the crystalline forms of bodies yet farther. He first ascertained the important fact of the constancy of the angles at which their faces meet; and observing further that many of them appear in several different shapes, first conceived the idea that these shapes might be reducible to one, appropriated in a peculiar manner to each _substance_, and modified by strict geometrical laws. Bergmann, reasoning on a fact imparted to him by his pupil Gahn, made a yet greater step, and showed how at least one species of crystal might be built up of thin laminæ ranged in a certain order, and following certain rules of superposition. He failed, however, in deducing just and general conclusions from this remark, which, correctly viewed, is the foundation of the most important law of crystallography, that which connects the primitive form with other forms capable of being exhibited by the same substance, by a certain fixed relation. An idea may be formed of what is meant by this sort of connection of one form with another, by considering a pointed pyramid built up of cubic stones, disposed in layers, each of which separately is a square plate of the thickness of a single stone. These layers, laid horizontally one on the other, and decreasing regularly in size from the bottom to the top, produce a pyramidal form with a rough or channeled surface; and if the layers are so extremely thin that the channels cease to be visible to the eye, the pyramid will seem smooth and perfect.

(262.) Very shortly after this, and without knowledge of what had been done by Gahn and Bergmann, the Abbé Haüy, instructed by the accidental fracture of a fine group of crystals, made the remark noticed already (in 67.), and reasoning on it with more caution and success, and pursuing it into all its detail, developed the general laws which regulate the superposition of the layers of particles of which he supposes all crystals to be built up, and which enable us, from knowing their primitive forms, to discover, previous to trial, what other forms they are capable of assuming; and which, according to this idea, are called derivative or secondary forms. Mohs and others have since imagined processes and systems by which the derivation of forms from each other is facilitated, and have corrected some errors of over-hasty generalization into which their predecessors had fallen, as well as advanced, by an extraordinary diligence of research, our knowledge of the forms which the various substances which occur in nature and art actually do assume.

(263.) In what manner a variety in point of external form may originate in a variety of figures in the ultimate particles of which a solid is composed, may very readily be imagined by considering what would happen if the bricks of which an edifice is constructed had all a certain _leaning_ or bias in one direction out of the perpendicular. Suppose every brick, for instance, when laid flat on its face, with its longer edges north and south, had its eastern and western faces upright, but its northern and southern ones leaning southwards at a certain inclination the same for each brick; a house built of such bricks would lean the same way, if the bricks fitted well together. If, _besides this_, the eastern and western faces of the bricks, instead of being truly upright, had an inclination eastward, the house would have a similar one, and all its four corners, instead of being upright, would lean to the south-east. Suppose, instead of a house, a pyramid were built of such oblique bricks, with the sides of its base directed to the four points of the compass; then its point, instead of being situated vertically over the centre of its base, would stand perpendicularly over some point to the south-east of that centre, and the pyramid itself would have its sides facing the south and the east, more highly inclined to the horizon than those towards the north and west.

(264.) Whatever conception we may form of the manner in which the particles of a crystal cohere and form masses, it is next to impossible to divest ourselves of the idea of a determinate figure common to them all. Any other supposition, indeed, would be incompatible with that exact similarity in all other respects which the phenomena of chemistry may be considered as having demonstrated. However, it must be borne in mind that this idea, plausible as it may appear, is yet in some degree hypothetical, and that the laws of crystallography, as determined from inductive observation, are quite independent of any supposition of the kind, or even of the existence of such things as ultimate particles or atoms at all.

(265.) Still, that peculiar internal constitution of solid bodies, whatever it be, which is indicated by the assumption of determinate figures, by their splitting easier in some directions than in others, and by their presenting glittering plane surfaces when broken into fragments, cannot but have an important influence on all their relations to external agents, as well as to their internal movements and the mutual actions of their parts on one another. Accordingly, the division of bodies into crystallized and uncrystallized, or imperfectly crystallized, is one of the most universal importance; and almost all the phenomena produced by those more intimate natural causes which act within small limits, and as it were on the immediate mechanism of solid substances, are remarkably modified by their crystalline structure. Thus, in transparent solids, the course taken by the rays of light, in traversing them, as well as the properties impressed upon them in so doing, are intimately connected with this structure. The recent experiments of M. Savart, too, have proved that this is also the case with their power of resistance to external force, on which depends their elasticity. Crystallized substances, according to the results of these experiments, resist compression with different degrees of elastic force, according to the direction in which it is attempted to compress them; and all the phenomena dependent on their elasticity are affected by this cause, especially those which relate to their vibratory movements and their conveyance of sound.

(266.) There can be little doubt that modifications, similarly depending on the internal structure of crystals, will be traced through every department of physics. In that interesting one which relates to the action of heat in expanding the dimensions of substances, a beginning has already been made by Professor Mitscherlich. It had long been known that all substances are dilated by heat, and no exception to this law has been found, so long as we regard the _bulk_ of the heated body. Thus, an iron rod when hot is both longer and thicker than when cold; and the difference of dimension, though but trifling in itself, is yet capable of being made sensible, and is of considerable consequence in engineering. Thus, too, the quicksilver in a common thermometer occupies a larger space when hot than when cold; and being confined by the glass ball, (which also expands, but _not so much in proportion_,) it is forced to rise in the tube. These and similar facts had long been known; and accurate measures of the total amount of dilatation of a variety of different bodies, under similar accessions of heat, had been obtained and registered in tables. But no one had suspected the important fact, that this expansion in crystallized bodies takes place under totally different circumstances from what obtains in uncrystallized ones. M. Mitscherlich has lately shown that such substances expand differently in different directions, and has even produced a case in which expansion in one direction is actually accompanied with contraction in another. This step, the most important beyond a doubt which has yet been made in pyrometry, can however only be regarded as the first in a series of researches which will occupy the next generation, and which promises to afford an abundant harvest of new facts, as well as the elucidation of some of the most obscure and interesting points in the doctrine of heat.

(267.) From what has been said, it is clear that if we look upon solid bodies as collections of particles or atoms, held together and kept in their places by the perpetual action of attractive and repulsive forces, we cannot suppose these forces, at least in crystallized substances, to act alike in all directions. Hence arises the conception of _polarity_, of which we see an instance, on a great scale, in the magnetic needle, but which, under modified forms, there is nothing to prevent us from conceiving to act among the ultimate atoms of solid or even fluid bodies, and to produce all the phenomena which they exhibit in their crystallized state, either when acting on each other, or on light, heat, &c. It is not difficult, if we give the reins to imagination, to conceive how attractive and repulsive atoms, bound together by some unknown tie, may form little machines or compound particles, which shall have many of the properties which we refer to polarity; and accordingly many ingenious suppositions have been made to that effect: but in the actual state of science it is certainly safest to wave these hypotheses, without however absolutely rejecting them, and regard the _polarity of matter_ as one of the ultimate phenomena to which the analysis of nature leads us, and of which it is our business fully to investigate the laws, before we endeavour to ascertain its causes, or trace the mechanism by which it is produced.

(268.) The mutual attractions and repulsions of the particles of matter, then, and their polarity, whether regarded as an original or a derivative property, are the forces which, acting with great energy, and within very confined limits, we must look to as the principles on which the intimate constitution of all bodies and many of their mutual actions depend. These are what are understood by the general term of _molecular forces_. Molecular attraction has been attempted to be confounded by some with the general attraction of gravity, which all matter exerts on all other matter; but this idea is refuted by the plainest facts.

CHAP. II.

OF THE COMMUNICATION OF MOTION THROUGH BODIES.--OF SOUND AND LIGHT.

(269.) The propagation of motion through all substances, whether of a single impulse, as a blow or thrust, or of one frequently and regularly repeated, such as a jarring or vibratory movement, depends wholly on these molecular forces; and it is on such propagation that sound and very probably light depend. To conceive the manner in which a motion may be conveyed from one part of a substance to another, whether solid or fluid, we may attend to what takes place when a wave is made to run along a stretched string, or the surface of still water. Every part of the string, or water, is in succession moved from its place, and agitated with a motion similar to that of the original impulse, leaving its place and returning to it, and when one part ceases to move the next receives as it were the impression, and forwards it onward. This may seem a slow and circuitous process in description; but when sound, for example, is conveyed through the air, we are to consider, 1st, that the air, the substance actually in motion, is extremely light and acted upon by a very powerful elasticity, so that the force which propagates the motion, or by which the particles adjacent act on, and urge forward, each other, is very great, compared with the quantity of materials set in motion by it: and the same is true, even in a greater degree, in liquids and solids; for in these the elastic forces are even greater, in proportion to the weight, than in air.

(270.) A general notion of the mode in which sounds are conveyed through the air was not altogether deficient among the ancients; but it is to Newton that we owe the first attempt to analyze the process, and show correctly what takes place in the communication of motion from particle to particle. Reasoning on the properties of the air as an elastic body, he showed the effect of an impulse on any portion of it to consist in a condensation of the air immediately adjacent in the direction of the impulse, which then, re-acting by its spring, drives back the portion which had advanced to its original place, and at the same time urges forward the portion before it, in the direction of the impulse, so that every particle alternately advances and retreats. But, in pursuing this idea into its details, Newton fell into some errors which were pointed out by Cramer, though their origin was not traced, nor the reasoning corrected, till the subject was resumed by Lagrange and Euler; nor is this any impeachment of the penetration of our immortal countryman. The mathematical theory of the propagation of sound, and of vibratory and undulatory motions in general, is one of the utmost intricacy; and, in spite of every exertion on the part of the most expert geometers, continues to this day to give continual occasion for fresh researches; while phenomena are constantly presenting themselves, which show how far we are from being able to deduce all the particulars, even of cases comparatively simple, by any direct reasoning from first principles.

(271.) Whenever an impulse of any kind is conveyed by the air, to our ears, it produces the impression of sound; but when such an impulse is regularly and uniformly repeated in extremely rapid succession, it gives us that of a musical note, the pitch of the note depending on the rapidity of the succession (see art. 153.). The sense of harmony, too, depends on the periodical recurrence of coincident impulses on the ear, and affords, perhaps, the only instance of a sensation for whose pleasing impression a distinct and intelligible reason can be assigned.

(272.) Acoustics, then, or the science of sound, is a very considerable branch of physics, and one which has been cultivated from the earliest ages. Even Pythagoras and Aristotle were not ignorant of the general mode of its transmission through the air, and of the nature of harmony; but as a branch of science, independent of its delightful application in the art of music, it could be hardly said to exist, till its nature and laws became a matter of experimental enquiry to Bacon and Galileo, Mersenne and Wallis; and of mathematical investigation to Newton, and his illustrious successors, Lagrange and Euler. From that time its progress, as a branch both of mathematical and experimental science, has been constant and accelerated. A curious and beautiful method of observation, due to Chladni, consists in the happy device of strewing sand over the surfaces of bodies in a state of sonorous vibration, and marking the figures it assumes. This has made their motions susceptible of ocular examination, and has been lately much improved on, and varied in its application, by M. Savart, to whom we also owe a succession of instructive researches on every point connected with the subject of sound, which may rank among the finest specimens of modern experimental enquiry. But the subject is far from being exhausted; and, indeed, there are few branches of physics which promise at once so much amusing interest, and such important consequences, in its bearings on other subjects, and especially, through the medium of strong analogies, on that of light.

_Light and Vision._

(273.) The nature of light has always been involved in considerable doubt and mystery. The ancients could scarcely be said to have any opinion on the subject, unless, indeed, it could be considered such to affirm that distant bodies could not be put into communication without an intermedium; and that, therefore, there must be _something_ between the eye and the thing seen. What that something is, however, they could only form crude and vague conjectures. One supposed that the eyes themselves emit rays or emanations of some unknown kind, by which distant objects are as it were felt; a singularly unfortunate idea, since it gives no reason why objects should not be equally well seen in the dark--no account, in short, of the part performed by _light_ in vision. Others imagined that all visible objects are constantly throwing out from them, in all directions, some sort of resemblances or spectral forms of themselves, which, when received by the eyes, produce an impression of the objects. Vague and clumsy as this hypothesis obviously is, it assigns to the object a power, and to light a diffusive propagation in all directions, which are, the one and the other, independent of our eyes, and therefore goes to separate the phenomena of _light_ from those of _vision_.

(274.) The hypothesis of Newton is a refinement and improvement on this idea. Instead of spectra or resemblances, he supposes luminous objects actually to dart out from them in all directions, particles, of inconceivable minuteness (as indeed they must be, having such an enormous velocity (see 17.), not to dash in pieces every thing they strike upon). These particles he supposes to be acted upon by attractive and repulsive forces, residing in all material bodies, the latter extending to some very small distance beyond their surfaces; and by the action of these forces to be turned aside from their natural straight-lined course, without ever coming in actual contact with the particles themselves of the bodies on which they fall, but either being turned back and _reflected_ by the repulsive forces before they reach them, or penetrating between their intervals, as a bird may be supposed to fly through the branches of a forest, and undergoing all their actions, to take at quitting them a direction finally determined by the position of the surface at which they emerge with respect to their course.

(275.) This hypothesis, which was discussed and reasoned upon by Newton in a manner worthy of himself, affords, by the application of the same dynamical laws which he had applied with so much success to the explanation of the planetary motions, not merely a plausible, but a perfectly reasonable and fair explanation of all the _usual_ phenomena of light known in his time. His own beautiful discoveries, too, of the different refrangibilities of the differently coloured rays, were perfectly well represented in this theory, by simply admitting a difference of velocity in the particles, which produce in the eye the sensations of different colours. And had the properties of light remained confined to these, there would have been no occasion to have resorted to any other mode of conceiving it.

(276.) A very different hypothesis had, however, been suggested about the same period by Huyghens, who supposed light to be produced in the same manner with sound, by the communication of a vibratory motion from the luminous body to a highly elastic fluid, which he imagined as filling all space, and as being less condensed within the limits of space occupied by matter, and that to a greater or less extent, according to the nature of the occupying substance. Thus, in place of any thing actually thrown off, he substituted waves, or vibrations, propagated in all directions from luminous bodies, through this medium, or ether, as he called it. Huyghens, being himself a consummate mathematician, was enabled to trace many of the consequences of this hypothesis, and to show that the ordinary laws of reflection and refraction were represented or accounted for by it, as well as by Newton’s. But the hypothesis of Huyghens has not been fully successful in accounting for what may be considered the chief of all optical facts, the production of colours in the ordinary refraction of light by a prism, of which the theory of Newton gives a complete and elegant explanation; and the discovery of which by him marks one of the greatest epochs in the annals of experimental science. This, which has been often urged in objection to it, remains still, if not quite unanswered, at least only imperfectly removed.

(277.) Other phenomena, however, were not wanting to afford a further trial of the _explanatory powers_ of either hypothesis. The diffraction or inflection of light, discovered by Grimaldi, a Jesuit of Bologna, seemed to indicate that the rays of light were turned aside from their straight course by merely passing near bodies of every description. These phenomena, which are very curious and beautiful, were minutely examined by Newton, and referred by him to the action of repulsive forces extending to a sensible distance from the surfaces of bodies; and his explanation, so far as the facts known to him are concerned, appears as satisfactory as could reasonably be then expected; and much more so than any thing which could at that time be produced on the side of the hypothesis of Huyghens, which, in fact, seemed incapable of giving any account whatever of them.

(278.) Another class of delicate and splendid optical phenomena, which had begun to attract attention somewhat previous to Newton’s time, seemed to leave both hypotheses equally at a loss. These were the colours exhibited by very thin films, either of a liquid (such as a soap-bubble), or of air, as when two glasses are laid together with only air between them. These colours were examined by Newton with a minuteness and care altogether unexampled in experimental philosophy at that time, and with which few researches undertaken since will bear to stand in competition. Their result was a theory of a very singular nature, which he grounded on an hypothesis of what he termed _fits of easy transmission and reflection_; and which supposed each ray of light to pass in its progress periodically through a succession of states such as would alternately dispose it to penetrate or be reflected back from the surface of a body on which it might fall. The simplest way in which the reader may conceive this hypothesis, is to regard every particle of light as a sort of little magnet revolving rapidly about its own centre while it advances in its course, and thus alternately presenting its attractive and repulsive pole, so that when it arrives at the surface of a body with its repulsive pole foremost, it is repelled and reflected; and when the contrary, attracted, so as to enter the surface. Newton, however, very cautiously avoided announcing his theory in this or any similar form, confining himself entirely to general language. In consequence, it has been confidently asserted by all his followers, that the doctrine of fits of easy reflection and transmission, as laid down by him, is substantially nothing more than a statement of facts. Were it so, it is clear that any other theory which should offer a just account of the same phenomena must ultimately involve and coincide with that of Newton. But this, as we shall presently see, is not the case; and this instance ought to serve to make us extremely cautious how we employ, in stating physical laws derived from experiment, language which involves any thing in the slightest degree theoretical, if we would present the laws themselves in a form which no future research shall modify or subvert.

(279.) A third class of optical phenomena, which were likewise discovered while Newton was yet engaged in his optical researches, was that exhibited by doubly refracting crystals. In what the phenomenon of double refraction consists, we have already had occasion to explain. The fact itself was first noticed by Erasmus Bartolin in the crystal called Iceland spar; and was studied with attention by Huyghens, who ascertained its laws, and referred it with remarkable ingenuity and success to his theory of light, by the additional hypothesis of such a constitution of his ethereal medium within the crystal as should enable it to convey an impulse faster in one direction than another: as if, for example’s sake, we should suppose a sound conveyed through the air with different degrees of rapidity in a vertical and horizontal direction.

(280.) Some remarkable facts accompanying the double refraction produced by Iceland spar, which Bartolin, Huyghens, and Newton, had observed, led the latter to conceive the singular idea that a ray of light after its emergence from such a crystal acquires _sides_, that is to say, distinct relations to surrounding space, which it carries with it through its whole subsequent course, and which give rise to all those curious and complicated phenomena which are now known under the name of the _polarization of light_. These results, however, appeared so extraordinary, and offered so little handle for further enquiry, that their examination dropped, as if by common consent; Newton himself resting content with urging strongly the apparent incompatibility of these properties with the Huyghenian doctrine, but without making any attempt to explain them by his own.

(281.) From the period of Newton’s optical discoveries to the commencement of the present century, no great accession to our knowledge of the nature of light was made, if we except one, which, from its invaluable practical application, must ever hold a prominent place in the annals both of art and science: we mean, the discovery of the principle of the achromatic telescope, which originated in a discussion between the celebrated geometer Euler, Klingenstierna, an eminent Swedish philosopher, and our own countryman, the admirable optician Dollond, on the occasion of certain abstract theoretical investigations of the former, which led him to speculate on its _possibility_, and which ultimately terminated in its complete and happy _execution_ by the latter; a memorable case in science, though not a singular one, where the speculative geometer in his chamber, apart from the world, and existing among abstractions, has originated views of the noblest practical application.[49]

(282.) The explanation which our knowledge of optical laws affords of the mechanism of the eye, and the process by which vision is performed, is as complete and satisfactory as that of hearing by the propagation of motion through the air. The camera obscura, invented by Baptista Porta in 1560, gave the first idea how the actual images of external objects might be conveyed into the eye, but it was not till after a considerable interval that Kepler, the immortal discoverer of those great laws which regulate the periods and motions of the planets, pointed out distinctly the offices performed by the several parts of the eye in the act of vision. From this to the invention of the telescope and microscope there would seem but a small step, but it is to accident rather than design that it is due; and its re-invention by Galileo, on a mere description of its effects, may serve, among a thousand similar instances, to show that inestimable practical applications lie open to us, if we can only once bring ourselves to conceive their possibility, a lesson which the invention of the achromatic telescope itself, as we have above related it, not less strongly exemplifies.

(283.) The little instrument with which Galileo’s splendid discoveries were made was hardly superior in power to an ordinary finder of the present day; but it was rapidly improved on, and in the hands of Huyghens attained to gigantic dimensions and very great power. It was to obviate the necessity of the enormous length required for these telescopes, and yet secure the same power, that Gregory and Newton devised the reflecting telescope, which has since become a much more powerful instrument than its original inventors probably ever contemplated.

(284.) The telescope, as it exists at present, with the improvements in its structure and execution which modern artists have effected, must assuredly be ranked among the highest and most refined productions of human art; that in which man has been able to approximate most closely to the workmanship of nature, and which has conferred upon him, if not another sense, at least an exaltation of one already possessed by him that merits almost to be regarded as a new one. Nor does it appear yet to have reached its ultimate perfection, to which indeed it is difficult to assign any bounds, when we take into consideration the wonderful progress which workmanship of every kind is making, and the delicacy, far superior to that of former times, with which materials may now be wrought, as well as the ingenious inventions and combinations which every year is bringing forth for accomplishing the same ends by means hitherto unattempted.[50]

(285.) After a long torpor, the knowledge of the properties of light began to make fresh progress about the end of the last century, advancing with an accelerated rapidity, which has continued unabated to the present time. The example was set by our late admirable and lamented countryman, Dr. Wollaston, who re-examined and verified the laws of double refraction in Iceland spar announced by Huyghens. Attention being thus drawn to the subject, the geometry of Laplace soon found a means of explaining at least one portion of the mystery of this singular phenomenon, by the Newtonian theory of light, applied under certain supposed conditions; and the reasoning which led him to the result (at that time quite unexpected), may justly be regarded as one of his happiest efforts. The prosecution of the subject, which had now acquired a high degree of interest, was encouraged by the offer of a prize on the part of the French Academy of Sciences; and it was in a memoir which received this honourable reward on that occasion, in 1810, that Malus, a retired officer of engineers in the French army, announced the great discovery of the _polarization of light_ by ordinary reflection at the surface of a transparent body.

(286.) Malus found that when a beam of light is reflected from the surface of such a body at a certain angle, it acquires precisely the same singular property which is impressed upon it in the act of double refraction, and which Newton had before expressed by saying that it possessed _sides_. This was the first circumstance which pointed out a connection between that hitherto mysterious phenomenon and any of the ordinary modifications of light; and it proved ultimately the means of bringing the whole within the limits, if not of a complete explanation, at least of a highly plausible theoretical representation. So true is, in science, the remark of Bacon, that no natural phenomenon can be adequately studied _in itself alone_, but, to be understood, must be considered _as it stands connected with all nature_.

(287.) The new class of phenomena thus disclosed were immediately studied with diligence and success, both abroad by Malus and Arago, and at home by our countryman Dr. Brewster, and their laws investigated with a care proportioned to their importance; when another and apparently still more extraordinary class of phenomena presented itself in the production of the most vivid and beautiful colours (every way resembling those observed by Newton in thin films of air or liquids, only infinitely more developed and striking,) in certain transparent crystallized substances, when divided into flat plates in particular directions, and exposed in a beam of polarized light. The attentive examination of these colours by Wollaston, Biot, and Arago, but more especially by Brewster, speedily led to the disclosure of a series of optical phenomena so various, so brilliant, and evidently so closely connected with the most important points relating to the intimate structure of crystallized bodies, as to excite the highest interest,--that sort of interest which is raised when we feel we are on the eve of some extraordinary discovery, and expect every moment that some leading fact will turn up, which will throw light on all that appears obscure, and reduce into order all that seems anomalous.

(288.) This expectation was not disappointed. So long before the time we are speaking of as the first year of the present century, our illustrious countryman, the late Dr. Thomas Young, had established a principle in optics, which, regarded as a physical law, has hardly its equal for beauty, simplicity, and extent of application, in the whole circle of science. Considering the manner in which the vibrations of two musical sounds arriving at once at the ear affect the sense with an impression of sound or silence according as they conspire or oppose each other’s effects, he was led to the idea that the same ought to hold good with light as with sound, if the theory which makes light analogous to sound be the true one; and that, therefore, two rays of light, setting off from the same origin, at the same instant, and arriving at the same place by different routes, ought to strengthen or wholly or partially destroy each other’s effects according to the difference in length of the routes described by them. That two lights should in any circumstances combine to produce darkness may be considered strange, but is _literally true_; and it had even been noticed long ago as a singular and unaccountable fact by Grimaldi, in his experiments on the inflection of light. The experimental means by which Dr. Young confirmed this principle, which is known in optics by the name of the _interference_ of the rays of light, were as simple and satisfactory as the principle itself is beautiful; but the verifications of it, drawn from the explanation it affords of phenomena apparently the most remote, are still more so. Newton’s colours of thin films were the first phenomena to which its author applied it with full success. Its next remarkable application was to those of diffraction, of which, in the hands of M. Fresnel, a late eminent French geometer, it also furnished a complete explanation, and that, too, in cases to which Newton’s hypothesis could not apparently be made to apply, and through a complication of circumstances which might afford a very severe test of any hypothesis.

(289.) A simple and beautiful experiment on the interferences of polarized light due to Fresnel and Arago enabled them to bring Dr. Young’s law to bear on the colours produced by crystallized plates in a polarized beam, and by so doing afforded a key to all the intricacies of these magnificent but complex phenomena. Nothing now was wanting to a rational theory of double refraction but to frame an hypothesis of some mode in which light might be conceived to be propagated through the elastic medium supposed to convey it in such a way as not to be contradictory to any of the facts, nor to the general laws of dynamics. This essential idea, without which every thing that had been before done would have been incomplete, was also furnished by Dr. Young, who, with a sagacity which would have done honour to Newton himself, had declared, that to accommodate the doctrine of Huyghens to the phenomena of polarized light it is necessary to conceive the mode of propagation of a luminous impulse through the ether, differently from that of a sonorous one through the air. In the latter, the particles of the air _advance_ and _recede_; in the former, those of the ether must be supposed to _tremble laterally_.

(290.) Taking this as the groundwork of his reasoning, Fresnel succeeded in erecting on it a theory of polarization and double refraction, so happy in its adaptation to facts, and in the coincidence with experience of results deduced from it by the most intricate analysis, that it is difficult to conceive it unfounded. If it be so, it is at least the most curiously artificial system that science has yet witnessed; and whether it be so or not, so long as it serves to group together in one comprehensive point of view a mass of facts almost infinite in number and variety, to reason from one to another, and to establish analogies and relations between them; on whatever hypothesis it may be founded, or whatever arbitrary assumptions it may make respecting structures and modes of action, it can never be regarded as other than a most real and important accession to our knowledge.

(291.) Still, it is by no means impossible that the Newtonian theory of light, if cultivated with equal diligence with the Huyghenian, might lead to an equally plausible explanation of phenomena now regarded as beyond its reach. M. Biot is the author of the hypothesis we have already mentioned of a rotatory motion of the particles of light about their axes. He has employed it only for a very limited purpose; but it might doubtless be carried much farther; and by admitting only the regular emission of the luminous particles at equal intervals of time, and in similar states of motion from the shining body, which does not seem a very forced supposition, all the phenomena of interference at least would be readily enough explained without the admission of an ether.

(292.) The optical examination of crystallized substances affords one among many fine examples of the elucidation which every branch of science is capable of affording to every other. The indefatigable researches of Dr. Brewster and others have shown that the phenomena exhibited by polarized light in its transmission through crystals afford a certain indication of the most important points relating to the structure of the crystals themselves, and thus become most valuable characters by which to recognise their internal constitution. It was Newton who first showed of what importance as a physical character,--as the indication of other properties,--the action of a body on light might become; but the characters afforded by the use of polarized light as an instrument of experimental enquiry are so marked and intimate, that they may almost be said to have furnished us with a kind of intellectual sense, by which we are enabled to scrutinize the internal arrangement of those wonderful structures which Nature builds up by her refined and invisible architecture, with a delicacy eluding our conception, yet with a symmetry and beauty which we are never weary of admiring. In this point of view the science of optics has rendered to mineralogy and crystallography services not less important than to astronomy by the invention of the telescope, or to natural history by that of the microscope; while the relations which have been discovered to exist between the optical properties of bodies and their crystalline forms, and even their chemical habitudes, have afforded numerous and beautiful instances of general laws concluded from laborious and painful induction, and curiously exemplifying the simplicity of nature as it emerges slowly from an entangled mass of particulars in which, at first, neither order nor connection can be traced.

CHAP. III.

OF COSMICAL PHENOMENA.

_Astronomy and Celestial Mechanics._

(293.) Astronomy, as has been observed in the former part of this discourse, as a science of observation, had made considerable progress among the ancients: indeed, it was the only branch of physical science which could be regarded as having been cultivated by them with any degree of assiduity or real success. The Chaldean and Egyptian records had furnished materials from which the motions of the sun and moon could be calculated with sufficient exactness for the prediction of eclipses; and some remarkable cycles, or periods of years in which the lunar eclipses return in very nearly the same order, had been ascertained by observation. Considering the extreme imperfection of their means of measuring time and space, this was, perhaps, as much as could have been expected at that early period, and it was followed up for a while in a philosophical spirit of just speculation, which, if continued, could hardly have failed to lead to sound and important conclusions.

(294.) Unfortunately, however, the philosophy of Aristotle laid it down as a principle, that the celestial motions were regulated by laws proper to themselves, and bearing no affinity to those which prevail on earth. By thus drawing a broad and impassable line of separation between celestial and terrestrial mechanics, it placed the former altogether out of the pale of experimental research, while it at the same time impeded the progress of the latter by the assumption of principles respecting natural and unnatural motions, hastily adopted from the most superficial and cursory remark, undeserving even the name of observation. Astronomy, therefore, continued for ages a science of mere record, in which theory had no part, except in so far as it attempted to conciliate the inequalities of the celestial motions with that assumed law of uniform circular revolution which was alone considered consistent with the perfection of the heavenly mechanism. Hence arose an unwieldy, if not self-contradictory, mass of hypothetical motions of sun, moon, and planets, in circles, whose centres were carried round in other circles, and these again in others without end,--“cycle on epicycle, orb on orb,”--till at length, as observation grew more exact, and fresh epicycles were continually added, the absurdity of so cumbrous a mechanism became too palpable to be borne. Doubts were expressed, to which the sarcasm of a monarch[51] gave a currency they might not have obtained in a period when men scarcely dared trust themselves to think; and at length Copernicus, promulgating his own, or reviving the Pythagorean doctrine, which places the sun in the centre of our system, gave to astronomy a simplicity which, contrasted with the complication of the preceding views, at once commanded assent.

(295.) An elegant writer[52], whom we have before had occasion to quote, has briefly and neatly accounted for the confused notions which so long prevailed respecting the constitution of our system, and the difficulty experienced in acquiring a true notion of the disposition of its parts. “We see it,” he observes, “not in _plan_, but in _section_.” The reason of this is, that our point of observation lies in its general plane, but the notion we aim at forming of it is not that of its section, but of its plan. This is as if we should attempt to read a book, or make out the countries on a map, with the eye on a level with the paper. We can only judge directly of the distances of objects by their sizes, or rather of their change of distance by their change of size; neither have we any means of ascertaining, otherwise than indirectly, even their positions, one among the other, from their apparent places as seen by us. Now, the variations in apparent size of the sun and moon are too small to admit of exact measure without the use of the telescope, and the bodies of the planets cannot even be distinguished as having any distinct size with the naked eye.

(296.) The Copernican system once admitted, however, this difficulty of conception, at least, is effectually got over, and it becomes a mere problem of geometry and calculation to determine, from the observed places of a planet, its real orbit about the sun, and the other circumstances of its motion. This Kepler accomplished for the orbit of Mars, which he ascertained to be an ellipse having the sun in one of its foci; and the same law, being extended by inductive analogy to all the planets, was found to be verified in the case of each. This with the other remarkable laws which are usually cited in physical astronomy by the name of Kepler’s laws, constitute undoubtedly the most important and beautiful system of geometrical relations which have ever been discovered by a mere inductive process, independent of any consideration of a theoretical kind. They comprise within them a compendium of the motions of all the planets, and enable us to assign their places in their orbits at any instant of time past or to come (disregarding their mutual perturbations), provided certain purely geometrical problems can be numerically resolved.

(297.) It was not, however, till long after Kepler’s time that the real importance of these laws could be felt. Regarded in themselves, they offered, it is true, a fine example of regular and harmonious disposition in the greatest of all the works of creation, and a striking contrast to the cumbersome mechanism of the cycles and epicycles which preceded them; but there their utility seemed to terminate, and, indeed, Kepler was reproached, and not without a semblance of reason, with having rendered the actual calculation of the places of the planets more difficult than before, the resources of geometry being then inadequate to resolve the problems to which the strict application of his laws gave rise.

(298.) The first result of the invention of the telescope and its application to astronomical purposes, by Galileo, was the discovery of Jupiter’s disc and satellites,--of a system offering a beautiful miniature of that greater one of which it forms a portion, and presenting to the eye of sense, at a single glance, that disposition of parts which in the planetary system itself is discerned only by the eye of reason and imagination (see 195.). Kepler had the satisfaction of seeing it ascertained, that the law which he had discovered to connect the times of revolution of the planets with their distances from the sun, holds good also when applied to the periods of circulation of these little attendants round the centre of their principal; thus demonstrating it to be something more than a mere empirical rule, and to depend on the intimate nature of planetary motion itself.

(299.) It had been objected to the doctrine of Copernicus, that, were it true, Venus should appear sometimes horned like the moon. To this he answered by admitting the conclusion, and averring that, should we ever be able to see its actual shape, it _would_ appear so. It is easy to imagine with what force the application would strike every mind when the telescope confirmed this prediction, and showed the planet just as both the philosopher and his objectors had agreed it ought to appear. The history of science affords perhaps only one instance analogous to this. When Dr. Hutton expounded his theory of the consolidation of rocks by the application of heat, at a great depth below the bed of the ocean, and especially of that of marble by actual fusion; it was objected that, whatever might be the case with others, with calcareous or marble rocks, at least, it was impossible to grant such a cause of consolidation, since heat decomposes their substance and converts it into quicklime, by driving off the carbonic acid, and leaving a substance perfectly infusible, and incapable even of agglutination by heat. To this he replied, that the pressure under which the heat was applied would prevent the escape of the carbonic acid; and that being retained, it might be expected to give that fusibility to the compound which the simple quicklime wanted. The next generation saw this anticipation converted into an observed fact, and verified by the direct experiments of Sir James Hall, who actually succeeded in melting marble, by retaining its carbonic acid under violent pressure.

(300.) Kepler, among a number of vague and even wild speculations on the causes of the motions whose laws he had developed so beautifully and with so much patient labour, had obtained a glimpse of the general law of the inertia of matter, as applicable to the great masses of the heavenly bodies as well as to those with which we are conversant on the earth. After Kepler, Galileo, while he gave the finishing blow to the Aristotelian dogmas which erected a barrier between the laws of celestial and terrestrial motion, by his powerful argument and caustic ridicule, contributed, by his investigations of the laws of falling bodies and the motions of projectiles, to lay the foundation of a true system of dynamics, by which motions could be determined from a knowledge of the forces producing them, and forces from the motions they produce. Hooke went yet farther, and obtained a view so distinct of the mode in which the planets might be retained in their orbits by the sun’s attraction, that, had his mathematical attainments been equal to his philosophical acumen, and his scientific pursuits been less various and desultory, it can hardly be doubted that he would have arrived at a knowledge of the law of gravitation.

(301.) But every thing which had been done towards this great end, before Newton, could only be regarded as smoothing some first obstacles, and preparing a state of knowledge, in which powers like his could be effectually exerted. His wonderful combination of mathematical skill with physical research enabled him to invent, at pleasure, new and unheard-of methods of investigating the effects of those causes which his clear and penetrating mind detected in operation. Whatever department of science he touched, he may be said to have formed afresh. Ascending by a series of close-compacted inductive arguments to the highest axioms of dynamical science, he succeeded in applying them to the complete explanation of all the great astronomical phenomena, and many of the minuter and more enigmatical ones. In doing this, he had every thing to create: the mathematics of his age proved totally inadequate to grapple with the numerous difficulties which were to be overcome; but this, so far from discouraging him, served only to afford new opportunities for the exertion of his genius, which, in the invention of the method of fluxions, or, as it is now more generally called, the differential calculus, has supplied a means of discovery, bearing the same proportion to the methods previously in use, that the steam-engine does to the mechanical powers employed before its invention. Of the optical discoveries of Newton we have already spoken; and if the magnitude of the objects of his astronomical discoveries excite our admiration of the mental powers which could so familiarly grasp them, the minuteness of the researches into which he there set the first example of entering, is no less calculated to produce a corresponding impression. Whichever way we turn our view, we find ourselves compelled to bow before his genius, and to assign to the name of NEWTON a place in our veneration which belongs to no other in the annals of science. His era marks the accomplished maturity of the human reason as applied to such objects. Every thing which went before might be more properly compared to the first imperfect attempts of childhood, or the essays of inexpert, though promising, adolescence. Whatever has been since performed, however great in itself, and worthy of so splendid and auspicious a beginning, has never, in point of intellectual effort, surpassed that astonishing one which produced the Principia.

(302.) In this great work, Newton shows all the celestial motions known in his time to be consequences of the simple law, that every particle of matter attracts every other particle in the universe with a force proportional to the product of their masses directly, and the square of their mutual distance inversely, and is itself attracted with an equal force. Setting out from this, he explains how an attraction arises between the great spherical masses of which our system consists, regulated by a law precisely similar in its expression; how the elliptic motions of planets about the sun, and of satellites about their primaries, according to the exact rules inductively arrived at by Kepler, result as necessary consequences from the same general law of force; and how the orbits of comets themselves are only particular cases of planetary movements. Thence proceeding to applications of greater difficulty, he explains how the perplexing inequalities of the moon’s motion result from the sun’s disturbing action; how tides arise from the unequal attraction of the sun as well as of the moon on the earth, and the ocean which surrounds it; and, lastly, how the precession of the equinoxes is a necessary consequence of the very same law.

(303.) The immediate successors of Newton found full occupation in verifying his discoveries, and in extending and improving the mathematical methods which it had now become manifest were to prove the keys to an inexhaustible treasure of knowledge. The simultaneous but independent discovery of a method of mathematical investigation in every respect similar to that of Newton, by Leibnitz, while it created a degree of national jealousy which can now only be regretted, had the effect of stimulating the continental geometers to its cultivation, and impressing on it a character more entirely independent of the ancient geometry, to which Newton was peculiarly attached. It was fortunate for science that it did so; for it was speedily found that (with one fine exception on the part of our countryman Maclaurin, followed up, after a long interval, by the late Professor Robison of Edinburgh, with equal elegance,) the geometry of Newton was like the bow of Ulysses, which none but its master could bend; and that, to render his methods available beyond the points to which he himself carried them, it was necessary to strip them of every vestige of that antique dress in which he had delighted to clothe them. This, however, the countrymen of Newton were very unwilling to do; and they paid the penalty in finding themselves condemned to the situation of lookers on, while their continental neighbours both in Germany and France were pushing forward in the career of mathematico-physical discovery with emulous rapidity.

(304.) The legacy of research which Newton may be said to have left to his successors was truly immense. To pursue, through all its intricacies, the consequences of the law of gravitation; to account for all the inequalities of the planetary movements, and the infinitely more complicated, and to us more important ones, of the moon; and to give, what Newton himself certainly never entertained a conception of, a demonstration of the stability and permanence of the system, under all the accumulating influence of its internal perturbations; this labour, and this triumph, were reserved for the succeeding age, and have been shared in succession by Clairaut, D’Alembert, Euler, Lagrange and Laplace. Yet so extensive is the subject, and so difficult and intricate the purely mathematical enquiries to which it leads, that another century may yet be required to go through with the task. The recent discoveries of astronomers have supplied matter for investigation, to the geometers of this and the next generation, of a difficulty far surpassing any thing that had before occurred. Five primary planets have been added to our system; four of them since the commencement of the present century, and these, singularly deviating from the general analogy of the others, and offering _cases of difficulty_ in theory, which no one had before contemplated. Yet even the intricate questions to which these bodies have given rise seem likely to be surpassed by those which have come into view, with the discovery of several comets revolving in elliptic orbits, like the planets, round the sun, in very moderate periods. But the resources of modern geometry seem, so far from being exhausted, to increase with the difficulties they have to encounter, and already, among the successors of Lagrange and Laplace, the present generation has to enumerate a powerful array of names, which promise to render it not less celebrated in the annals of physico-mathematical research than that which has just passed away.

(305.) Meanwhile the positions, figures, and dimensions of all the planetary orbits, are now well known, and their variations from century to century in great measure determined; and it has been generally demonstrated, that all the changes which the mutual actions of the planets on each other can produce in the course of indefinite ages, are _periodical_, that is to say, increasing to a certain extent (and that never a very great one), and then again decreasing; so that the system can never be destroyed or subverted by the mutual action of its parts, but keeps constantly oscillating, as it were, round a certain mean state, from which it can never deviate to any ruinous extent. In particular the researches of Laplace, Lagrange, and Poisson, have shown the ultimate invariability of the mean distance of each planet from the sun, and consequently of its periodic time. Relying on these grand discoveries, we are enabled to look forward, from the point of time which we now occupy, many thousands of years into futurity, and predict the state of our system without fear of material error, but such as may arise from causes whose existence at present we have no reason to suppose, or from interference which we have no right to anticipate.

(306.) A correct enumeration and description of the fixed stars in catalogues, and an exact knowledge of their position, supply the only effectual means we can have of ascertaining what changes they are liable to, and what motions, too slow to deprive them of their usual epithet, _fixed_, yet sufficient to produce a sensible change in the lapse of ages, may exist among them. Previous to the invention of the compass, they served as guides to the navigator by night; but for this purpose, a very moderate knowledge of a few of the principal ones sufficed. Hipparchus was the first astronomer, who, excited by the appearance of a new star, conceived the idea of forming a catalogue of the stars, with a view to its use as an astronomical record, “by which,” says Pliny, “posterity will be able to discover, not only whether they are born and die, but also whether they change their places, and whether they increase or decrease.” His catalogue, containing more than 1000 stars, was constructed about 128 years before Christ. It was in the course of the laborious discussion of his own and former observations of them, undertaken with a view to the formation of this catalogue, that he first recognised the fact of that slow, general advance of all the stars eastward, when compared with the place of the equinox, which is known under the name of the precession of the equinoxes, and which Newton succeeded in referring to a motion in the earth’s axis, produced by the attraction of the sun and moon.

(307.) Since Hipparchus, at various periods in the history of astronomy, catalogues of stars have been formed, among which that of Ulugh Begh, comprising about 1000 stars, constructed in 1437, is remarkable as the production of a sovereign prince, working personally in conjunction with his astronomers; and that of Tycho Brahe, containing 777 stars, constructed in 1600, as having originated in a phenomenon similar to that which drew the attention of Hipparchus. In more recent times, astronomers provided with the finest instruments their respective eras could supply, and established in observatories, munificently endowed by the sovereigns and governments of different European nations, have vied and are still vying with each other, in extending the number of registered stars, and giving the utmost possible degree of accuracy to the determination of their places. Among these, it would be ungrateful not to claim especial notice for the superb series of observations which, under a succession of indefatigable and meritorious astronomers, has, for a very long period, continued to emanate from our own national observatory of Greenwich.

(308.) The distance of the fixed stars is so immense, that every attempt to assign a limit, _within which_ it _must_ fall, has hitherto failed. The enquiries of astronomers of all ages have been directed to ascertain this distance, by taking the dimensions of our own particular system of sun and planets, or of the earth itself, as the unit of a scale on which it might be measured. But although many have imagined that their observations afforded grounds for the decision of this interesting point, it has uniformly happened either that the phenomena on which they relied have proved to be referable to other causes not previously known, and which the superior accuracy of their researches has for the first time brought to light; or to errors arising from instrumental imperfections and unavoidable defects of the observations themselves.

(309.) The only indication we can expect to obtain of the actual distance of a star, would consist in an annual change in its apparent place corresponding to the motion of the earth round the sun, called its _annual parallax_, and which is nothing more than the measure of the apparent size of the earth’s orbit as seen from the star. Many observers have thought they have detected a measurable amount of this parallax; but as astronomical instruments have advanced in perfection, the quantity which they have successively assigned to it has been continually reduced within narrower and narrower limits, and has invariably been commensurate with the errors to which the instruments used might fairly be considered liable. The conclusion this strongly presses on us is, that it is really a quantity too small to admit of distinct measurement in the present state of our means for that purpose; and that, therefore, the distance of the stars must be a magnitude of such an order as the imagination almost shrinks from contemplating. But this increase in our scale of dimension calls for a corresponding enlargement of conception in all other respects. The same reasoning which places the stars at such immeasurable remoteness, exalts them at the same time into glorious bodies, similar to, and even far surpassing, our own sun, the centres perhaps of other planetary systems, or fulfilling purposes of which we can have no idea, from any analogy in what passes immediately around us.

(310.) The comparison of catalogues, published at different periods, has given occasion to many curious remarks, respecting changes both of place and brightness among the stars, to the discovery of variable ones which lose and recover their lustre periodically, and to that of the disappearance of several from the heavens so completely as to have left no vestige discernible even by powerful telescopes. In proportion as the construction of astronomical and optical instruments has gone on improving, our knowledge of the contents of the heavens has undergone a corresponding extension, and, at the same time, attained a degree of precision which could not have been anticipated in former ages. The places of all the principal stars in the northern hemisphere, and of a great many in the southern, are now known to a degree of nicety which must infallibly detect any real motions that may exist among them, and has in fact done so, in a great many instances, some of them very remarkable ones.

(311.) It is only since a comparatively recent date, however, that any great attention has been bestowed on the smaller stars, among which there can be no doubt of the most interesting and instructive phenomena being sooner or later brought to light. The minute examination of them with powerful telescopes, and with delicate instruments for the determination of their places, has, indeed, already produced immense catalogues and masses of observations, in which thousands of stars invisible to the naked eye are registered; and has led to the discovery of innumerable important and curious facts, and disclosed the existence of whole classes of celestial objects, of a nature so wonderful as to give room for unbounded speculation on the extent and construction of the universe.

(312.) Among these, perhaps the most remarkable are the revolving double stars, or stars which, to the naked eye or to inferior telescopes, appear single; but, if examined with high magnifying powers, are found to consist of two individuals placed almost close together, and which, when carefully watched, are (many of them) found to revolve in regular elliptic orbits about each other; and so far as we have yet been able to ascertain, to obey the same laws which regulate the planetary movements. There is nothing calculated to give a grander idea of the scale on which the sidereal heavens are constructed than these beautiful systems. When we see such magnificent bodies united in pairs, undoubtedly by the same bond of mutual gravitation which holds together our own system, and sweeping over their enormous orbits, in periods comprehending many centuries, we admit at once that they must be accomplishing ends in creation which will remain for ever unknown to man; and that we have here attained a point in science where the human intellect is compelled to acknowledge its weakness, and to feel that no conception the wildest imagination can form will bear the least comparison with the intrinsic greatness of the subject.

_Geology._

(313.) The researches of physical astronomy are confessedly incompetent to carry us back to the origin of our system, or to a period when its state was, in any great essential, different from what it is at present. So far as the causes now in action go, and so far as our calculations will enable us to estimate their effects, we are equally unable to perceive in the general phenomena of the planetary system either the evidence of a beginning, or the prospect of an end. Geometers, as already stated, have demonstrated that, in the midst of all the fluctuations which can possibly take place in the elements of the orbits of the planets, by reason of their mutual attraction, the general balance of the parts of the system will always be preserved, and every departure from a mean state periodically compensated. But neither the researches of the physical astronomer, nor those of the geologist, give us any ground for regarding our system, or the globe we inhabit, as of eternal duration. On the contrary, there are circumstances in the physical constitution of our own planet which at least obscurely point to an origin and a formation, however remote, since it has been found that the figure of the earth is not globular but elliptical, and that its attraction is such as requires us to admit the interior to be more dense than the exterior, and the density to increase with some degree of regularity from the surface towards the centre, and _that_, in layers arranged elliptically round the centre, circumstances which could scarcely happen without some such successive deposition of materials as would enable pressure to be propagated with a certain degree of freedom from one part of the mass to another, even if we should hesitate to admit a state of primitive fluidity.

(314.) But from such indications nothing distinct can be concluded; and if we would speculate to any purpose on a former state of our globe and on the succession of events which from time to time may have changed the condition and form of its surface, we must confine our views within limits far more restricted, and to subjects much more within the reach of our capacity, than either the creation of the world or its assumption of its present figure. These, indeed, were favourite speculations with a race of geologists now extinct; but the science itself has undergone a total change of character, even within the last half century, and is brought, at length, effectually within the list of the inductive sciences. Geologists now no longer bewilder their imaginations with wild theories of the formation of the globe from chaos, or its passage through a series of hypothetical transformations, but rather aim at a careful and accurate examination of the records of its former state, which they find indelibly impressed on the great features of its actual surface, and to the evidences of former life and habitation which organised remains imbedded and preserved in its strata indisputably afford.

(315.) Records of this kind are neither few nor vague; and though the obsoleteness of their language when we endeavour to interpret it too minutely, may, and no doubt often does, lead to misapprehension, still its general meaning is, on the whole, unequivocal and satisfactory. Such records teach us, in terms too plain to be misunderstood, that the whole or nearly the whole of our present lands and continents were formerly at the bottom of the sea, where they received deposits of materials from the wearing and degradation of other lands not now existing, and furnished receptacles for the remains of marine animals and plants inhabiting the ocean above them, as well as for similar spoils of the land washed down into its bosom.

(316.) These remains are occasionally brought to light; and their examination has afforded indubitable evidence of the former existence of a state of animated nature widely different from what now obtains on the globe, and of a period anterior to that in which it has been the habitation of man, or rather, indeed, of a series of periods, of unknown duration, in which both land and sea teemed with forms of animal and vegetable life, which have successively disappeared and given place to others, and these again to new races approximating gradually more and more nearly to those which now inhabit them, and at length comprehending species which have their counterparts existing.

(317.) These wrecks of a former state of nature, thus wonderfully preserved (like ancient medals and inscriptions in the ruins of an empire), afford a sort of rude chronology, by whose aid the successive depositions of the strata in which they are found may be marked out in epochs more or less definitely terminated, and each characterized by some peculiarity which enables us to recognise the deposits of any period, in whatever part of the world they may be found. And, so far as has been hitherto investigated, the _order_ of succession in which these deposits have been formed appears to have been the same in every part of the globe.

(318.) Many of the strata which thus bear evident marks of having been deposited at the bottom of the sea, and of course in a horizontal state, are now found in a position highly inclined to the horizon, and even occasionally vertical. And they often bear no less evident marks of violence, in their bending and fracture, the dislocation of parts which were once contiguous, and the existence of vast collections of broken fragments which afford every proof of great violence having been used in accomplishing some at least of the changes which have taken place.

(319.) Besides the rocks which carry this internal evidence of submarine deposition, are many which exhibit no such proofs, but on the contrary hold out every appearance of owing their origin to volcanoes or to some other mode of igneous action; and in every part of the world, and among strata of all ages, there occur evidences of such action so abundant, and on such a scale, as to point out the volcano and the earthquake as agents which may have been instrumental in the production of those changes of level, and those violent dislocations which we perceive to have taken place.

(320.) At all events, in accounting for those changes, geologists have no longer recourse, as formerly, to causes purely hypothetical, such as a shifting of the earth’s axis of rotation, bringing the sea to overflow the land, by a change in the place of the longer and shorter diameters of the spheroidal figure, nor to tides produced by the attraction of comets suddenly approaching very near the earth, nor to any other fanciful and arbitrarily assumed hypotheses; but rather endeavour to confine themselves to a careful consideration of causes evidently in action at present, with a view to ascertain how far they, in the first instance, are capable of accounting for the facts observed, and thus legitimately bringing into view, as residual phenomena, those effects which cannot be so accounted for. When this shall have been in some measure accomplished, we shall be able to pronounce with greater security than at present respecting the necessity of admitting a long succession of tremendous and ravaging catastrophes and cataclysms,--epochs of terrific confusion and violence which many geologists (perhaps with justice) regard as indispensable to the explanation of the existing features of the world. We shall learn to distinguish between the effects which require for their production the sudden application of convulsive and fracturing efforts, and those, probably not less extensive, changes which may have been produced by forces equally or more powerful, but acting with less irregularity, and so distributed over time as to produce none of those _interregnums_ of chaotic anarchy which we are apt to think (perhaps erroneously) great disfigurements of an order so beautiful and harmonious as that of nature.

(321.) But to estimate justly the effects of causes now in action in geology is no easy task. There is no _à priori_ or deductive process by which we can estimate the amount of the annual erosion, for instance, of a continent by the action of meteoric agents, rain, wind, frost, &c., nor the quantity of destruction produced on its coasts by the direct violence of the sea, nor the quantity of lava thrown up _per annum_ by volcanoes over the whole surface of the earth, nor any similar effect. And to consult experience on all such points is a slow and painful process if rightly gone into, and a very fallible one if only partially executed. Much, then, at present must be left to opinion, and to that sort of clear-judging tact which sometimes anticipates experience; but this ought not to stand in the way of our making every possible effort to obtain accurate information on such points, by which alone geology can be rendered, if not an experimental science, at least a science of that kind of active observation which forms the nearest approach to it, where actual experiment is impossible.

(322.) Let us take, for example, the question, “What is the actual direction in which changes of relative level are taking place between the existing continents and seas?” If we consult partial experience, that is, _all_ the information that we possess respecting ancient sea-marks, soundings, &c., we shall only find ourselves bewildered in a mass of conflicting, because imperfect, evidence. It is obvious that the only way to decide the point is to ascertain, by very precise and careful observations at proper stations on coasts, selected at points where there exist natural marks not liable to change in the course of at least a century, the true elevation of such marks above the _mean_ level of the sea, and to multiply these stations sufficiently over the whole globe to be capable of affording real available knowledge. Now, this is not a very easy operation (considering the accuracy required); for the _mean_ level of the sea can be determined by no single observation, any more than the mean height of the barometer at a given station, being affected both by periodical and accidental fluctuations due to tides, winds, waves, and currents. Yet if an instrument adapted for the purpose were constructed, and rendered easily attainable, and rules for its use carefully drawn up, there is little doubt we should soon (by the industry of observers scattered over the world) be in possession of a most valuable mass of information, which could not fail to afford a point of departure for the next generation, and furnish ground for the only kind of argument which ever can be conclusive on such subjects.

(323.) Geology, in the magnitude and sublimity of the objects of which it treats, undoubtedly ranks, in the scale of the sciences, next to astronomy; like astronomy, too, its progress depends on the continual accumulation of observations carried on for ages. But, unlike astronomy, the observations on which it depends, when the whole extent of the subject to be explored is taken into consideration, can hardly yet be said to be more than commenced. Yet, to make up for this, there is another important difference, that while in the latter science it is impossible to recall the past or anticipate the future, and observation is in consequence limited to a single fact in a single moment; in the former, the records of the past are always present;--they may be examined and re-examined as often as we please, and require nothing but diligence and judgment to put us in possession of their whole contents. Only a very small part of the surface of our globe has, however, been accurately examined in detail, and of that small portion we are only able to scratch the mere exterior, for so we must consider those excavations which we are apt to regard as searching the bowels of the earth; since the deepest mines which have been sunk penetrate to a depth hardly surpassing the ten thousandth part of the distance between its surface and its centre. Of course inductions founded on such limited examination can only be regarded as provisional, except in those remarkable cases where the same great formations in the same order have been recognised in very distant quarters, and without exception. This, however, cannot long be the case. The spirit with which the subject has been prosecuted for many years in our own country has been rewarded with so rich a harvest of surprising and unexpected discoveries, and has carried the investigation of our island into such detail, as to have excited a corresponding spirit among our continental neighbours; while the same zeal which animates our countrymen on their native shore accompanies them in their sojourns abroad, and has already begun to supply a fund of information respecting the geology of our Indian possessions, as well as of every other point where English intellect and research can penetrate.

(324.) Nothing can be more desirable than that every possible facility and encouragement should be afforded for such researches, and indeed to the pursuits of the enlightened resident or traveller in every department of science, by the representatives of our national authority wherever our power extends. By these only can our knowledge of the actual state of the surface of the globe, and that of the animals and vegetables of the ancient continents and seas, be extended and perfected, while more complete information than we at present possess of the habits of those actually existing, and the influence of changes of climate, food, and circumstances, on them, may be expected to render material assistance to our speculations respecting those which have become extinct.

CHAP. IV.

OF THE EXAMINATION OF THE MATERIAL CONSTITUENTS OF THE WORLD.

_Mineralogy._

(325.) The consideration of the history and structure of our globe, and the examination of the fossil contents of its strata, lead us naturally to consider the materials of which it consists. The history of these materials, their properties as objects of philosophical enquiry, and their application to the useful arts and the embellishments of life, with the characters by which they can be certainly distinguished one from another, form the object of mineralogy, taken in its most extended sense.

(326.) There is no branch of science which presents so many points of contact with other departments of physical research, and serves as a connecting link between so many distant points of philosophical speculation, as this. To the geologist, the chemist, the optician, the crystallographer, the physician, it offers especially the very elements of their knowledge, and a field for many of their most curious and important enquiries. Nor, with the exception of chemistry, is there any which has undergone more revolutions, or been exhibited in a greater variety of forms. To the ancients it could scarcely be said to be at all known, and up to a comparatively recent period, nothing could be more imperfect than its descriptions, or more inartificial and unnatural than its classification. The more important minerals in the arts, indeed, those used for economical purposes and those from which metals were extracted, had a certain degree of attention paid to them, for the sake of their utility and commercial value, and the precious stones for that of ornament. But until their crystalline forms were attentively observed and shown to be determinate characters on which dependence could be placed, no mineralogist could give any correct account of the real distinction between one mineral and another.

(327.) It was only, however, when chemical analysis had acquired a certain degree of precision and universal applicability that the importance of mineralogy as a science began to be recognized, and the connection between the external characters of a stone and its ingredient constituents brought into distinct notice. Among these characters, however, none were found to possess that eminent distinctness which the crystalline form offers; a character, in the highest degree geometrical, and affording, as might be naturally supposed, the strongest evidence of its necessary connection with the intimate constitution of the substance. The full importance of this character was, however, not felt until its connection with the texture or cleavage of a mineral was pointed out, and even then it required numerous and striking instances of the critical discernment of Haüy and other eminent mineralogists in predicting from the measurements of the angles of crystals which had been confounded together that differences would be found to exist in their chemical composition, all which proved fully justified in their result before the essential value of this character was acknowledged. This was no doubt in great measure owing to the high importance set by the German mineralogists on those external characters of touch, sight, weight, colour, and other sensible qualities, which are little susceptible, with the exception of weight, of exact determination, and which are subject to material variations in different specimens of the same mineral. By degrees, however, the necessity of ascribing great weight to a character so definite was admitted, especially when it was considered that the same step which pointed out the intimate connection of external form with internal structure furnished the mineralogist with the means of reducing all the forms of which a mineral is susceptible under one general type, or primitive form, and afforded grounds for an elegant theoretical account of the assumption of definite figures _ab initio_.

(328.) A simple and elegant invention of Dr. Wollaston, the reflecting goniometer, gave a fresh impulse to that view of mineralogy which makes the crystalline form the essential or leading character, by putting it in the power of every one, by the examination of even the smallest portion of a broken crystal, to ascertain and verify that essential character on which the identity of a mineral in the system of Haüy was made to depend. The application of so ready and exact a method speedily led to important results, and to a still nicer discrimination of mineral species than could before be attained; and the confirmation given to these results by chemical analysis stamped them with a scientific and decided character which they have retained ever since.

(329.) Meanwhile the progress made in chemical analysis had led to the important conclusion that every chemical compound susceptible of assuming the solid state assumed with it a determinate crystalline form; and the progress of optical science had shown that the fundamental crystalline form, in the case at least of transparent bodies, drew with it a series of optical properties no less curious than important in relation to the affections of light in its passage through such substances. Thus, in every point of view, additional importance became added to this character; and the study of the crystalline forms of bodies in general assumed the form of a separate and independent branch of science, of which the geometrical forms of the mineral world constituted only a particular case. Mineralogy, however, as a branch of natural history, remains still distinct either from optics or crystallography. The mineralogist is content, and thinks he has performed his task, if not as a natural historian at least as a classifier and arranger, if he only gives such a characteristic description of a mineral as shall effectually distinguish it from every other, and shall enable any one who may encounter such a body in any part of the world to impose on it its name, assign it a place in his system, and turn to his books for a further description of all that the chemist, the optician, the lapidary, or the artist, may require to know. Still this is no easy matter: the laborious researches of the most eminent mineralogists can hardly yet be said to have effectually accomplished it; and its difficulty may be appreciated by the small number of simple minerals, or minerals of perfectly definite and well-marked characters, which have been hitherto made out. Nor can this indeed be wondered at, when we consider that by far the greater portion of the rocks and stones which compose the external crust of the globe consists of nothing more than the accumulated _detritus_ of older rocks, in which the fragments and powder of an infinite variety of substances are mingled together, in all sorts of varying proportions, and in such a way as to defy separation. Many of these rocks, however, so compounded, occur with sufficient frequency and uniformity of character to have acquired names and to have been usefully applied; indeed, in the latter respect, minerals of this description far surpass all the others. As objects of natural history, therefore, they are well worthy of attention, however difficult it may be to assign them a place in any artificial arrangement.

(330.) This paucity of simple minerals, however, is probably rather apparent than real, and in proportion as the researches of the chemist and crystallographer shall be extended throughout nature, they will no doubt become much more numerous. Indeed, in the great laboratories of nature it can hardly be doubted that almost every kind of chemical process is going forwards, by which compounds of every description are continually forming. Accordingly, it is remarked, that the lavas and ejected scoriæ of volcanoes are receptacles in which mineral products previously unknown are constantly discovered, and that the primitive formations, as they are called in geology, which bear no marks of having been produced by the destruction of others, are also remarkable for the beauty and distinctness of character of their minerals.

(331.) The great difficulty which has been experienced in attempts to classify mineral substances by their chemical constituents has arisen from the observed presence, in some specimens of minerals bearing that general resemblance in other respects as well as agreement in form which would seem to entitle them to be considered as alike, of ingredients foreign to the usual composition of the species, and that occasionally in so large a proportion as to render it unjustifiable to refer their occurrence to accidental impurities. These cases, as well as some anomalies observed in the classification of minerals by their crystalline forms, which seemed to show that the same substance might occasionally appear under two distinct forms, as well as some remarkable coincidences between the forms of substances quite distinct from each other in a chemical point of view, have within a recent period given rise to a branch of the science of crystallography of a very curious and important nature. The _isomorphism_ of certain groups of chemical elements has already afforded us an example illustrative of the manner in which inductions sometimes receive unexpected verifications (see 180.). The laws and relations thus brought to light are among the most curious and interesting parts of modern science, and seem likely in their further developement to afford ample scope for the exercise of chemical and mineralogical research. They have already afforded innumerable fine examples of that important step in science by which anomalies disappear, and occasional incongruities become reconciled under more general expressions of physical laws, and thus unite in affording support to those very views which they promised, when first observed, to overset. Nothing, indeed, can be more striking than to see the very ingredient which every previous chemist and mineralogist would agree to disregard and reject as a mere casual impurity brought forward and appealed to in support of a theory expressly directed to the object of rescuing science from the imputation of disregarding, under any circumstances, the plain results of direct experiment.

_Chemistry._

(332.) The laws which concern the intimate constitution of bodies, not as respects their _structure_ or the manner in which their parts are put together, but as regards their _materials_ or the ingredients of which those parts are composed, form the objects of chemistry. A solid body may be regarded as a fabric, more or less regularly and artificially constructed, in which the materials and the workmanship may be separately considered, and in which, though the latter be ruined and confounded by violence, the former remain unchanged in their nature, though differently arranged. In liquid or aërial bodies, too, though there prevails a less degree of difference in point of structure, and a greater facility of dispersion and dissipation, than in solids, yet an equal diversity of _materials_ subsists, giving to them properties differing extremely from each other.

(333.) The inherent activity of matter is proved not only by the production of motion by the mutual attractions and repulsions of distant or contiguous masses, but by the changes and apparent transformations which different substances undergo in their sensible qualities by mere mixture. If water be added to water, or salt to salt, the effect is an increase of quantity, but no change of quality. In this case, the mutual action of the particles is entirely mechanical. Again, if a blue powder and a yellow one, each perfectly dry, be mixed and well shaken together, a green powder will be produced; but this is a mere effect arising in the eye from the intimate mixture of the yellow and blue light separately and independently reflected from the minute particles of each; and the proof is had by examining the mixture with a microscope, when the yellow and blue grains will be seen separate and each quite unaltered. If the same experiment be tried with coloured liquids, which are susceptible of mixing without chemical action, a compound colour is likewise produced, but no examination with magnifiers is in that case sufficient to detect the ingredients; the reason obviously being, the excessive minuteness of the parts, and their perfect intermixture, produced by agitating two liquids together. From the mixture of two powders, extreme patience would enable any one, by picking out with a magnifier grain after grain, to separate the ingredients. But when liquids are mixed, no mechanical separation is any longer practicable; the particles are so minute as to elude all search. Yet this does not hinder us from regarding such a compound as still a mere mixture, and its properties are accordingly intermediate between those of the liquids mixed. But this is far from being the case with all liquids. When a solution of potash, for example, and another of tartaric acid, each perfectly liquid, are mixed together in proper proportions, a great quantity of a solid saline substance falls to the bottom of the containing vessel, which is quite different from either potash or tartaric acid, and the liquid from which it subsided offers no indications by its taste or other sensible qualities of the ingredients mixed, but of something totally different from either. It is evident that this is a phenomenon widely different from that of mere mixture; there has taken place a great and radical change in the intimate nature of the ingredients, by which a new substance is produced which had no existence before. And it has been produced by the _union_ of the ingredients presented to each other; for when examined it is found that nothing has been _lost_, the weight of the whole mixture being the sum of the weights mixed. Yet the potash and tartaric acid have disappeared entirely, and the weight of the new product is found to be exactly equal to that of the tartaric acid and potash employed, taken together, abating a small portion held in solution in the liquid, which may be obtained however by evaporation. They have therefore combined, and adhere to one another with a cohesive force sufficient to form a solid out of a liquid; a force which has thus been called into action by merely presenting them to each other in a state of solution.

(334.) It is the business of chemistry to investigate these and similar changes, or the reverse of such changes, where a single substance is resolved into two or more others, having different properties from it, and from each other, and to enquire into all the circumstances which can influence them; and either determine, modify, or suspend their accomplishment, whether such influence be exercised by heat or cold, by time and rest, or by agitation or pressure, or by any of those agents of which we have acquired a knowledge, such as electricity, light, magnetism, &c.

(335.) The wonderful and sudden transformations with which chemistry is conversant, the violent activity often assumed by substances usually considered the most inert and sluggish, and, above all, the insight it gives into the nature of innumerable operations which we see daily carried on around us, have contributed to render it the most popular, as it is one of the most extensively useful, of the sciences; and we shall, accordingly, find none which have sprung forward, during the last century, with such extraordinary vigour, and have had such extensive influence in promoting corresponding progress in others. One of the chief causes of its popularity is, perhaps, to be sought for in this, that it is, of all the sciences, perhaps, the most completely an experimental one; and even its theories are, for the most part, of that generally intelligible and readily applicable kind, which demand no intense concentration of thought, and lead to no profound mathematical researches. The simple process of inductive generalization, grounded on the examination of numerous facts, all of them presenting considerable intrinsic interest, has sufficed, in most instances, to lead, by a clear and direct road, to its highest laws yet known. But, on the other hand, these laws, when stated, are not yet fully sufficient to lead us, except in very limited cases, to a deductive knowledge of particulars never before examined, at least, not without great caution, and constant appeal to experiment as a check on our reasoning; so that we are justified in regarding the _axioms_ of chemistry, the true handles of deductive reasoning, as still unknown, and, perhaps, likely long to remain so. This is no fault of its cultivators, who have comprised in their list the highest and most varied talents and industry, but of the inherent complexity of the subject, and the infinite multitude of causes which are concerned in the production of every, even the simplest, chemical phenomenon.

(336.) The history of chemistry (on which, however, we are not about to enlarge,) is one of great interest to those who delight to trace the steps by which mankind advance to the discovery of truth through a series of mistakes and failures. It may be divided, 1st, into the period of the alchemists, a lamentable epoch in the annals of intellectual wandering; 2dly, that of the phlogistic doctrines of Beccher and Stahl, in which, as if to prove the perversity of the human mind, of two possible roads the wrong was chosen; and a theory obtained universal credence on the strength of an induction, valid as such, but wrongly interpreted, which is negatived, _in every instance_, by an appeal to the balance. This, too, happened, not by reason of unlucky coincidences, or individual oversights, but of necessity, and from an inherent defect of the theory itself, which thus impeded the progress of the science, as far as a science of experiment can be impeded by a false theory, by perplexing its cultivators with the appearance of contradictions in their experiments where none really subsisted, by destroying all their confidence in the numerical exactness of their own results, and by involving the subject in a mist of visionary and hypothetical causes in place of the true acting principles. Thus, in the combustion of any substance which is incapable of flying away in fumes, an increase of weight takes place,--the ashes are heavier than the fuel. Whenever this was observed, however, it was passed carelessly over as arising from the escape of phlogiston, or the principle of inflammability, which was considered as being either the element of fire itself, or in some way combined with it, and thus essentially _light_. It is now known that the increase of weight is owing to the absorption of, and combination with, a quantity of a peculiar ingredient called _oxygen_, from the air, a principle essentially _heavy_. So far as weight is concerned, it makes no difference whether a body having weight enters, or one having levity escapes; but there is this plain difference in a philosophical point of view, that oxygen is a real producible substance, and phlogiston is no such thing: the former is a _vera causa_, the latter an hypothetical being, introduced to account for what the other accounts for much better.

(337.) The third age of chemistry--that which may be called emphatically modern chemistry--commenced (in 1786) when Lavoisier, by a series of memorable experiments, extinguished for ever this error, and placed chemistry in the rank of one of the exact sciences,--a science of number, weight, and measure. From that epoch to the present day it has constantly advanced with an accelerated progress, and at this moment may be regarded as more progressive than ever. The principal features in this progress may be comprised under the following general heads:--

1. The discovery of the proximate, if not the ultimate, elements of all bodies, and the enlargement of the list of known elements to its present extent of between fifty and sixty substances.

2. The developement of the doctrine of latent heat by Black, with its train of important consequences, including the scientific theory of the steam-engine.

3. The establishment of Wenzel’s law of definite proportions on his own experiments, and those of Richter, a discovery subsequently merged in the more general wording and better development of Dalton’s atomic theory.

4. The precise determination of the atomic weights of the different chemical elements, mainly due to the astonishing industry of Berzelius, and his unrivalled command of chemical resources, as well as to the researches of the other chemists of the Swedish and German school.

5. The assimilation of gases and vapours, by which we are led to regard the former, universally, as particular cases of the latter, a generalization resulting chiefly from the experiments of Faraday on the condensation of the gases, and those of Gay-Lussac and Dalton, on the laws of their expansion by heat compared with that of vapours.

6. The establishment of the laws of the combination of gases and vapours by definite volumes, by Gay-Lussac.

7. The discovery of the chemical effects of electricity, and the decomposing agency of the Voltaic pile, by Nicholson and Carlisle; the investigation of the laws of such decompositions, by Berzelius and Hisinger: the decomposition of the alkalies by Davy, and the consequent introduction into chemistry of new and powerful agents in their metallic bases.

8. The application of chemical analysis to all the objects of organized and unorganized nature, and the discovery of the ultimate constituents of all, and the proximate ones of organic matter, and the recognisance of the important distinctions which appear to divide these great classes of bodies from each other.

9. The applications of chemistry to innumerable processes in the arts, and among other useful purposes to the discovery of the essential medical principles in vegetables, and to important medicaments in the mineral kingdom.

10. The establishment of the intimate connection between chemical composition and crystalline form, by Haüy and Vauquelin, with the successive rectifications the statement of that connection has undergone in the hands of Mitscherlich, Rose, and others, with the progress of chemical and crystallographical knowledge.

(338.) To pursue these several heads into detail would lead us into a treatise on chemistry; but a few remarks on one or two of them, as they bear upon the general principles of all scientific enquiry, will not be irrelevant. And first, then, with reference to the discovery of new elements, it will be observed, that philosophical chemistry no more aims at determining the one essential element out of which all matter is framed--the one ultimate principle of the universe--than astronomy at discovering the origin of the planetary movements in the application of a determinate projectile force in a determinate direction, or geology at ascending to the creation of the earth. There may be such an element. Some singular relations which have been pointed out in the atomic weights of bodies seem to suggest to minds fond of speculation that there is; but philosophical chemistry is content to wait for some striking fact, which may either occur unexpectedly or be led to by the slow progress of enlarged views, to disclose to us its existence. Still, the multiplication of so-considered elementary bodies has been considered by some as an inconvenience. We confess we do not coincide with this view. Whatever they be, the obstinacy with which they resist decomposition shows that they are ingredients of a very high and primary importance in the economy of nature; and such as, in any state of science, it would be indispensably necessary to be perfectly familiar with. Like particular theorems in geometry, which, though not rising to the highest point of generality, have yet their several scopes and ranges of extensive application, they must be well and perfectly understood in all their bearings. Should we ever arrive at an analysis of these bodies, the chemical properties of the new elements which will then come into view will be known only by our knowledge of these, or of other compounds of the same class, which they may be capable of forming. Not but that such an analysis would be a most important and indeed triumphant achievement, and change the face of chemistry; but it would undo nothing that has been done, and render useless no point of knowledge which we have yet arrived at.

(339.) The atomic theory, or the law of definite proportions, which is the same thing presented in a form divested of all hypothesis, after the laws of mechanics, is, perhaps, the most important which the study of nature has yet disclosed. The extreme simplicity which characterizes it, and which is itself an indication, not unequivocal, of its elevated rank in the scale of physical truths, had the effect of causing it to be announced at once by Mr. Dalton, in its most general terms, on the contemplation of a few instances[53], without passing through subordinate stages of painful inductive ascent by the intermedium of subordinate laws, such as, had the contrary course been pursued by him, would have been naturally preparatory to it, and such as would have led others to it by the prosecution of Wenzel’s and Richter’s researches, had they been duly attended to. This is, in fact, an example, and a most remarkable one, of the effect of that natural propensity to generalize and simplify (noticed in 171.), which, if it occasionally leads to over-hasty conclusions, limited or disproved by further experience, is yet the legitimate parent of many of our most valuable and soundest results. Instances like this, where great and, indeed, immeasurable steps in our knowledge of nature are made at once, and almost without intellectual effort, are well calculated to raise our hopes of the future progress of science, and, by pointing out the simplest and most obvious combinations as those which are actually found to be agreeable to the harmony of creation, to hold out the cheering prospect of difficulties diminishing as we advance, instead of thickening around us in increasing complexity.

(340.) A consequence of this immediate presentation of the law of definite proportions in its most general form is, that its subordinate laws--those which limit its generality in particular cases, which diminish the number of combinations abstractly possible, and restrain the indiscriminate mixture of elements,--remain to be discovered. Some such limitations have, in fact, been traced to a certain extent, but by no means so far as the importance of the subject requires; and we have here abundant occupation for chemists for some time.

(341.) The determination of the atomic weights of the chemical elements, like that of other standard physical data, with the utmost exactness, is in itself a branch of enquiry not only of the greatest importance, but of extreme difficulty. Independent of the general reasons for desiring accuracy in this respect, there is one peculiar to the subject. It has been suggested (by Dr. Prout), and strongly insisted on (by Dr. Thomson), that all the numbers representing these weights, constituting a scale of great extent, in which the extremes already known are in proportion to each other, as 1 to upwards of 200, are simple even multiples of the least of them. If this be really the case, it opens views of such importance as to justify any degree of labour and pains in the verification of the law as a purely inductive one. But in the actual state of chemical analysis, with all deference to such high authority, we confess it appears to us to stand in great need of further confirmation, since it seems doubtful whether such accuracy has yet been attained as to enable us to answer positively for a fraction not exceeding the three or four hundredth part of the whole quantity to be determined: at least the results of the first experimenters, obtained with the greatest care, differ often by a greater amount; and this degree of exactness, at least, would be required to verify the law satisfactorily in the higher parts of the scale.

(342.) The mere agitation of such a question, however, points out a class of phenomena in physical science of a remote and singular kind, and of a very high and refined order, which could never become known but in an advanced state of science, not only practical, but theoretical,--we mean, such as consist in observed relations among the _data_ of physics, which show them to be quantities not _arbitrarily_ assumed, but depending on laws and causes which they may be the means of at length disclosing. A remarkable instance of such a relation is the curious law which Bode observed to obtain in the progression of the magnitudes of the several planetary orbits. This law was interrupted between Mars and Jupiter, so as to induce him to consider a planet as wanting in that interval;--a deficiency long afterwards strangely supplied by the discovery of _four_ new planets in that very interval, all of whose orbits conform in dimension to the law in question, within such moderate limits of error as may be due to causes independent of those on which the law itself ultimately rests.[54]

(343.) Neither is it irrelevant to our subject to remark, that the progress which has been made in this department of chemistry, and the considerable exactness actually attainable in chemical analysis, have been owing, in great measure, to a circumstance which might at first have been hardly considered likely to exercise much influence on the progress of a science,--the discovery of platina. Without the resources placed at the ready disposal of chemists by this invaluable metal, it is difficult to conceive that the multitude of delicate analytical experiments which have been required to construct the fabric of existing knowledge could have ever been performed. This, among many such lessons, will teach us that the most important uses of natural objects are not those which offer themselves to us most obviously. The chief use of the moon for man’s immediate purposes remained unknown to him for five thousand years from his creation. And, since it cannot but be that innumerable and most important uses remain to be discovered among the materials and objects already known to us, as well as among those which the progress of science must hereafter disclose, we may hence conceive a well-grounded expectation, not only of constant increase in the physical resources of mankind, and the consequent improvement of their condition, but of continual accessions to our power of penetrating into the arcana of nature, and becoming acquainted with her highest laws.

CHAP. V.

OF THE IMPONDERABLE FORMS OF MATTER.

_Heat._

(344.) One of the chief agents in chemistry, on whose proper application and management the success of a great number of its enquiries depends, and many of whose most important laws are disclosed to us by phenomena of a chemical nature, is HEAT. Although some of its effects are continually before our eyes as matters of the most common occurrence, insomuch that there is scarcely any process in the useful arts and manufactures which does not call for its intervention, and although, independent of this high utility, and the proportionate importance of a knowledge of its nature and laws, it presents in itself a subject of the most curious speculation; yet there is scarcely any physical agent of which we have so imperfect a knowledge, whose intimate nature is more hidden, or whose laws are of such delicate and difficult investigation.

(345.) The word heat generally implies the sensation which we experience on approaching a fire; but, in the sense it carries in physics, it denotes the cause, whatever it be, of that sensation, and of all the other phenomena which arise on the application of fire, or of any other heating cause. We should be greatly deceived if we referred only to sensation as an indication of the presence of this cause. Many of those things which excite in our organs, and especially of those of taste, a sensation of heat, owe this property to chemical stimulants, and not at all to their being actually _hot_. This error of judgment has produced a corresponding confusion of language, and hence had actually at one period[55] crept into physical philosophy a great many illogical and absurd conclusions. Again, there are a number of chemical agents, which, from their corroding, blackening, and dissolving, or drying up the parts of some descriptions of bodies, and producing on them effects not generally unlike (though intrinsically very different from) those produced by heat, are said, in loose and vulgar language, to burn them; and this error has even become rooted into a prejudice, by the fact that some of these agents are capable of becoming actually and truly _hot_ during their action on moist substances, by reason of their combination with the water the latter contain. Thus, quicklime and oil of vitriol both exercise a powerful corrosive action on animal and vegetable substances, and both become violently hot by their combination with water. They are, therefore, set down in vulgar parlance as substances of a hot nature; whereas, in their relations to the physical cause of heat, they agree with the generality of bodies similarly constituted.

(346.) The nature of heat has hitherto been chiefly studied under the general heads of--

1st, Its sources, or the phenomena which it usually accompanies.

2d, Its communication from its sources to substances capable of receiving it, and from these to others, with a view to discover the laws which regulate its distribution through space or through the bodies which occupy it.

3d, Its effects, on our senses, and on the bodies to which it is communicated in its various degrees of intensity, by which, means are afforded us of measuring these degrees.

4th, Its intimate relations to the atoms of matter, as exhibited in its capability of acquiring a latent state under certain circumstances, and of entering into something like chemical combinations.

(347.) The most obvious sources of heat are, the sun, fire, animal life, fermentations, violent chemical actions of all kinds, friction, percussion, lightning, or the electric discharge, in whatever manner produced, the sudden condensation of air, and others, so numerous, and so varied, as to show the extensive and important part it has to perform in the economy of nature. The discoveries of chemists, however, have referred most of these to the general head of chemical combination. Thus, fire, or the combustion of inflammable bodies, is nothing more than a violent chemical action attending the combination of their ingredients with the oxygen of the air. Animal heat is, in like manner, referable to a process bearing no remote analogy to a slow combustion, by which a portion of carbon, an inflammable principle existing in the blood, is united with the oxygen of the air in respiration; and thus carried off from the system: fermentation is nothing more than a decomposition of chemical elements loosely united, and their re-union in a more permanent state of combination. The analogy between the sun and terrestrial fire is so natural as to have been chosen by Newton to exemplify the irresistible force of an inference derived from that principle. But the nature of the sun and the mode in which its wonderful supply of light and heat is maintained are involved in a mystery which every discovery that has been made either in chemistry or optics, so far from elucidating, seems only to render more profound. Friction as a source of heat is well known: we rub our hands to warm them, and we grease the axles of carriage-wheels to prevent their setting fire to the wood; an accident which, in spite of this precaution, does sometimes happen. But the effect of friction, as a means of producing heat with little or no consumption of materials, was not fully understood till made the subject of direct experiment by count Rumford, whose results appear to have established the extraordinary fact, that an unlimited supply of heat may be derived by friction from the same materials. Condensation, whether of air by pressure, or of metals by percussion, is another powerful source of heat. Thus, iron may be so dexterously hammered as to become red-hot, and the rapid condensation of a confined portion of air will set tinder on fire.

(348.) The most violent heats known are produced by the concentration of the solar rays by burning glasses,--by the combustion of oxygen and hydrogen gases mixed in the exact proportion in which they combine to produce water,--and by the discharge of a continued and copious current of electricity through a small conductor. As these three sources of heat are independent of each other, and each capable of being brought into action in a very confined space, there seems no reason why they might not all three be applied at once at the same point, by which means, probably, effects would be produced infinitely surpassing any hitherto witnessed.

(349.) Heat is communicated either by _radiation_ between bodies at a distance, or by _conduction_ between bodies in contact, or between the contiguous parts of one and the same body. The laws of the radiation of heat have been studied with great attention, and have been found to present strong analogies with that of light in some points, and singular differences in others. Thus, the heat which accompanies the sun’s rays comports itself, in all respects, like light; being subject to similar laws of reflection, refraction, and even of polarization, as has been shown by Berard. Yet they are not identical with each other; Sir William Herschel having shown, by decisive experiments, verified by those of Sir H. Englefield, that there exist in a solar beam both rays of heat which are not luminous, and rays of light which have no heating power.

(350.) The heat, radiated by terrestrial fires, and by bodies _obscurely_ hot, by whatever means they have acquired their heat (even by exposure to the sun’s rays), differs very materially from solar heat in their power of penetrating transparent substances. This singular and important difference was first noticed by Mariotte, and afterwards made the subject of many curious and interesting experiments by Scheele, who found that terrestrial heat, or that radiated from fires or heated bodies, is intercepted and detained by glass or other transparent bodies, while solar heat is not; and that, being so detained, it heats them: which the latter, as it passes freely through them, is incapable of doing. The more recent researches of Delaroche, however, have shown that this detention is complete only when the temperature of the source of heat is low; but that, as that temperature is higher, a portion of the heat radiated acquires a power of penetrating glass; and that the quantity which does so bears continually a larger and larger proportion to the whole, as the heat of the radiant body is more intense. This discovery is very important, as it establishes a community of nature between solar and terrestrial heat; while at the same time it leads us to regard the actual temperature of the sun as far exceeding that of any earthly flame.

(351.) A variety of theories have been framed to account for these curious phenomena; but the subject stands rather in need of further elucidation from experiment, and is one which merits, and will probably amply repay, the labours of those who may hereafter devote their attention to it. The theory of the radiation of heat, in general, which seems to agree best with the known phenomena, is that of M. Prevost, who considers all bodies as constantly radiating out heat in all directions, and receiving it by a similar means of communication from others, and thus tending, in any space filled, wholly or in part, with bodies at various temperatures, to establish an equilibrium or equality of heat in all parts. The application of this idea to the explanation of the phenomenon of dew we have already seen (see 167.). The laws of such radiation, under various circumstances, have been lately investigated in a beautiful series of experiments on the cooling of bodies by their own radiation in vacuo, by Messrs. Dulong and Petit, which offer some of the best examples in science of the inductive investigation of quantitative laws.

(352.) The communication of heat between bodies in contact, or between the different parts of the same body, is performed by a process called conduction. It is, in fact, only a particular case of radiation, as has been explained above (217.); but a case _so_ particular as to require a separate and independent investigation of its laws. The most important consideration introduced into the enquiry by this peculiarity is that of time. The communication of heat by conduction is performed, for the most part, with extreme slowness, while that performed by direct radiation is probably not less rapid than the propagation of light itself. The analysis of the delicate and difficult points which arise in the investigation of this subject in its reduction to direct geometrical treatment has been executed with admirable success by the late Baron Fourrier, whose recent lamented death has deprived science of an ornament it could ill spare, thinned as its ranks have been within the last few years. This acute philosopher and profound mathematician has developed, in a series of elaborate memoirs presented to the French Institute, the laws of the communication of heat through the interior of solid masses, placed under the influence of any external heating and cooling causes, and has in particular applied his results to the conditions on which the maintenance of the actual observed temperature on the earth’s surface depends; to the possible influence of a supposed central heat on our climates; and to the determination of the actual amount of the heat, derived to us from the sun, or at least that portion of it on which the difference of the seasons depends.

(353.) The principal effects of heat are the sensations of warmth or cold consequent on its entry or egress into or out of our bodies; the dilatation it causes in the dimensions of all substances in which it is accumulated; the changes of state it produces in the melting of solids, and the conversion of them and of liquids into vapour; and the chemical changes it performs by actual decompositions effected in the intimate molecules of various substances, especially those of which vegetables and animals are composed; to which we may add, the production of electric phenomena under certain circumstances in the contact of metals, and the developement of electric polarity in crystallised substances.

(354.) Cold has been considered by some as a positive quality, the effect of a cause antagonist to that of heat; but this idea seems now (with perhaps a single exception) to be universally abandoned. The sensation of cold is as easily explicable by the passage of heat outwards through the surface of the body as that of heat by its ingress from without; and the experiments cited in proof of a radiation of cold are all perfectly explained by Prevost’s theory of reciprocal interchange. It is remarkable, however, how very limited our means of producing intense cold are, compared with those we possess of effecting the accumulation of heat in bodies. This is one of the strongest arguments adducible in favour of the doctrines of those who maintain the possibility of exhausting the heat of a body altogether, and leaving it in a state absolutely devoid of it. But we ought to consider, that the known methods of generating heat chiefly turn on the production of chemical combinations: we may easily conceive, therefore, that, to obtain equally powerful corresponding frigorific effects, we ought to possess the means of effecting a disunion equally extensive and rapid between such elements, actually combined, as have already produced heat by their union. This, however, we can only accomplish by engaging them in combinations still more energetic, that is to say, in which we may reasonably expect more heat to be produced by the new combination than would be destroyed or abstracted by the proposed decomposition. Chemistry, however, (unaided by electric agency,) affords no means of suddenly breaking the union of two elements, and presenting _both_ in an uncombined state. A certain analogy to such disunion, however, and its consequences, may be traced in the sudden expansion of condensed gases from a liquid state into vapour, which is the most powerful source of cold known.

(355.) The dilatation of bodies by heat forms the subject of that branch of science called pyrometry. There is no body but is capable of being penetrated by heat, though some with greater, others with less rapidity; and being so penetrated, all bodies (with a very few exceptions, and those depending on very peculiar circumstances,) are dilated by it in bulk, though with a great diversity in the amount of dilatation produced by the same degree of heat. Of the several forms of natural bodies, gases and vapours are observed to be most dilatable; liquids next, and solids least of all. The dilatation of solids has been made a subject of repeated and careful measurement by several experimenters; among whom, Smeaton, Lavoisier, and Laplace, are the principal. The remarkable discovery of the unequal dilatation of crystallised bodies by Mitscherlich has already been spoken of. (266.) That of gases and vapours was examined about the same time by Dalton and Gay-Lussac, who both arrived independently at the conclusion of an equal dilatability subsisting in them all, which constitutes one of the most remarkable points in their history.

(356.) The dilatation of air by heat affords, perhaps, the most unexceptionable means known of measuring degrees of heat. The thermometer, as originally constructed by Cornelius Drebell, was an air thermometer. Those now in common use measure accessions of heat not by the degree of dilatation of air but of mercury. It has been shown, by the researches of Dulong and Petit, that its indications coincide exactly with that of the air-thermometer in moderate temperatures; though at very elevated ones they exhibit a sensible, and even considerable, deviation. By this instrument, which owes its present convenience and utility to the happy idea of Newton, who first thought of fixing determinate points on its scale, we are enabled to estimate, or at least identify, the degrees of heat; and thereby to investigate with accuracy the laws of its communication and its other properties. Were we sure that equal additions of heat produced equal increments of dimension in any substance, the indications of a thermometer would afford a true and secure _measure_ of the quantity present; but this is so far from being the case, that we are nearly in total ignorance on this important point; a circumstance which throws the greatest difficulty in the way of all theoretical reasoning, and even of experimental enquiry. The laws of the dilatation of liquids, in consequence of this deficiency of necessary preliminary knowledge, are still involved in great obscurity, notwithstanding the pains which have been bestowed on them by the elaborate experiments and calculations of Gilpin, Blagden, Deluc, Dalton, Gay-Lussac, and Biot.

(357.) The most striking and important of the effects of heat consist, however, in the liquefaction of solid substances, and the conversion of the liquids so produced into vapour. There is no solid substance known which, by a sufficiently intense heat, may not be melted, and finally dissipated in vapour; and this analogy is so extensive and cogent, that we cannot but suppose that all those bodies which are liquid under ordinary circumstances, owe their liquidity to heat, and would freeze or become solid if their heat could be sufficiently reduced. In many we see this to be the case in ordinary winters; for some, severe frosts are requisite; others freeze only with the most intense artificial colds; and some have hitherto resisted all our endeavours; yet the number of these last is few, and they will probably cease to be exceptions as our means of producing cold become enlarged.

(358.) A similar analogy leads us to conclude that all aëriform fluids are merely liquids kept in the state of vapour by heat. Many of them have been actually condensed into the liquid state by cold accompanied with violent pressure; and as our means of applying these causes of condensation have improved, more and more refractory ones have successively yielded. Hence we are fairly entitled to extend our conclusion to those which we have not yet been able to succeed with; and thus we are led to regard it as a general fact, that the liquid and aëriform or vaporous states are entirely dependent on _heat_; that were it not for this cause, there would be nothing but solids in nature; and that, on the other hand, nothing but a sufficient intensity of heat is requisite to destroy the cohesion of every substance, and reduce all bodies, first to liquids, and then into vapour.

(359.) But solids, themselves, by the abstraction of heat shrink in dimension, and at the same time become harder, and more brittle; yielding less to pressure, and permitting less separation between their parts by tension. These facts, coupled with the greater compressibility of liquids, and the still greater of gases, strongly induce us to believe that it is heat, and heat alone, which holds the particles of all bodies at that distance from each other which is necessary to allow of compression; which in fact gives them their elasticity, and acts as the antagonist force to their mutual attraction, which would otherwise draw them into actual contact, and retain them in a state of absolute immobility and impenetrability. Thus we learn to regard heat as one of the great maintaining powers of the universe, and to attach to all its laws and relations a degree of importance which may justly entitle them to the most assiduous enquiry.

(360.) It was first ascertained by Dr. Black that when heat produces the liquefaction of a solid, or the conversion of a liquid into vapour, the liquid or the vapour resulting is no _hotter_ than the solid or liquid from which it was produced, though a great deal of heat has been expended in producing this effect, and has actually entered into the substance.

(361.) Hence he drew the conclusion that it has become _latent_, and continues to exist in the product, maintaining it in its new state, without increasing its temperature. He further proved, that when the vapour condenses, or the liquid freezes, this latent heat is again given out from it. This great discovery, with its natural and hardly less important concomitant, that of the difference of specific heats in different bodies, or the different quantities of heat they require to raise their temperature equally, are the chief reasons for regarding heat as a material substance in a more decided manner than light, with which in its radiant state it holds so close an analogy.

(362.) The subject of latent heat has been far less attentively studied than its great practical importance would appear to demand, when we consider that it is to this part of physical science that the theory of the steam-engine is mainly referable, and that material improvements may not unreasonably be expected in that wonderful instrument, from a more extended knowledge than we possess of the latent heats of different vapours. This is not the case, however, with the subject of specific heat, which was followed up immediately after its first promulgation with diligence by Irvine; and, after a brief interval, by Lavoisier and Laplace, as well as by our countryman Crawfurd, who determined the specific heats of many substances, both solid and liquid. After a considerable period of inactivity, the subject was again resumed by Delaroche and Berard, and subsequently by Dulong and Petit: the result of whose investigations has been the inductive establishment of one of those simple and elegant physical laws which carry with them, if not their own evidence, at least their own recommendation to our belief, as being in unison with every thing we know of the harmony of nature. The law to which we allude is this:--that the atoms of all the simple chemical elements have exactly the same capacity for heat, or are all equally heated or cooled by equal accessions or abstractions of heat. It is only among laws like this that we can expect to find a clew capable of guiding us to a knowledge of the true nature of heat, and its relations to ponderable matter.

_Magnetism and Electricity._

(363.) These two subjects, which had long maintained a distinct existence, and been studied as separate branches of science, are at length effectually blended. This is, perhaps, the most satisfactory result which the experimental sciences have ever yet attained. All the phenomena of magnetic polarity, attraction, and repulsion, have at length been resolved into one general fact, that two currents of electricity, moving in the same direction repel, and in contrary directions attract, each other. The phenomena of the communication of magnetism and what is called its induced state, alone remain unaccounted for; but the interesting theory which has been developed by M. Ampere, under the name of Electro-dynamics, holds out a hope that this difficulty will also in its turn give way, and the whole subject be at length completely merged, as far as the consideration of the acting causes goes, in the more general one of electricity. This, however, does not prevent magnetism from maintaining its separate importance as a department of physical enquiry, having its own peculiar laws and relations of the highest practical interest, which are capable of being studied quite apart from all consideration of its electrical origin. And not only so, but to study them with advantage, we must proceed as if that origin were totally unknown, and, at least up to a certain point, and that a considerably advanced one, conduct our enquiries into the subject on the same inductive principles as if this branch of physics were absolutely independent of all others.

(364.) Iron, and its oxides and alloys, were for a long time the only substances considered susceptible of magnetism. The loadstone was even one of the examples produced by Bacon of that class of physical instances to which he applies the term “Instantiæ monodicæ”--_singular instances_. And the history of magnetism affords a beautiful comment on his remark on instances of this sort. “Nor should our enquiries,” he observes, “into their nature be broken off, till the properties and qualities found in such things as may be esteemed wonders in nature are reduced and comprehended under some certain law; so that all irregularity or singularity may be found to depend upon some common form, and the wonder only rest in the exact differences, degrees, or extraordinary concurrence, and not in the species itself.” The discovery of the magnetism of nickel, which though inferior to that of iron, is still considerable; that of cobalt, yet feebler, and that of titanium, which is only barely perceptible, have effectually broken down the imaginary limit between iron and the other materials of the world, and established the existence of that general law of continuity which it is one chief business of philosophy to trace throughout nature. The more recent discoveries of M. Arago (mentioned in 160.) have completed this generalization, by showing that there is no substance but which, under proper circumstances, is capable of exhibiting unequivocal signs of the magnetic virtue. And to obliterate all traces of that line of separation which was once so broad, we are now enabled, by the great discovery of Oërsted, to communicate at and during pleasure to a coiled wire of any metal indifferently all the properties of a magnet;--its attraction, repulsion, and polarity; and _that_ even in a more intense degree than was previously thought to be possible in the best natural magnets. In short, in this case, and in this case only, perhaps, in science, have we arrived at that point which Bacon seems to have understood by the discovery of “forms.” “The _form_ of any nature,” says he, “is such, that where it is, the given nature must infallibly be. The form, therefore, is perpetually present when that nature is present; ascertains it universally, and accompanies it every where. Again, this form is such, that when removed, the given nature infallibly vanishes. Lastly, a true form is such as can deduce a given nature from some essential property, which resides in many things.”

(365.) Magnetism is remarkable in another important point of view. It offers a prominent, or “_glaring instance_” of that quality in nature which is termed _polarity_ (267.), and that under circumstances which peculiarly adapt it for the study of this quality. It does not appear that the ancients had any knowledge of this property of the magnet, though its attraction of iron was well known to them. The first mention of it in modern times cannot be traced earlier than 1180, though it was probably known to the Chinese before that time. The polarity of the magnet consists in this, that if suspended freely, one part of it will invariably direct itself towards a certain point in the horizon, the other towards the opposite point; and that, if two magnets, so suspended, be brought near each other, there will take place a mutual action, in consequence of which, the positions of both will be disturbed, in the same manner as would happen if the corresponding parts of each repelled, and those oppositely directed attracted, each other; and by properly varying the experiment, it is found that they really do so. If a small magnet, freely suspended, be brought into the neighbourhood of a larger one, it will take a position depending on the position of the _poles_ of the larger one, with respect to its point of suspension. And it has been ascertained that these and all other phenomena exhibited by magnets in their mutual attractions and repulsions are explicable on the supposition of two forces or virtues lodged in the particles of the magnets, the one predominating at one end, the other at the other; and such that each particle shall attract those in which the _opposite_ virtue to its own prevails, and repel those in which a _similar_ one resides with a force proportional to the inverse square of their mutual distance.

(366.) The direction in which a magnetic bar, or needle of steel, freely suspended, places itself, has been ascertained to be different at different points of the earth’s surface. In some places it points exactly north and south, in others it deviates from this direction more or less, and at some actually stands at right angles to it. This remarkable phenomenon, which is called the variation of the needle, and which was discovered by Sebastian Cabot in the year 1500, is accompanied with another called the dip, noticed by Robert Norman in 1576. It consists in a tendency of a needle, nicely balanced on its centre, when unmagnetized, to _dip_ or point downwards when rendered magnetic, towards a point below the horizon, and situated within the earth. By tracing the variation and dip over the whole surface of the globe, it has been found that these phenomena take place as they would do if the earth itself were a great magnet, having its poles deeply situated below the surface,--and, what is very remarkable, possessing a slow motion within it, in consequence of which neither the variation nor dip remain constantly the same at the same place. The laws of this motion are at present unknown; but the discovery of electro-magnetism, by rendering it almost certain that the earth’s magnetism is merely an effect of the continual circulation of great quantities of electricity round it, in a direction generally corresponding with that of its rotation, have dissipated the greater part of the mystery which hung over these phenomena; since a variety of causes, both geological and others, may be imagined which may produce considerable deviations in the intensity, and partial ones in the direction, of such electric currents. The unequal distribution of land and sea in the two hemispheres, by affecting the operation of the sun’s heat in producing evaporation from the latter, which is probably one of the great sources of terrestrial electricity, may easily be conceived to modify the general tendency of such currents, and to produce irregularities in them, which may render a satisfactory account of whatever still appears anomalous in the phenomena of terrestrial magnetism. This branch of science thus becomes connected, on a great scale, with that of meteorology, one of the most complicated and difficult, but at the same time interesting, subjects of physical research; one, however, which has of late begun to be studied with a diligence which promises the speedy disclosure of relations and laws of which at present we can form but a very imperfect notion.

(367.) The communication of magnetism from the earth to a magnetic body, or from one magnetic body to another, is performed by a process to which the name of induction has been given, and the laws and properties of such induced magnetism have been studied with much perseverance and success,--practically, by Gilbert, Boyle, Knight, Whiston, Cavallo, Canton, Duhamel, Rittenhouse, Scoresby, and others; and theoretically, by Æpinus, Coulomb, and Poisson, and in our own country by Messrs. Barlow and Christie, who have investigated with great care the curious phenomena which take place when masses of iron are presented successively, in different positions, by rotation on an axis, to the influence of the earth’s magnetism. The magnetism of crystallized bodies (partly from the extreme rarity of such as are susceptible of any considerable magnetic virtue) has not hitherto been at all examined, but would probably afford very curious results.

(368.) To electricity the views of the physical enquirer now turn from almost every quarter, as to one of those universal powers which Nature seems to employ in her most important and secret operations. This wonderful agent, which we see in intense activity in lightning, and in a feebler and more diffused form traversing the upper regions of the atmosphere in the northern lights, is present, probably in immense abundance, in every form of matter which surrounds us, but becomes sensible only when disturbed by excitements of peculiar kinds. The most effectual of these is friction, which we have already observed to be a powerful source of heat. Everybody is familiar with the crackling sparks which fly from a cat’s back when stroked. These, by proper management, may be accumulated in bodies suitably disposed to receive them, and, although then no longer visible, give evidence of their existence by the exhibition of a vast variety of extraordinary phenomena,--producing attractions and repulsions in bodies at a distance,--admitting of being transferred by contact, or by sudden and violent transilience of the interval of separation, from one body to another, under the form of sparks and flashes;--traversing with perfect facility the substance of the densest metals, and a variety of other bodies called conductors, but being detained by others, such as glass, and especially _air_, which are thence called non-conductors,--producing painful shocks and convulsive motions, and even death itself if in sufficient quantity, in animals through which they pass, and finally imitating, on a small scale, all the effects of lightning.

(369.) The study of these phenomena and their laws until a comparatively recent period occupied the entire attention of electricians, and constituted the whole of the science of electricity. It appears, as the result of their enquiries, that all the phenomena in question are explicable on the supposition that electricity consists in a rare, subtle, and highly elastic fluid, which in its tendency to expand and diffuse itself pervades with more or less facility the substance of conductors, but is obstructed and detained from expansion more or less completely by non-conductors. It is supposed, moreover, that this electric fluid possesses a power of attraction for the particles of all ponderable matter, together with that of a repulsion for particles of its own kind. Whether it has weight, or is rather to be regarded as a species of matter distinct from that of which ponderable bodies consist, is a question of such delicacy, that no direct experiments have yet enabled us to decide it; but at all events its _inertia_ compared with its elastic force must be conceived excessively small, so that it is to be regarded as a fluid in the highest degree _active_, obeying every impulse, internal or external, with the greatest promptitude; in short, a fluid whose energies can only be compared with those of the ethereal medium by which, in the undulatory doctrine, light is supposed to be conveyed. The properties of hydrogen gas compared with those of the denser aëriform fluids will, in some slight degree, aid our conception of the excessive mobility and penetrating activity of a fluid so constituted. Electricity, however, must be regarded as differing in some remarkable points from all those fluids to which we have hitherto been accustomed to apply the epithet elastic, such as air, gases, and vapours. In these, the repulsive force of the particles on which their elasticity depends is considered as extending only to very small distances, so as to affect only those in the immediate vicinity of each other, while their attractive power, by which they obey the general gravitation of all matter, extends to any distance. In electricity, on the other hand, the very reverse must be admitted. The force by which its particles repel each other extends to great distances, while its force of adhesion to ponderable matter must be regarded as limited in its extent to such minute intervals as escape observation.

(370.) The conception of a single fluid of this kind, which when accumulated in excess in bodies tends constantly to escape, and seek a restoration of equilibrium by communicating itself to any others where there may be a deficiency, is that which occurs most naturally to the mind, and was accordingly maintained by Franklin, to whom the science of electricity is under great obligations for those decisive experiments which informed us respecting the true nature of lightning. The same theory was afterwards advocated by Æpinus, who first showed how the laws of equilibrium of such a fluid might be reduced to strict mathematical investigation. But there are phenomena accompanying its transfer from body to body and the state of equilibrium it affects under various circumstances, which appear to require the admission of _two distinct fluids_ antagonist to each other, each attracting the other, and repelling itself; but each, alike, susceptible of adhesion to material substances, and of transfer more or less rapid from particle to particle of them. These fluids in the natural undisturbed state are conceived to exist in a state of combination and mutual saturation; but this combination may be broken, and either of them separately accumulated in a body to any amount without the other, provided its escape be properly obstructed by surrounding it with non-conductors. When so accumulated, its repulsion for its own kind and attraction of the opposite species in neighbouring bodies tends to disturb the natural equilibrium of the two fluids present in them, and to produce phenomena of a peculiar description, which are termed _induced_ electricity. Curious and artificial as this theory may appear, there has hitherto been produced no phenomenon of which it will not afford at least a plausible, and in by far the majority of cases a very satisfactory, explanation. It has one character which is extremely valuable in any theory, that of admitting the application of strict mathematical reasoning to the conclusions we would draw from it. Without this, indeed, it is scarcely possible that any theory should ever be fairly brought to the test by a comparison with facts. Accordingly, the mathematical theory of electrical equilibrium, and the laws of the distribution of the electric fluids over the surfaces of bodies in which they are accumulated, have been made the subject of elaborate geometrical investigation by the most expert mathematicians, and have attained a degree of extent and elegance which places this branch of science in a very high rank in the scale of mathematico-physical enquiry. These researches are grounded on the assumption of a law of attraction and repulsion similar to those of gravity and magnetism, and which by the general accordance of the results with facts, as well as by experiments instituted for the express purpose of ascertaining the laws in question, are regarded as sufficiently demonstrated.

(371.) The most obscure part of the subject is no doubt the original mode of disturbance of electrical equilibrium, by which electricity is excited in the first instance, either by friction or by any other of those causes which have been ascertained to produce such an effect: analogies, it is true, are not wanting[56]; but it must be allowed that hitherto nothing decisive has been offered on the subject; and that conjectural modes of action have in this instance too often usurped the place of those to which a careful examination of facts alone can lead us.

(372.) Philosophers had long been familiar with the effects of electricity above referred to, and with those which it produces in its sudden and violent transfer from one body to another, in rending and shattering the parts of the substances through which it passes, and where in great quantity, producing all the effect of intense heat, igniting, fusing, and volatilizing metals, and setting fire to inflammable bodies; even its occasional influence in destroying or altering the polarity of the magnetic needle had been noticed: but as heat was known to be produced by mechanical violence, and as magnetism was also known to be greatly affected by the same cause, these effects were referred rather to that cause than to any thing in the peculiar nature of the electric matter, and regarded rather as an indirect consequence of its mode of action than as connected with its intimate nature. In short, electricity seemed destined to furnish another in addition to many instances of subjects insulated from the rest of philosophy, and capable of being studied only in its own internal relations, when the great discoveries of Galvani and Volta placed a new power at the command of the experimenter, by whose means those effects which had before been crowded within an inappreciable instant could be developed in detail and studied at leisure; and those forces which had previously exhibited themselves only in a state of uncontrollable intensity were tamed down, as it were, and made to distribute their efficacy over an indefinite time, and to regulate their action at the will of the operator. It was then soon ascertained that electricity in the act of its passage along conductors, produces a variety of wonderful effects, which had never been previously suspected; and these of such a nature, as to afford points of contact with several other branches of physical enquiry, and to throw new and unexpected lights on some of the most obscure operations of nature.

(373.) The history of this grand discovery affords a fine illustration of the advantage to be derived in physical enquiry from a close and careful attention to any phenomenon, however apparently trifling, which may at the moment of observation appear inexplicable on received principles. The convulsive motions of a dead frog in the neighbourhood of an electric discharge, which originally drew Galvani’s attention to the subject, had been noticed by others nearly a century before his time, but attracted no further remark than as indicating a peculiar sensibility to electrical excitement depending on that remnant of vitality which is not extinguished in the organic frame of an animal by the deprivation of actual life. Galvani was not so satisfied. He analysed the phenomenon; and in investigating all the circumstances connected with it was led to the observation of a peculiar electrical excitement which took place when a circuit was formed of three distinct parts, a muscle, a nerve, and a metallic conductor, each placed in contact with the other two, and which was manifested by a convulsive motion produced in the muscle. To this phenomenon he gave the name of animal electricity, an unfortunate epithet, since it tended to restrict enquiry into its nature to the class of phenomena in which it first became apparent. But this circumstance, which in a less enquiring age of science might have exercised a fatal influence on the progress of knowledge, proved happily no obstacle to the further developement of its principles, the subject being immediately taken up with a kind of prophetic ardour by Volta, who at once generalized the phenomena, rejecting the physiological considerations introduced by Galvani, as foreign to the enquiry, and regarding the contraction of the muscles as merely a delicate means of detecting the production of electrical excitements too feeble to be rendered sensible by any other means. It was thus that he arrived at the knowledge of a general fact, that of the disturbance of electrical equilibrium by the mere contact of different bodies, and the circulation of a current of electricity in one constant direction, through a circuit composed of three different conductors. To increase the intensity of the very minute and delicate effect thus observed became his next aim, nor did his enquiry terminate till it had placed him in possession of that most wonderful of all human inventions, the pile which bears his name, through the medium of a series of well conducted and logically combined experiments, which has rarely, if ever, been surpassed in the annals of physical research.

(374.) Though the original pile of Volta was feeble compared to those gigantic combinations which were afterwards produced, it sufficed, however, to exhibit electricity under a very different aspect from any thing which had gone before, and to bring into view those peculiar modifications in its action which Dr. Wollaston was the first to render a satisfactory account of, by referring them to an increase of _quantity_, accompanied with a diminution of _intensity_ in the supply afforded. The discovery had not long been made public, and the instrument in the hands of chemists and electricians, before it was ascertained that the electric current, transmitted by it through conducting liquids, produces in them chemical decompositions. This capital discovery appears to have been made, in the first instance, by Messrs. Nicholson and Carlisle, who observed the decomposition of water so produced. It was speedily followed up by the still more important one of Berzelius and Hisinger, who ascertained it as a general law, that, in all the decompositions so effected, the acids and oxygen become transferred to, and accumulated around, the positive,--and hydrogen, metals, and alkalies round the negative, pole of a Voltaic circuit; being transferred in an invisible, and, as it were, a latent or torpid state, by the action of the electric current, through considerable spaces, and even through large quantities of water or other liquids, again to re-appear with all their properties at their appropriate resting-places.

(375.) It was in this state of things that the subject was taken up by Davy, who, seeing that the strongest chemical affinities were thus readily subverted by the decomposing action of the pile, conceived the happy idea of bringing to bear the intense power of the enormous batteries of the Royal Institution on those substances which, though strongly suspected to be compounds, had resisted all attempts to decompose them--the alkalies and earths. They yielded to the force applied, and a total revolution was thus effected in chemistry; not so much by the introduction of the new elements thus brought to light, as by the mode of conceiving the nature of chemical affinity, which from that time has been regarded (as Davy broadly laid it down, in a theory which was readily adopted by the most eminent chemists, and by none more readily than by Berzelius himself,) as entirely due to electric attractions and repulsions, those bodies combining most intimately whose particles are habitually in a state of the most powerful electrical antagonism, and dispossessing each other, according to the amount of their difference in this respect.

(376.) The connection of magnetism and electricity had long been suspected, and innumerable fruitless trials had been made to determine, in the affirmative or negative, the question of such connection. The phenomena of many crystallized minerals which become electric by heat, and develope opposite electric poles at their two extremities, offered an analogy so striking to the polarity of the magnet, that it seemed hardly possible to doubt a closer connection of the two powers. The developement of a similar polarity in the Voltaic pile pointed strongly to the same conclusion; and experiments had even been made with a view to ascertain whether a pile in a state of excitement might not manifest a disposition to place itself in the magnetic meridian; but the essential condition had been omitted, that of allowing the pile to discharge itself freely, a condition which assuredly never would have occurred of itself to any experimenter. Of all the philosophers who had speculated on this subject, none had so pertinaciously adhered to the idea of a necessary connection between the phenomena as Oërsted. Baffled often, he returned to the attack; and his perseverance was at length rewarded by the complete disclosure of the wonderful phenomena of electro-magnetism. There is something in this which reminds us of the obstinate adherence of Columbus to his notion of the necessary existence of the New World; and the whole history of this beautiful discovery may serve to teach us reliance on those general analogies and parallels between great branches of science by which one strongly reminds us of another, though no direct connection appears; as an indication not to be neglected of a community of origin.

(377.) It is highly probable that we are still ignorant of many interesting features in electrical science, which the study of the Voltaic circuit will one day disclose. The violent mechanical effects produced by it on mercury, placed under conducting liquids which have been referred by Professor Erman to a modified form of capillary attraction, but which a careful and extended view of the phenomena have led others[57] to regard in a very different light, as pointing out a primary action of a dynamical rather than a statical character, deserve, in this point of view, a further investigation; and the curious relations of electricity to heat, as exhibited in the phenomena of what has been called thermo-electricity, promise an ample supply of new information.

(378.) Among the remarkable effects of electricity disclosed by the researches of Galvani and Volta, perhaps the most so consisted in its influence on the nervous system of animals. The origin of muscular motion is one of those profound mysteries of nature which we can scarcely venture to hope will ever be fully explained. Physiologists, however, had long entertained a general conception of the conveyance of some subtle fluid or spirit from the brain to the muscles of animals along the nerves; and the discovery of the rapid transmission of electricity along conductors, with the violent effects produced by shocks, transmitted through the body, on the nervous system, would very naturally lead to the idea that this nervous fluid, if it had any real existence, might be no other than the electrical. But until the discoveries of Galvani and Volta, this could only be looked upon as a vague conjecture. The character of a _vera causa_ was wanting to give it any degree of rational plausibility, since no reason could be imagined for the disturbance of the electrical equilibrium in the animal frame, composed as it is entirely of conductors, or rather, it seemed contrary to the then known laws of electrical communication to suppose any such. Yet one strange and surprising phenomenon might be adduced indicative of the possibility of such disturbance, viz. the powerful shock given by the torpedo and other fishes of the same kind, which presented so many analogies with those arising from electricity, that they could hardly be referred to a different source, though _besides_ the shock neither spark nor any other indication of electrical tension could be detected in them.

(379.) The benumbing effect of the torpedo had been ascertained to depend on certain singularly constructed organs composed of membranous columns, filled from end to end with laminæ, separated from each other by a fluid: but of its mode of action no satisfactory account could be given; nor was there any thing in its construction, and still less in the nature of its materials, to give the least ground for supposing it an electrical apparatus. But the pile of Volta supplied at once the analogies both of structure and of effect, so as to leave little doubt of the electrical nature of the apparatus, or of the power, a most wonderful one certainly, of the animal, to determine, by an effort of its will, that concurrence of conditions on which its activity depends. This remained, as it probably ever will remain, mysterious and inexplicable; but the principle once established, that there exists in the animal economy a power of determining the developement of electric excitement, capable of being transmitted along the nerves, and it being ascertained, by numerous and decisive experiments, that the transmission of Voltaic electricity along the nerves of even a dead animal is sufficient to produce the most violent muscular action, it became an easy step to refer the origin of muscular motion in the living frame to a similar cause; and to look to the brain, a wonderfully constituted organ, for which no mode of action possessing the least plausibility had ever been devised, as the source of the required electrical power.[58]

(380.) It is not our intention, however, to enter into any further consideration of physiological subjects. They form, it is true, a most important and deeply interesting province of philosophical enquiry; but the view that we have taken of physical science has rather been directed to the study of inanimate nature, than to that of the mysterious phenomena of organization and life, which constitute the object of physiology. The history of the animal and vegetable productions of the globe, as affording objects and materials for the convenience and use of man, and as dependent on and indicative of the general laws which determine the distribution of heat, moisture, and other natural agents, over its surface, and the revolutions it has undergone, are of course intimately connected with our subject, and will, therefore, naturally afford room for some remarks, but not such as will long detain the reader’s attention.

(381.) In _zoology_, the connection of peculiar modes of life and food, with peculiarities of structure, has given rise to systems of classification at once obvious and natural; and the great progress which has been made in comparative anatomy has enabled us to trace a graduated scale of organization almost through the whole chain of animal being; a scale not without its intervals, but which every successive discovery of animals heretofore unknown has tended to fill up. The wonders disclosed by microscopic observation have opened to us a new world, in which we discover, with astonishment, the extremes of minuteness and complexity of structure united; while, on the other hand, the examination of the fossil remains of a former state of creation has demonstrated the existence of animals far surpassing in magnitude those now living, and brought to light many forms of being which have nothing analogous to them at present, and many others which afford important connecting links between existing genera. And, on the other hand, the researches of the comparative anatomist and conchologist have thrown the greatest light on the studies of the geologist, and enabled him to discern, through the obscure medium of a few relics, scattered here and there through a stratum, circumstances connected with the formation of the stratum itself which he could have recognised by no other indication. This is one among many striking instances of the unexpected lights which sciences, however apparently remote, may throw upon each other.

(382.) To _botany_ many of the same remarks apply. Its artificial systems of classification, however convenient, have not prevented botanists from endeavouring to group together the objects of their science in natural classes having a community of character more intimate than those which determine their place in the Linnean or any similar system; a community of character extending over the whole habit and properties of the individuals compared. The important chemical discoveries which have been lately made of peculiar proximate principles which, in an especial manner, characterize certain families of plants, hold out the prospect of a greatly increased field of interesting knowledge in this direction, and not only interesting, but in a high degree important, when it is considered that the principles thus brought into view are, for the most part, very powerful medicines, and are, in fact, the essential ingredients on which the medical virtues of the plants depend. The law of the distribution of the generic forms of plants over the globe, too, has, within a comparatively recent period, become an object of study to the naturalist; and its connection with the laws of climate constitutes one of the most interesting and important branches of natural-historical enquiry, and one on which great light remains to be thrown by future researches. It is this which constitutes the chief connecting link between botany and geology, and renders a knowledge of the vegetable fossils, of any portion of the earth’s surface, indispensable to the formation of a correct judgment of the circumstances under which it existed in its ancient state. Fossil botany is accordingly cultivated with great and increasing ardour; and the subterraneous “Flora” of a geological formation is, in many instances, studied with a degree of care and precision little inferior to that which its surface exhibits.

CHAP. VI.

OF THE CAUSES OF THE ACTUAL RAPID ADVANCE OF THE PHYSICAL SCIENCES COMPARED WITH THEIR PROGRESS AT AN EARLIER PERIOD.

(383.) There is no more extraordinary contrast than that presented by the slow progress of the physical sciences, from the earliest ages of the world to the close of the sixteenth century, and the rapid developement they have since experienced. In the former period of their history, we find only small additions to the stock of knowledge, made at long intervals of time; during which a total indifference on the part of the mass of mankind to the study of nature operated to effect an almost complete oblivion of former discoveries, or, at best, permitted them to linger on record, rather as literary curiosities, than as possessing, in themselves, any intrinsic interest and importance. A few enquiring individuals, from age to age, might perceive their value, and might feel that irrepressible thirst after knowledge which, in minds of the highest order, supplies the absence both of external stimulus and opportunity. But the total want of a right direction given to enquiry, and of a clear perception of the objects to be aimed at, and the advantages to be gained by systematic and connected research, together with the general apathy of society to speculations remote from the ordinary affairs of life, and studiously kept involved in learned mystery, effectually prevented these occasional impulses from overcoming the inertia of ignorance, and impressing any regular and steady progress on science. Its objects, indeed, were confined in a region too sublime for vulgar comprehension. An earthquake, a comet, or a fiery meteor, would now and then call the attention of the whole world, and produce from all quarters a plentiful supply of crude and fanciful conjectures on their causes; but it was never supposed that sciences could exist among common objects, have a place among mechanical arts, or find worthy matter of speculation in the mine or the laboratory. Yet it cannot be supposed, that all the indications of nature continually passed unremarked, or that much good observation and shrewd reasoning on it failed to perish unrecorded, before the invention of printing enabled every one to make his ideas known to all the world. The moment this took place, however, the sparks of information from time to time struck out, instead of glimmering for a moment, and dying away in oblivion, began to accumulate into a genial glow, and the flame was at length kindled which was speedily to acquire the strength and rapid spread of a conflagration. The universal excitement in the minds of men throughout Europe, which the first out-break of modern science produced, has been already spoken of. But even the most sanguine anticipators could scarcely have looked forward to that steady, unintermitted progress which it has since maintained, nor to that rapid succession of great discoveries which has kept up the interest of the first impulse still vigorous and undiminished. It may truly, indeed, be said, that there is scarcely a single branch of physical enquiry which is either stationary, or which has not been, for many years past, in a constant state of advance, and in which the progress is not, at this moment, going on with accelerated rapidity.

(384.) Among the causes of this happy and desirable state of things, no doubt we are to look, in the first instance, to that great increase in wealth and civilization which has at once afforded the necessary leisure and diffused the taste for intellectual pursuits among numbers of mankind, which have long been and still continue steadily progressive in every principal European state, and which the increase and fresh establishment of civilized communities in every distant region are rapidly spreading over the whole globe. It is not, however, merely the increased number of cultivators of science, but their enlarged opportunities, that we have here to consider, which, in all those numerous departments of natural research that require local information, is in fact the most important consideration of all. To this cause we must trace the great extension which has of late years been conferred on every branch of natural history, and the immense contributions which have been made, and are daily making, to the departments of zoology and botany, in all their ramifications. It is obvious, too, that all the information that can possibly be procured, and reported, by the most enlightened and active travellers, must fall infinitely short of what is to be obtained by individuals actually resident upon the spot. Travellers, indeed, may make collections, may snatch a few hasty observations, may note, for instance, the distribution of geological formations in a few detached points, and now and then witness remarkable local phenomena; but the resident alone can make continued series of regular observations, such as the scientific determination of climates, tides, magnetic variations, and innumerable other objects of that kind, requires; can alone mark all the details of geological structure, and refer each stratum, by a careful and long continued observation of its fossil contents, to its true epoch; can alone note the habits of the animals of his country, and the limits of its vegetation, or obtain a satisfactory knowledge of its mineral contents, with a thousand other particulars essential to that complete acquaintance with our globe as a whole, which is beginning to be understood by the extensive designation of physical geography. Besides which, ought not to be omitted multiplied opportunities of observing and recording those extraordinary phenomena of nature which offer an intense interest, from the rarity of their occurrence as well as the instruction they are calculated to afford. To what, then, may we not look forward, when a spirit of scientific enquiry shall have spread through those vast regions in which the process of civilization, its sure precursor, is actually commenced and in active progress? And what may we not expect from the exertions of powerful minds called into action under circumstances totally different from any which have yet existed in the world, and over an extent of territory far surpassing that which has hitherto produced the whole harvest of human intellect? In proportion as the number of those who are engaged on each department of physical enquiry increases, and the geographical extent over which they are spread is enlarged, a proportionately increased facility of communication and interchange of knowledge becomes essential to the prosecution of their researches with full advantage. Not only is this desirable, to prevent a number of individuals from making the same discoveries at the same moment, which (besides the waste of valuable time) has always been a fertile source of jealousies and misunderstandings, by which great evils have been entailed on science; but because methods of observation are continually undergoing new improvements, or acquiring new facilities, a knowledge of which, it is for the general interest of science, should be diffused as widely and as rapidly as possible. By this means, too, a sense of common interest, of mutual assistance, and a feeling of sympathy in a common pursuit, are generated, which proves a powerful stimulus to exertion; and, on the other hand, means are thereby afforded of detecting and pointing out mistakes before it is too late for their rectification.

(385.) Perhaps it may be truly remarked, that, next to the establishment of institutions having either the promotion of science in general, or, what is still more practically efficacious in its present advanced state, that of particular departments of physical enquiry, for their express objects, nothing has exercised so powerful an influence on the progress of modern science as the publication of monthly and quarterly scientific journals, of which there is now scarcely a nation in Europe which does not produce several. The quick and universal circulation of these, places observers of all countries on the same level of perfect intimacy with each other’s objects and methods, while the abstracts they from time to time (if well conducted) contain of the most important researches of the day consigned to the more ponderous tomes of academical collections, serve to direct the course of general observation, as well as to hold out, in the most conspicuous manner, models for emulative imitation. In looking forward to what may hereafter be expected from this cause of improvement, we are not to forget the powerful effect which must in future be produced by the spread of elementary works and digests of what is actually known in each particular branch of science. Nothing can be more discouraging to one engaged in active research, than the impression that all he is doing may, very likely, be labour taken in vain; that it may, perhaps, have been already done, and much better done, than, with his opportunities, or his resources, he can hope to perform it; and, on the other hand, nothing can be more exciting than the contrary impression. Thus, by giving a connected view of what has been done, and what remains to be accomplished in every branch, those digests and bodies of science, which from time to time appear, have, in fact, a very important weight in determining its future progress, quite independent of the quantity of information they communicate. With respect to elementary treatises, it is needless to point out their utility, or to dwell on the influence which their actual abundance, contrasted with their past remarkable deficiency, is likely to exercise over the future. It is only by condensing, simplifying, and arranging, in the most lucid possible manner, the acquired knowledge of past generations, that those to come can be enabled to avail themselves to the full of the advanced point from which they will start.

(386.) One of the means by which an advanced state of physical science contributes greatly to accelerate and secure its further progress, is the exact knowledge acquired of physical data, or those normal quantities which we have more than once spoken of in the preceding pages (222.); a knowledge which enables us not only to appretiate the accuracy of experiments, but even to correct their results. As there is no surer criterion of the state of science in any age than the degree of care bestowed, and discernment exhibited, in the choice of such data, so as to afford the simplest possible grounds for the application of theories, and the degree of accuracy attained in their determination, so there is scarcely any thing by which science can be more truly benefited than by researches directed expressly to this object, and to the construction of tables exhibiting the true numerical relations of the elements of theories, and the actual state of nature, in all its different branches. It is only by such determinations that we can ascertain what changes are slowly and imperceptibly taking place in the existing order of things; and the more accurate they are, the _sooner_ will this knowledge be acquired. What might we not now have known of the motions of the (so-called) fixed stars, had the ancients possessed the means of observation we now possess, and employed them as we employ them now?

(387.) In any enumeration of causes which have contributed to the recent rapid advancement of science, we must not forget the very important one of improved and constantly improving means of observation, both in instruments adapted for the exact measurement of quantity, and in the general convenience and well-judged adaptation to its purposes, of every description of scientific apparatus. In the actual state of science there are few observations which can be productive of any great advantage but such as afford accurate measurement; and an increased refinement in this respect is constantly called for. The degree of delicacy actually attained, we will not say in the most elaborate works of the highest art, but in such ordinary apparatus as every observer may now command, is such as could not have been arrived at unless in a state of the mechanical arts, which in its turn (such is the mutual re-action of cause and effect) requires for its existence a very advanced state of science. What an important influence may be exercised over the progress of a single branch of science by the invention of a ready and convenient mode of executing a definite measurement, and the construction and common introduction of an instrument adapted for it cannot be better exemplified than by the instance of the reflecting goniometer. This simple, cheap, and portable little instrument, has changed the face of mineralogy, and given it all the characters of one of the exact sciences.

(388.) Our means of perceiving and measuring minute quantities, in the important relations of weight, space, and time, seem already to have been carried to a point which it is hardly conceivable they should surpass. Balances have been constructed which have rendered sensible the millionth part of the whole quantity weighed; and to turn with the thousandth part of a grain is the performance of balances pretending to no very extraordinary degree of merit. The elegant invention of the sphærometer, by substituting the sense of touch for that of sight in the measurement of minute objects, permits the determination of their dimensions with a degree of precision which is fully adequate to the nicest purposes of scientific enquiry. By its aid an inch may be readily subdivided into ten or even twenty thousand parts; and the lever of contact, an instrument in use among the German opticians, enables us to appretiate quantities of space even yet smaller. For the subdivision of time, too, the perfection of modern mechanism has furnished resources which leave very little to be desired. By the aid of clocks and chronometers, as they are now constructed, a few tenths of a second is all the error that need be apprehended in the subdivision of a day; and for the further subdivision of smaller portions of time, instruments have been imagined which admit of almost unlimited precision, and permit us to appreciate intervals to the nicety of the hundredth, or even the thousandth part of a single second.[59] When the precision attainable by such means is contrasted with what could be procured a few generations ago, by the rude and clumsy workmanship of even the early part of the last century, it will be no matter of astonishment that the sciences which depend on exact measurements should have made a proportional progress. Nor will any degree of nicety in physical determinations appear beyond our reach, if we consider the inexhaustible resources which science itself furnishes, in rendering the quantities actually to be determined by measure great multiples of the elements required for the purposes of theory, so as to diminish in the same proportion the influence of any errors which may be committed on the final results.

(389.) Great, indeed, as have been of late the improvements in the construction of instruments, both as to what regards convenience and accuracy, it is to the discovery of improved _methods_ of observation that the chief progress of those parts of science which depend on exact determinations is owing. The balance of torsion, the ingenious invention of Cavendish and Coulomb, may be cited as an example of what we mean. By its aid we are enabled not merely to render sensible, but to subject to precise measurement and subdivision, degrees of force infinitely too feeble to affect the nicest balance of the usual construction, even were it possible to bring them to act on it. The galvanometer, too, affords another example of the same kind, in an instrument whose range of utility lies among electric forces which we have no other means of rendering sensible, much less of estimating with exactness. In determinations of quantities less minute in themselves, the methods devised by Messrs. Arago and Fresnel, for the measurement of the refractive powers of transparent media by means of the phenomenon of diffraction, may be cited as affording a degree of precision limited only by the wishes of the observer, and the time and patience he is willing to devote to his observation. And in respect of the direction of observations to points from which real information is to be obtained, and positive conclusions drawn, the hygrometer of Daniell may be cited as an elegant example of the introduction into general use of an instrument substituting an indication founded on strict principles for one perfectly arbitrary.

(390.) In speculating on the future prospects of physical science, we should not be justified in leaving out of consideration the probability, or rather certainty, of the occasional occurrence of those happy accidents which have had so powerful an influence on the past; occasions, where a fortunate combination opportunely noticed may admit us in an instant to the knowledge of principles of which no suspicion might occur but for some such casual notice. Boyle has entitled one of his essays thus remarkably,--“_Of Man’s great Ignorance of the Uses of natural Things; or that there is no one Thing in Nature whereof the Uses to human Life are yet thoroughly understood_.”[60] The whole history of the arts since Boyle’s time has been one continued comment on this text; and if we regard among the uses of the works of nature, _that_, assuredly the noblest of all, which leads us to a knowledge of the Author of nature through the contemplation of the wonderful means by which he has wrought out his purposes in his works, the sciences have not been behind hand in affording their testimony to its truth. Nor are we to suppose that the field is in the slightest degree narrowed, or the chances in favour of such fortunate discoveries at all decreased, by those which have already taken place: on the contrary, they have been incalculably extended. It is true that the ordinary phenomena which pass before our eyes have been minutely examined, and those more striking and obvious principles which occur to superficial observation have been noticed and embodied in our systems of science; but, not to mention that by far the greater part of natural phenomena remain yet unexplained, every new discovery in science brings into view whole classes of facts which would never otherwise have fallen under our notice at all, and establishes relations which afford to the philosophic mind a constantly extending field of speculation, in ranging over which it is next to impossible that he should not encounter new and unexpected principles. How infinitely greater, for instance, are the mere chances of discovery in chemistry among the innumerable combinations with which the modern chemist is familiar, than at a period when two or three imaginary elements, and some ten or twenty substances, whose properties were known with an approach to distinctness, formed the narrow circle within which his ideas had to revolve? How many are the instances where a new substance, or a new property, introduced into familiar use, by being thus brought into relation with all our actual elements of knowledge, has become the means of developing properties and principles among the most common objects, which could never have otherwise been discovered? Had not platina (to take an instance) been an object of the most ordinary occurrence in a laboratory, would a suspicion have ever occurred that a lamp could be constructed to burn without flame; and should we have ever arrived at a knowledge of those curious phenomena and products of semi-combustion which this beautiful experiment discloses?

(391.) Finally, when we look back on what has been accomplished in science, and compare it with what remains to be done, it is hardly possible to avoid being strongly impressed with the idea that we have been and are still executing the labour by which succeeding generations are to profit.[61] In a few instances only have we arrived at those general axiomatic laws which admit of direct deductive inference, and place the solutions of physical phenomena before us as so many problems, whose principles of solution we fully possess, and which require nothing but acuteness of reasoning to pursue even into their farthest recesses. In fewer still have we reached that command of abstract reasoning itself which is necessary for the accomplishment of so arduous a task. Science, therefore, in relation to our faculties, still remains boundless and unexplored, and, after the lapse of a century and a half from the æra of Newton’s discoveries, during which every department of it has been cultivated with a zeal and energy which have assuredly met their full return, we remain in the situation in which he figured himself,--standing on the shore of a wide ocean, from whose beach we may have culled some of those innumerable beautiful productions it casts up with lavish prodigality, but whose acquisition can be regarded as no diminution of the treasures that remain.

(392.) But this consideration, so far from repressing our efforts, or rendering us hopeless of attaining any thing intrinsically great, ought rather to excite us to fresh enterprise, by the prospect of assured and ample recompense from that inexhaustible store which only awaits our continued endeavours. “It is no detraction from human capacity to suppose it incapable of infinite exertion, or of exhausting an infinite subject.”[62] In whatever state of knowledge we may conceive man to be placed, his progress towards a yet higher state need never fear a check, but must continue till the last existence of society.

(393.) It is in this respect an advantageous view of science, which refers all its advances to the discovery of general laws, and to the inclusion of what is already known in generalizations of still higher orders; inasmuch as this view of the subject represents it, as it really is, essentially incomplete, and incapable of being fully embodied in any system, or embraced by any single mind. Yet it must be recollected that, so far as our experience has hitherto gone, every advance towards generality has at the same time been a step towards simplification. It is only when we are wandering and lost in the mazes of particulars, or entangled in fruitless attempts to work our way downwards in the thorny paths of applications, to which our reasoning powers are incompetent, that nature appears complicated:--the moment we contemplate it as it is, and attain a position from which we can take a commanding view, though but of a small part of its plan, we never fail to recognise that sublime simplicity on which the mind rests satisfied that it has attained the truth.

INDEX.

Acoustics cultivated by Pythagoras and Aristotle, page 248.

Æpinus, his laws of equilibrium of electricity, 332.

Aëriform fluids, liquids kept in a state of vapour, 321.

Agricola, George, his knowledge of mineralogy and metallurgy, 112.

Air, compressibility and elasticity of; limitation to the repulsive tendency of, 226. Weight of, unknown to the ancients, 228. First perceived by Galileo, 228. Proved by a crucial instance, 229. Equilibrium of, established, 231. Dilatation of, by heat, 319.

Air-pump, discovery of, 230.

Airy, his experiments in Dolcoath mine, 187.

Alchemists, advantages derived from, 11.

Algebra, 19.

Ampere, his electro-dynamic theory, 202. Utility of, 203, 324.

Analysis of force, 86. Of motion, 87. Of complex phenomena, 88.

Anaxagoras, philosophy of, 107.

Animal electricity, 337.

Arago, M., his experiment with a magnetic needle and a plate of copper, 157.

Archimedes, his practical application of science, 72. His knowledge of hydrostatics, 231.

Arfwedson, his discovery of lithia, 158.

Aristotle, his knowledge of natural history, 109. His works condemned, and subsequently studied with avidity, 111. His philosophy overturned by the discoveries of Copernicus, Kepler, and Galileo, 113.

Arithmetic, 19.

Art, empirical and scientific, differences between, 71. Remarks on the language, terms, or signs, used in treating of it, 70.

Assurances, life, utility and abuses of, 58.

Astronomy, cause of the slow progress of our knowledge of, 78. Theory and practical observations distinct in, 132. An extensive acquaintance with science and every branch of knowledge necessary to make a perfect observer in, 132. Five primary planets added to our system, 274. Positions, figures, and dimensions of all the planetary orbits now well known, 275.

Atomic theory, 305. Advantage of, 306.

Atomic weights of chemical elements, 306.

Attraction, capillary, or capillarity, investigated by Laplace and Young, 234.

Bacon, celebrated in England for his knowledge of science, 72. Benefits conferred on Natural Philosophy by him, 104. His Novum Organum, 105. His reform in philosophy proves the paramount importance of induction, 114. His prerogative of facts, 181. Illustrated by the fracture of a crystallized substance, 183. His collective instances, 184. Importance of, 185. His experiment on the weight of bodies, 186. Travelling instances of, frontier instances of, 188. His difference between liquids and aëriform fluids, 233.

Bartolin, Erasmus, first discovers the phenomena exhibited by doubly refracting crystals, 254.

Beccher, phlogistic doctrines of, 300.

Bergmann, his advancement in crystallography, 239.

Bernoulli, experiments of, in hydrodynamical science, 181.

Biot, his hypothesis of a rotatory motion of the particles of light about their axes, 262.

Black, Dr., his discovery of latent heat, 322.

Bode, his curious law observed in the progression of the magnitudes of the several planetary orbits, 308.

Bodies, natural constitution of, 221. Division of, into crystallized and uncrystallized, 242.

Bones, dry, a magazine of nutriment, 65.

Borda, his invention for subdivision, 128.

Botany, general utility of, 345.

Boyle, Robert, his enthusiasm in the pursuit of science, 115. His improvement on the air-pump, 230.

Brain, hypothesis of its being an electric pile, 343.

Bramah’s press, principle and utility of, 233.

Brewster, Dr., his improvement on lenses for lighthouses, 56. His researches prove that the phenomena exhibited by polarized light, in its transmission through crystals, afford a certain indication of the most important points relating to the structure of crystals themselves, 263.

Cabot, Sebastian, his discovery of the variation of the needle, 327.

Cagnard, Baron de la Tour, utility of his experiments, 234.

Causes and consequences directors of the will of man, 6.

Causes, proximate, discovery of, called by Newton _veræ causæ_, 144.

Celestial mechanics, 265.

Chaldean records, 265.

Chemistry furnishes causes of sudden action, also fulminating compositions, 62. Analogy of the complex phenomena of, with those of physics, 92. Benefits arising from the analysis of, 94. Axioms of, analogous to those of geometry, 95. Many of the new elements of, detected in the investigation of residual phenomena, 158. The most general law of, 209. Illustration of, 210. Between fifty and sixty elements in, 211. Objects of, 296. General heads of the principal improvements in, 302. Remarks on those general heads, 304.

Chemistry, Stahlian, cause of the mistakes and confusions of, 123.

Chladni, experiments of, in dynamical science, 181.

Chlorine, disinfectant powers of, 56.

Clarke, Dr., his experiments on the arseniate and phosphate of soda, 170. His success in producing a new phosphate of soda, 171.

Climate, change of, in large tracts of the globe, alleged cause of, 145.

Coals, power of a bushel of, properly consumed, 59. Quantity consumed in London, 60.

Cohesion, an ultimate phenomenon, 90.

Cold, qualities of, 318.

Compass, mariner’s, 55.

Condensation, a source of heat, 313.

Conduction of heat, laws of, 205.

Copernicus, effect of his discoveries on the Aristotelian philosophy, 113. Objections to his astronomical doctrines, 269.

Crystallography, laws of, 123, 239. A determinate figure supposed to be common to all the particles of a crystal, 242.

D’Alembert, his improvements in hydrodynamics, 236.

Dalton, his announcement of the atomic theory, 305. His examination of gases and vapours, 319.

Davy, Sir H., brings the voltaic pile to bear upon the earths and alkalies, 339.

Deduction, utility of, 174.

De l’Isle, Romé, his study of crystalline bodies, 239.

Dew, causes of, investigated, 159. Effects of, on different substances, 160. Objects capable of contracting it, 161. A cloudless sky favourable to its production, 162. General proximate cause of, 163.

Drummond, lieutenant, his improvement on lenses for lamps of lighthouses, 56.

Dynamics, importance of, 96, 223.

Earth, the orbit of,--diminution of its eccentricity round the sun, 147.

Economy, political, 73.

Egypt, great pyramid of, height, weight, and ground occupied by it, 60. Accuracy of the astronomical records of, 265.

Elasticity, an ultimate phenomenon, 90.

Electricity may be the cause of magnetism, 93. Universality of, 329. Effects of, 330. Activity of, 331. Equilibrium of, 332. Productive of chemical decomposition, 338.

Empirical laws, 178. Evils resulting from, 179.

Encke, professor, his prediction of the return of the comet so many times in succession, 156.

Englefield, sir H., his analysis of a solar beam, 314.

Equilibrium maintained by force, 222.

Erman, professor, his opinion of the effects of the voltaic circuit, 340.

Euler, his improvement on Newton’s theory of sound, 247.

Experience, source of our knowledge of nature’s laws, 76.

Experiment, a means of acquiring experience, 76. Utility of, 151.

Facts, the observation of, 118.

Faujas de St. Fond, imaginary craters of, 131.

Fluids, laws of the motion of, 181. Compressibility of, 225. Consideration of the motions of, more complicated than that of equilibrium, 235.

Force, analysis of, 86. The cause of motion, 149. Phenomena of, 221. Molecular forces, 245.

Fourier, baron, his opinion that the celestial regions have a temperature, independent of the sun, not greatly inferior to that at which quicksilver congeals, 157. His analysis of the laws of conduction and radiation of heat, 317.

Franklin, Dr., his experiments on electricity, 332.

Fresnel, M., his mathematical explanation of the phenomena of double refraction, 32. His improvement on lenses for lamps of lighthouses, 56. His opinions on the nature of light, 207. His experiments on the interference of polarized light, 261. His theory of polarization, 262.

Friction, a source of heat, 313.

Galileo, celebrity of, for his knowledge of science, 72. His exposition of the Aristotelian philosophy, 110. His refutation of Aristotle’s dogmas respecting motion, his persecution in consequence of it, 113. His knowledge of the accelerating power of gravity, 168. His knowledge of the weight of the atmosphere, 228.

Galvani, utility of his discoveries in electricity, 335. His application of it to animals, 336.

Gay-Lussac, his examination of gases and vapours, 319.

Generalization, inductive, 1, 90.

Geology, 281. Its rank as a science, 287.

Geometry, axioms of, an appeal to experience, not corporeal, but mental, 95.

Gilbert, Dr., of Colchester, his knowledge of magnetism and electricity, 112.

Gravitation, law of, a physical axiom of a very high and universal kind, 98. Influence of, decreases in the inverse ratio of the square of the distance, 123.

Greece, philosophers of, their extraordinary success in abstract reasoning, and their careless consideration of external nature, 105. Their general character, 106. Philosophy of, 108.

Grimaldi, a jesuit of Bologna, his discovery of diffraction, or inflection of light, 252.

Guinea and feather experiment, 168.

Gunpowder, invention of, 55. A mechanical agent, 62.

Haarlem lake, draining of, 61.

Harmony, sense of, 248.

Head, captain, anecdote of, 84.

Heat, 193. Radiation and conduction of, 205. One of the chief agents in chemistry, 310. Our ignorance of the nature of, 310. Abuse of the sense of the term, 311. The general heads under which it is studied, 312. Its most obvious sources, 312. Animal heat, to what process referable, 313. Radiation and conduction of, 314. Solar heat differs from terrestrial fires, or hot bodies, 315. Principal effects of, 317. The antagonist to mutual attraction, 322. Latent heat, 322. Specific heat, 323.

Herschel, sir William, his analysis of a solar beam, 314.

Hipparchus, his catalogue of stars, 276.

Holland drained of water by windmills, 61.

Hooke almost the rival of Newton, 116.

Huel Towan, steam-engine at, 59.

Huyghens, his doctrine of light, 207. Ascertains the laws of double refraction, 254.

Hydrostatics, first step towards a knowledge of, made by Archimedes, 231. Law of the equal pressure of liquids, 232. General applicability of, 232.

Hypothesis, not to be deterred from framing them, 196. Conditions on which they should be framed, 197. Illustrated by the laws of gravitation, 198. Use and abuse of, 204.

Induction, different ways of carrying it on, 102. Steps by which it is arrived at on a legitimate and extensive scale, 118. First stage of, 144. Verification of, 164. Instanced in astronomy, 166. Must be followed into all its consequences, and applied to all those cases which seem even remotely to bear upon the subject of enquiry, 173. Nature of the inductions by which quantitative laws are arrived at, 176. Necessity of induction embracing a series of cases which absolutely include the whole scale of variation of which the quantities in question admit, 177.

Induced electricity, 333.

Inertia, 223.

Iodine, discovery of, 50. Efficacy of, in curing goître, 51.

Isomorphism, law of, 170.

Kepler, effect of his discoveries on the Aristotelian philosophy, 113. Nature of his laws of the planetary system, 178. Proofs of the Newtonian system, 179.

Knowledge, physical facts illustrative of the utility of, 45. Diffusion of, how to take advantage of in the investigation of nature, 138.

Lagrange, his improvements on Newton’s theory of sound, 247. His astronomical researches, 275.

Lamp, safety, 55.

Laplace, his explanation of the residual velocity of sound and confirmation of the general law of the developement of heat by compression, 172. His astronomical research, 275. His experiments on the dilatation of bodies by heat, 319. His study of specific heat, 323. Latent heat, 323.

Laws, inductive, 171. General, 198. How applicable, 199. Illustrated by the planetary system, 201. Empirical laws, 178.

Lavoisier, his improvements in chemical science, 302. Experiments on dilatation of bodies by heat, 319. His investigation on specific heat, 323.

Light, refraction of, 30. Double refraction of, 31. Polarization of, 254.

Light and vision, ignorance of the ancients respecting, 249.

Lighthouse, 56.

Lightning, how to judge philosophically of it, 120. Returning stroke of, 121.

Liquids, cohesion, attraction and repulsion of the particles of, 227. Differ from aëriform fluids by their cohesion, 233. The Florentine experiment on; experiments by Canton, Perkins, Oërsted, and others on, 235. Obscurity of the laws of dilatation of, 320.

Linnæus, his knowledge of crystalline substances, 239.

Logic, 19.

Lyell’s Principles of Geology, extract from, 146.

Magnetism may be caused by electricity, 93. Offers a “glaring instance” of polarity, 326. Experiments illustrative of, 327.

Malus, a French officer of engineers, discovers the polarization of light, 132, 258.

Man, regarded as a creature of instinct, 1. Of reason and speculation, 3. His will determined by causes and consequences, 6. Advantages to, from the study of science, 7. His necessity to study the laws of nature illustrated, 66. Happiness and the opposite state of man in the aggregate, 67. Advantages conferred on, by the augmentation of physical resources, 68. Advantages from intellectual resources, 69.

Mariotte, his law of equilibrium of an elastic fluid recently verified by the Royal Academy of Paris, 231. His difference between solar and other heat, 315.

Matter, indestructibility of; Divided by grinding, 40. By fire, 41. Dilated by heat, 193. Inertia of, 202. Polarity of, one of the ultimate phenomena to which the analysis of nature leads us, 245. Inherent activity of, 297. Causes of the polarity of, 299. Imponderable forms of, 310.

Measure, the standard, difficulty of preserving it unaltered, 128. How to be assisted in measurement, 129. Our conclusions from, should be conditional, 130.

Menai Bridge, weight and height of, 60.

Mechanics, practical, 63.

Mètre, the French, 126.

Microscopes, power of, 191.

Millstones, method of making in France, 48.

Mind, its transition from the little to the great, and _vice versâ_, illustrated, 172.

Mineralogy unknown to the ancients, 79. Prejudiced by the rage for nomenclature, 139. Benefited by the progress of chemical analysis, 293.

Minerals, simple, apparent paucity of, 294. Difficulty in classing them, 295.

Mitscherlich, his law of isomorphism, 170. His experiments on the expansion of substances by heat, 243.

Motion, 87. Simplicity and precision of the laws of, 179.

Nature, laws of, 37. Immutability of, 42. Harmony of, and advantage of studying them, 43. Prove the impossibility of attaining the declared object of the alchemist. How they serve mankind generally, 44. Illustrated by mining, 45. Economy derived from a knowledge of, 65. How to be regarded, 100, 101.

Nature, objects of, an enumeration and nomenclature of, useful in the study of, 135. Mechanism of, on too large or too small a scale to be immediately cognisable by our senses, 191.

Newton, his proof of Galileo’s laws of gravitation by an experiment with a hollow glass pendulum, 160. His foundation to hydrodynamical science, 181. Fixes the division between statics and dynamics, 223. His investigation of the law of equilibrium of elastic fluids, 231. His law of hydrostatics, 232. His foundation of hydrodynamics 236. His analysis of sound, 247. Hypothesis of light, 250. Examination of a soap-bubble, 252. His hypothesis of fits of easy transmission and reflection, 253. His combination of mathematical skill with physical research, 271. His Principia, 272. His successors; his geometry, 273.

Nomenclature, importance of, to science, 136. More a consequence than a cause of extended knowledge, 138. Prejudicial to mineralogy, 139.

Norman, Robert, his discovery of the dip of the needle, 327.

Numerical precision, necessity of, in science, 122.

Objects, and their mutual actions, subjects of contemplation, 118.

Observation, a means of acquiring experience, 76. Passive and active, 77. Recorded observation, 120. Necessity of, to acquire precise physical data, 215. Illustrated by the barometer, 216.

Oërsted, his discoveries in electricity and magnetism, 132. Of electro-magnetism, 340.

Opacity, 189.

Otto von Guericke of Magdeburgh, his invention of the air-pump, 230.

Paracelsus, power of his chemical remedies; his use of mercury, opium, and tartar, 112.

Pascal, his crucial instances proving the weight of air, 229.

Pendulum, 126.

Phenomena, analysis of, illustrated by musical sounds, the sensation of taste, 85. The ultimate and inward process of nature in the production of, 86. Analysis of complex phenomena, 88. Ultimate phenomena, 90. How the analysis of, is useful, 97. A transient phenomenon, how to judge of, 122. Method of explaining one when it presents itself, 148. How to discover the cause of one, 150. Two, or many, theories, maintained as the origin of, in physics, 195. Cosmical phenomena, 265.

Philosophy, natural, unfounded objections to the study of, 7. Advantages derivable from the study of, 10. Pleasure and happiness, the consequences of the study of, 15.

Phlogistic doctrines of Beccher and Stahl, 300.

Physical data, necessity of, 209. Great importance of, 211. Illustrated by the erection of observatories, 213. Necessity of an exact knowledge of, 214. More precise than the observations by which we acquire them, 215.

Physics, axioms of; analysis of, 102.

Planets, circumjovial, 186.

Platina, discovery of, 308.

Pliny, his knowledge of quartz and diamond, 239.

Pneumatics, 228.

Political economy, 73.

Prejudices of opinion and sense, 80. Conditions on which such are injurious, 81. Illustrated by the division of the rays of light, by the moon at the horizon, and by ventriloquism, 82. By the transition of the hand from heat to cold, 83.

Prevost, M., his theory of heat, 316. His theory of reciprocal interchanges, a proof of the radiation of cold, 318.

Printing, the art of, 193. Performed by steam, 194.

Probabilities, doctrine of, 217. Illustrated by shooting at a wafer, 218.

Prout, Dr., his opinion of the atomic weights, 307.

Pyrometry, 319.

Pythagoras, philosophy of, 107.

Quinine, sulphate of, comparative comfort and health resulting from the use of, 56.

Radiation of heat, laws of, 205.

Repulsion in fluids and solids, 227.

Rules, general, for guiding and facilitating our search among a great mass of assembled facts, 151.

Rumford, count, experiments of, on gunpowder, 62.

Savart, M., his experiments on solids, 243. His researches on sound, 249.

Science, abstract, a preparation for the study of physics, 19. Not indispensable to the study of physical laws, 25. Instances illustrative of, 27.

Science, physical, nature and objects, immediate and collateral, as regarded in itself and in its application to the practical purposes of life, and its influence on society, 35. State of, previous to the age of Galileo and Bacon, 104. Causes of the rapid advance of, compared with the progress at an earlier period, 347.

Science, natural, cause and effect, the ultimate relations of, 76.

Sciences and Arts, remarks on the language, terms, or signs used in treating of them, 70. Receive an impulse by the Baconian philosophy, 114.

Sensation, cause of, 91.

Senses, inadequate to give us direct information for the exact comparison of quantity, 124. Substitutes for the inefficiency of, 125.

Seringapatam, method of breaking blocks from the quarries of, 47.

Shells found in rocks at a great height above the sea, supposed cause of, 145.

Smeaton, his experiments on bodies dilated by heat, 319.

Solids, transparent, exhibit periodical colours when exposed to polarized light, 99. Influence of, on the Mind, 101.

Solids in general, nature of, 236. Constitution of, complicated, 237. Toughness of, distinct from hardness; tenacity of, 238. Become liquefied by the addition of heat, 321.

Sounds, musical, illustrative of the analysis of phenomena, 85. Means of having a knowledge of, 89. Propagation of, through the air, 246. Newton’s analysis of, 247.

Standard measurement, necessity of, 125. Laws of nature used as such, illustrated by the rotation of the earth, 126.

Substances all subject to dilatation by the addition of heat, 243.

Sun, the character of the heat of, 315.

Thales, philosophy of, 107.

Theories, how to estimate the value of, 204. Best arrived at by the consideration of general laws, 208. Explanatory of the phenomena of nature; on what their application ought to be grounded, 209.

Thomson, Dr., his opinion of the atomic weights, 307.

Thermometer, air, 319.

Thermo-electricity, 341.

Time, division of, 126, 127.

Torricelli, pupil of Galileo, his experiments proving the weight of atmosphere, 229.

Torpedo, shock of, 341, 342.

Ulugh Begh, his catalogue of stars, 277.

Vaccination, success of, as a preventive to small-pox, 52.

Vision and light, ignorance of the ancients respecting, 249.

Volta, his discoveries in electricity, 335. Electric pile of, 337.

Voltaic circuit, 338.

Water, effects of the power of, 61.

Whewell, his experiments, 187.

Wells, Dr., his theory of dew, 163.

Wind, effects of the power of, 61.

Wire steel, magnetized masks of, used by needle-makers, 57.

Wollaston, Dr., his verification of the laws of double refraction in Iceland spar, 258. His invention of the goniometer, 292.

World, the materials of the, 290.

Young, Dr., his experiments on the interference of the rays of light, 260.

Zoology, fossil, 344.

THE END.

LONDON PRINTED BY SPOTTISWOODE AND CO. NEW-STREET SQUARE.

FOOTNOTES

[1] Hooke’s Posthumous Works. Lond. 1705.--p. 472 and p. 458.

[2] Wealth of Nations, book i. chap. i. p. 15.

[3] On this subject, we cannot forbear citing a passage from one of the most profound but at the same time popular writers of our time, on a subject unconnected it is true with our own, but bearing strongly on the point before us. “But, if science be manifestly incomplete, and yet of the highest importance, it would surely be most unwise to restrain enquiry, conducted on just principles, even where the immediate practical utility of it was not visible. In mathematics, chemistry, and every branch of natural philosophy, how many are the enquiries necessary for their improvement and completion, which, taken separately, do not appear to lead to any specifically advantageous purpose! how many useful inventions, and how much valuable and improving knowledge, would have been lost, if a rational curiosity, and a mere love of information, had not generally been allowed to be a sufficient motive for the search after truth!”--Malthus’s Principles of Political Economy, p. 16.

[4] Λογος, _ratio_, reason.

[5] Λογος, _verbum_, a word.

[6] It were much to be wished that navigators would be more cautious in laying themselves open to a similar censure. On looking hastily over a map of the world we see three Melville Islands, two King George’s Sounds, and Cape Blancos innumerable.

[7] Young. Lectures on Nat. Phil. ii. 627. See also Phil. Trans. 1801-2.

[8] Captain Basil Hall, R. N.

[9] We must caution our readers who would assure themselves of it by trial, that it is an experiment of some delicacy, and not to be made without several precautions to ensure success. For these we must refer to our original authority (Fresnel. Mémoire sur la Diffraction de la Lumiere, p. 124.); and the principles on which they depend will of course be detailed in that volume of the Cabinet Cyclopædia which is devoted to the subject of LIGHT.

[10] Little reels used in cotton mills to twist the thread.

[11] Such a block would weigh between four and five hundred thousand pounds. See Dr. Kennedy’s “Account of the Erection of a Granite Obelisk of a Single Stone about Seventy Feet high, at Seringapatam.”--_Ed. Phil. Trans._ vol. ix, p. 312.

[12] Dr. Coindet of Geneva.

[13] Journal of a Voyage to the South Seas, &c. &c. under the Command of Commodore George Anson, in 1740-1744, by Pascoe Thomas, Lond. 1745, So tremendous were the ravages of scurvy, that, in the year 1726, admiral Hosier sailed with seven ships of the line to the West Indies, and buried his ships’ companies twice, and died himself in consequence of a broken heart. Dr. Johnson, in the year 1778, could describe a sea-life in such terms as these:--“As to the sailor, when you look down from the quarter deck to the space below, you see the utmost extremity of human misery, such crowding, such filth, such stench!”--“A ship is a prison with the chance of being drowned--it is worse--worse in every respect--worse room, worse air, worse food--worse company!” Smollet, who had personal experience of the horrors of a seafaring life in those days, gives a lively picture of them in his Roderick Random.

[14] Lemon juice was known to be a remedy for scurvy far superior to all others 200 years ago, as appears by the writings of Woodall. His work is entitled “The Surgeon’s Mate, or Military and Domestic Medicine. By John Woodall, Master in Surgery London, 1636,” p. 165. In 1600, Commodore Lancaster sailed from England with three other ships for the Cape of Good Hope, on the 2d of April, and arrived in Saldanha Bay on the 1st of August, the commodore’s own ship being in perfect health, from the administration of three table-spoonsfull of lemon juice every morning to each of his men, whereas the other ships were so sickly as to be unmanageable for want of hands, and the commander was obliged to send men on board to take in their sails and hoist out their boats. (Purchas’s Pilgrim, vol. i. p. 149.) A Fellow of the college, and an eminent practitioner, in 1753 published a tract on sea scurvy, in which he adverts to the superior virtue of this medicine; and Mr. A. Baird, surgeon of the Hector sloop of war, states, that from what he had seen of its effects on board of that ship, he “thinks he shall not be accused of presumption in pronouncing it, if properly administered, a _most infallible remedy_, both in the cure and prevention of scurvy.” (Vide Trotter’s Medicina Nautica.) The precautions adopted by captain Cook in his celebrated voyages, had fully demonstrated by their complete success the practicability of keeping scurvy under in the longest voyages, but a uniform system of prevention throughout the service was still deficient.

It is to the representations of Dr. Blair and sir Gilbert Blane, in their capacity of commissioners of the board for sick and wounded seamen, in 1795, we believe, that its _systematic introduction into nautical diet_, by a general order of the admiralty, is owing. The effect of this wise measure (taken, of course, in conjunction with the general causes of improved health,) may be estimated from the following facts:--In 1780, the number of cases of scurvy received into Haslar hospital was 1457; in 1806 _one_ only, and in 1807 _one_. There are now many surgeons in the navy who have never seen the disease.

[15] Throughout France the conductor is recognised as a most valuable and useful instrument; and in those parts of Germany where thunder-storms are still more common and tremendous they are become nearly universal. In Munich there is hardly a modern house unprovided with them, and of a much better construction than ours--several copper wires twisted into a rope.

[16] We have been informed by an eminent physician in Rome, (Dr. Morichini) that a vast quantity of the sulphate of quinine is manufactured there and consumed in the Campagna, with an evident effect in mitigating the severity of the malarious complaints which affect its inhabitants.

[17] Dr. Johnson, Memoirs of the Medical Society, vol. v.

[18] The engine at Huel Towan. See Mr. Henwood’s Statement “of the performance of steam-engines in Cornwall for April, May, and June, 1829.” Brewster’s Journal, Oct. 1829.--The _highest_ monthly average of this engine extends to 79 millions of pounds.

[19] However, this is not quite a fair statement; a man’s daily labour is about 4 lbs. of coals. The extreme toil of this ascent arises from other obvious causes than the mere height.

[20] Its surface is about 40,000 acres, and medium depth about 20 feet. It was proposed to drain it by running embankments across it, and thus cutting it up into more manageable portions to be drained by windmills.

[21] No one doubts the _practicability_ of the undertaking. Eight or nine thousand chaldrons of coals duly burnt would evacuate the whole contents. But many doubt whether it would be profitable, and some, considering that a few hundreds of fishermen who gain their livelihood on its waters would be dispossessed, deny that it would be _desirable_.

[22] “Experiments to determine the Force of fired Gunpowder.” Phil. Trans. vol. lxxxvii. p. 254. et seq.

[23] See a very ingenious application of this kind in Mr. Babbage’s article on Diving in the Encyc. Metrop.--Others will readily suggest themselves. For instance, the ballast in reserve of a balloon might consist of materials capable of evolving great quantities of hydrogen gas in proportion to their weight, should such be found.

[24] The sulphuric. Bracconot, Annales de Chimie, vol. xii. p. 184.

[25] D’Arcet, Annales de l’Industrie, Fevrier, 1829.

[26] See Dr. Prout’s account of the experiments of professor Autenrieth of Tubingen. Phil. Trans. 1827, p. 381. This discovery, which renders famine next to _impossible_, deserves a higher degree of celebrity than it has obtained.

[27] Greenwich.

[28] Maskelyne’s.

[29] Thomson’s First Principles of Chemistry, vol. ii. p. 68.

[30] Galileo exposes unsparingly the Aristotelian style of reasoning. The reader may take the following from him as a specimen of its quality. The object is to prove the immutability and incorruptibility of the heavens; and thus it is done:--

I. Mutation is either generation or corruption.

II. Generation and corruption only happen between contraries.

III. The motions of contraries are contrary.

IV. The celestial motions are circular.

V. Circular motions have no contraries.

α. Because there can be but three simple motions. 1. To a centre. 2. Round a centre. 3. From a centre.

β. Of three things, one only can be contrary to one.

γ. But a motion to a centre is manifestly the contrary to a motion from a centre.

δ. Therefore a motion _round_ a centre (_i. e._ a circular motion) remains without a contrary.

VI. _Therefore_ celestial motions have no contraries--_therefore_ among celestial _things_ there are no contraries--_therefore_ the heavens are eternal, immutable, incorruptible, and so forth.

It is evident that all this string of nonsense depends on the excessive vagueness of the notions of generation, corruption, contrariety, &c. on which the changes are rung.--_See_ GALILEO, _Systema Cosmicum_, Dial. i. p. 30.

[31] Macquer justly observes, that the alchemists would have rendered essential service to chemistry had they only related their unsuccessful experiments as clearly as they have obscurely related those which they pretend to have been successful.--_Macquer’s Dictionary of Chemistry_, i. x.

[32] Paracelsus performed most of these cures by mercury and opium, the use of which latter drug he had learned in Turkey. Of mercurial preparations the physicians of his time were ignorant, and of opium they were afraid, as being “cold in the fourth degree.” Tartar was likewise a great favourite of Paracelsus, who imposed on it that name, “because it contains the water, the salt, the oil, and the acid, which burn the patient as hell does:” in short, a kind of counterbalance to his opium.

[33] See the Life of Galileo Galilei, by Mr. Drinkwater, with Illustrations of the Advancement of Experimental Philosophy.

[34] The temporary star in Cassiopeia observed by Cornelius Gemma, in 1572, was so bright as to be seen at noon-day. That in Serpentarius, first seen by Kepler in 1604, exceeded in brilliancy all the other stars and planets.

[35] Edinburgh Phil. Journ. 1819, vol. i. p. 8.

[36] The abstract principle of repetition in matters of measurement (viz. juxta-position of units without error) is applicable to a great variety of cases in which quantities are required to be determined to minute nicety. In chemistry, in determining the standard atomic weights of bodies, it seems easily and completely applicable, by a process which will suggest itself at once to every chemist, and seems the only thing wanting to place the exactness of chemical determinations on a par with astronomical measurements.

[37] Accurate and _perfectly_ authentic copies of the yard and pound, executed in platina, and hermetically sealed in glass, should be deposited deep in the interior of the massive stone-work of some great public building, whence they could only be rescued with a degree of difficulty sufficient to preclude their being disturbed unless on some very high and urgent occasion. The fact should be publicly recorded, and its memory preserved by an inscription. Indeed, how much valuable and useful information of the actual existing state of arts and knowledge at any period might be transmitted to posterity in a distinct, tangible, and imperishable form, if, instead of the absurd and useless deposition of a few coins and medals under the foundations of buildings, specimens of ingenious implements or condensed statements of scientific truths, or processes in arts and manufactures, were substituted. Will books infallibly preserve to a remote posterity all that we may desire should be hereafter known of ourselves and our discoveries, or all that posterity would wish to know? and may not a useless ceremony be thus transformed into an act of enrolment in a perpetual archive of what we most prize, and acknowledge to be most valuable?

[38] In the system alluded to, the name of quartz is assigned to iolite and obsidian; that of mica to plumbago, chlorite, and uranite; sulphur, to orpiment and realgar, &c. See Mohs’s System of Mineralogy, translated by Haidinger.

[39] The following passage, from Lindley’s Synopsis of the British Flora, characterises justly the respective merits, in a philosophical point of view, of natural and artificial systems of classification in general, though limited in its expression to his own immediate science:--“After all that has been effected, or is likely to be accomplished hereafter, there will always be more difficulty in acquiring a knowledge of the natural system of botany than of the Linnæan. The latter skims only the surface of things, and leaves the student in the fancied possession of a sort of information which it is easy enough to obtain, but which is of little value when acquired: the former requires a minute investigation of every part and every property known to exist in plants; but when understood has conveyed to the mind a store of real information, of the utmost use to man in every station of life. Whatever the difficulties may be of becoming acquainted with plants according to this method, they are inseparable from botany, which cannot be usefully studied without encountering them.” Schiller has some beautiful lines on this, entitled “Menschliches Wissen” (or Human Knowledge); Gedichte, vol. i. p. 72. Leipzig, 1800.

[40] Lyell’s Principles of Geology, vol. i. Fourrier, Mém. de l’Acad. des Sciences, tom. vii. p. 592. “L’établissement et le progrès des sociétés humaines, l’action des forces naturelles, peuvent changer notablement, et dans de vastes contrées, l’état de la surface du sol, la distribution des eaux, et les grands mouvemens de l’air. De tels effets sont propres à faire varier, dans le cours de plusieurs siècles, le dégré de la chaleur moyenne; car les expressions analytiques comprennent des coefficiens qui se rapportent à l’état superficiel, et qui influent beaucoup sur la valeur de la température.” In this enumeration, by M. Fourrier, of causes which may vary the general relation of the surface of extensive continents to heat, it is but justice to Mr. Lyell to observe, that the gradual shifting of the _places_ of the continents themselves on the surface of the globe, by the abrading action of the sea on the one hand, and the elevating agency of subterranean forces on the other, does not expressly occur and cannot be fairly included in the general sense of the passage, which confines itself to the consideration of such changes as may take place on the existing surface of the land.

[41] The reader will find this subject further developed in a paper lately communicated to the Geological Society.

[42] Phil. Trans. 1824.

[43] Wells on Dew.

[44] Principia, book iii. prop. 6.

[45] A very curious instance of the pursuit of a law completely empirical into an extreme case is to be found in Newton’s rule for the dilatation of his coloured rings seen between glasses at great obliquities. Optics, book ii. part i. obs. 7.

[46] See Phil. Trans. 1819.

[47] “When we are told that Saturn moves in his orbit more than 22,000 miles an hour, we fancy the motion to be swift; but when we find that he is more than three hours moving his own diameter, we must then think it, as it really is, slow.” Thirty Letters on various Subjects, by William Jackson, 1795.

[48] Thomson’s First Principles of Chemistry.

[49] There seems no doubt, however, that an achromatic telescope had been constructed by a private amateur, a Mr. Hall, some time before either Euler or Dollond ever thought of it.

[50] We allude to the recently invented achromatic combinations of Messrs. Barlow and Rogers, and the dense glasses of which Mr. Faraday has recently explained the manufacture in a memoir full of the most beautiful examples of delicate and successful chemical manipulation, and which promise to give rise to a new era in optical practice, by which the next generation at least may benefit. See Phil. Trans. 1830.

[51] Alphonso of Castile, 1252.

[52] Jackson, Letters on Various Subjects, &c.

[53] Thomson’s First Principles of Chemistry, Introduction.

[54] The progress of astronomical discovery has since shown that this law cannot be relied on (1851).

[55] Novum Organum, part ii. table 2. (24), (30), &c. on the form or nature of heat.

[56] We will mention one which we do not remember to have seen noticed elsewhere in the case of a disturbance of the equilibrium of heat produced by means purely mechanical, and by a process depending entirely on a certain order and sequence of events, and the operation of known causes. Suppose a quantity of air enclosed in a metallic reservoir, of some good conductor of heat, and suddenly compressed by a piston. After giving time for the heat developed by the condensation to be communicated from the air to the metal which will be thereby more or less raised in temperature _above_ the surrounding atmosphere, let the piston be suddenly retracted and the air restored to its original volume in an instant. The whole apparatus is now precisely in its initial situation, as to the disposition of its material parts, and the whole quantity of heat it contains remains unchanged. But it is evident that the distribution of this heat within it is now very different from what it was before; for the air in its sudden expansion cannot re-absorb in an instant of time all the heat it had parted with to the metal: it will, therefore, have a temperature _below_ that of the general atmosphere, while the metal yet retains one above it. Thus, a subversion of the equilibrium of temperature has been _bonâ fide_ effected. Heat has been driven from the air into the metal, while every thing else remains unchanged.

We have here a means by which, it is evident, heat may be obtained, to any extent, from the air, without fuel. For if, in place of withdrawing the piston and letting the _same_ air expand, within the reservoir, it be allowed to escape so suddenly as not to re-absorb the heat given off, and fresh air be then admitted and the process repeated, any quantity of air may thus be _drained_ of its heat.

[57] See Phil. Trans. 1824.

[58] If the brain be an electric pile, constantly in action, it may be conceived to discharge itself at regular intervals, when the tension of the electricity developed reaches a certain point, along the nerves which communicate with the heart, and thus to excite the pulsations of that organ. This idea is forcibly suggested by a view of that elegant apparatus, the dry pile of Deluc; in which the successive accumulations of electricity are carried off by a suspended ball, which is kept by the discharges in a state of regular pulsation for any length of time. We have witnessed the action of such a pile maintained in this way for whole years in the study of the above-named eminent philosopher. The same idea of the cause of the pulsation of the heart appears to have occurred to Dr. Arnott; and is mentioned in his useful and excellent work on physics, to which however, we are not indebted for the suggestion, it having occurred to us independently many years ago.

[59] See a description of a contrivance of this kind by Dr. Young, Lectures, vol. i. p. 191.

[60] Boyle’s Works, folio, vol. iii. Essay x. p. 185.

[61] Jackson, The Four Ages, p. 52. London: Cadell and Davies, 1798. 8vo.

[62] Jackson, The Four Ages, p. 90.

Transcriber’s Notes

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