Chapter 15

_The Unknown_.--There is nothing more difficult than a good definition, for it is scarcely possible to express in a few words the abstracted view of an infinite variety of facts. Dr. Black has defined chemistry to be that science which treats of the changes produced in bodies by motions of their ultimate particles or atoms, but this definition is hypothetical, for the ultimate particles or atoms are mere creations of the imagination. I will give you a definition, which will have the merit of novelty and which is probably general in its application. Chemistry relates to those operations by which the intimate nature of bodies is changed, or by which they acquire new properties. This definition will not only apply to the effects of mixture, but to the phenomena of electricity, and, in short, to all the changes which do not merely depend upon the motion or division of masses of matter. However difficult it may have been to have given you a definition of chemistry, it is still more difficult to give you a detail of all the qualities necessary for a chemical philosopher. I will not name as many as Athenaeus has named for a cook, who, he says, ought to be a mathematician, a theoretical musician, a natural philosopher, a natural historian, &c., though you had a disposition just now to make chemistry merely subservient to the uses of the kitchen. But I will seriously mention some of the studies fundamental to the higher departments of this science; a man may be a good practical chemist perhaps without possessing them, but he never can become a great chemical philosopher. The person who wishes to understand the higher departments of chemistry, or to pursue them in their most interesting relations to the economy of Nature, ought to be well-grounded in elementary mathematics; he will oftener have to refer to arithmetic than algebra, and to algebra than to geometry. But all these sciences lend their aid to chemistry; arithmetic, in determining the proportions of analytical results and the relative weights of the elements of bodies; algebra, in ascertaining the laws of the pressure of elastic fluids, the force of vapour as dependent upon temperature, and the effects of masses and surfaces on the communication and radiation of heat; the applications of geometry are principally limited to the determination of the crystalline forms of bodies, which constitute the most important type of their nature, and often offer useful hints for analytical researches respecting their composition. The first principles of natural philosophy or general physics ought not to be entirely unknown to the chemist. As the most active agents are fluids, elastic fluids, heat, light, and electricity, he ought to have a general knowledge of mechanics, hydrodynamics, pneumatics, optics, and electricity. Latin and Greek among the dead and French among the modern languages are necessary, and, as the most important after French, German and Italian. In natural history and in literature what belongs to a liberal education, such as that of our universities, is all that is required; indeed, a young man who has performed the ordinary course of college studies which are supposed fitted for common life and for refined society, has all the preliminary knowledge necessary to commence the study of chemistry. The apparatus essential to the modern chemical philosopher is much less bulky and expensive than that used by the ancients. An air pump, an electrical machine, a voltaic battery (all of which may be upon a small scale), a blow-pipe apparatus, a bellows and forge, a mercurial and water-gas apparatus, cups and basins of platinum and glass, and the common reagents of chemistry, are what are required. All the implements absolutely necessary may be carried in a small trunk, and some of the best and most refined researches of modern chemists have been made by means of an apparatus which might with ease be contained in a small travelling carriage, and the expense of which is only a few pounds. The facility with which chemical inquiries are carried on, and the simplicity of the apparatus, offer additional reasons, to those I have already given, for the pursuit of this science. It is not injurious to the health; the modern chemist is not like the ancient one, who passed the greater part of his time exposed to the heat and smoke of a furnace and the unwholesome vapours of acids and alkalies and other menstrua, of which, for a single experiment, he consumed several pounds. His processes may be carried on in the drawing-room, and some of them are no less beautiful in appearance than satisfactory in their results. It was said, by an author belonging to the last century, of alchemy, "that its beginning was deceit, its progress labour, and its end beggary." It may be said of modern chemistry, that its beginning is pleasure, its progress knowledge, and its objects truth and utility. I have spoken of the scientific attainments necessary for the chemical philosopher; I will say a few words of the intellectual qualities necessary for discovery or for the advancement of the science. Amongst them patience, industry, and neatness in manipulation, and accuracy and minuteness in observing and registering the phenomena which occur, are essential. A steady hand and a quick eye are most useful auxiliaries; but there have been very few great chemists who have preserved these advantages through life; for the business of the laboratory is often a service of danger, and the elements, like the refractory spirits of romance, though the obedient slave of the magician, yet sometimes escape the influence of his talisman and endanger his person. Both the hands and eyes of others, however, may be sometimes advantageously made use of. By often repeating a process or an observation, the errors connected with hasty operations or imperfect views are annihilated; and, provided the assistant has no preconceived notions of his own, and is ignorant of the object of his employer in making the experiment, his simple and bare detail of facts will often be the best foundation for an opinion. With respect to the higher qualities of intellect necessary for understanding and developing the general laws of the science, the same talents I believe are required as for making advancement in every other department of human knowledge; I need not be very minute. The imagination must be active and brilliant in seeking analogies; yet entirely under the influence of the judgment in applying them. The memory must be extensive and profound; rather, however, calling up general views of things than minute trains of thought. The mind must not be, like an encyclopedia, a burthen of knowledge, but rather a critical dictionary which abounds in generalities, and points out where more minute information may be obtained. In detailing the results of experiments and in giving them to the world, the chemical philosopher should adopt the simplest style and manner; he will avoid all ornaments as something injurious to his subject, and should bear in mind the saying of the first king of Great Britain respecting a sermon which was excellent in doctrine but overcharged with poetical allusions and figurative language, "that the tropes and metaphors of the speaker were like the brilliant wild flowers in a field of corn--very pretty, but which did very much hurt the corn." In announcing even the greatest and most important discoveries, the true philosopher will communicate his details with modesty and reserve; he will rather be a useful servant of the public, bringing forth a light from under his cloak when it is needed in darkness, than a charlatan exhibiting fireworks and having a trumpeter to announce their magnificence. I see you are smiling, and think what I am saying in bad taste; yet, notwithstanding, I will provoke your smiles still further by saying a word or two on his other moral qualities. That he should be humble-minded, you will readily allow, and a diligent searcher after truth, and neither diverted from this great object by the love of transient glory or temporary popularity, looking rather to the opinion of ages than to that of a day, and seeking to be remembered and named rather in the epochas of historians than in the columns of newspaper writers or journalists. He should resemble the modern geometricians in the greatness of his views and the profoundness of his researches, and the ancient alchemists in industry and piety. I do not mean that he should affix written prayers and inscriptions of recommendations of his processes to Providence, as was the custom of Peter Wolfe, and who was alive in my early days, but his mind should always be awake to devotional feeling, and in contemplating the variety and the beauty of the external world, and developing its scientific wonders, he will always refer to that infinite wisdom through whose beneficence he is permitted to enjoy knowledge; and, in becoming wiser, he will become better, he will rise at once in the scale of intellectual and moral existence, his increased sagacity will be subservient to a more exalted faith, and in proportion as the veil becomes thinner through which he sees the causes of things he will admire more the brightness of the divine light by which they are rendered visible.

DIALOGUE THE SIXTH. POLA, OR TIME.

During our stay in Illyria, I made an excursion by water with the Unknown, my preserver, now become my friend, and Eubathes, to Pola, in Istria. We entered the harbour of Pola in a felucca when the sun was setting; and I know no scene more splendid than the amphitheatre seen from the sea in this light. It appears not as a building in ruins, but like a newly erected work, and the reflection of the colours of its brilliant marble and beautiful forms seen upon the calm surface of the waters gave to it a double effect--that of a glorious production of art and of a magnificent picture. We examined with pleasure the remains of the arch of Augustus and the temple, very perfect monuments of imperial grandeur. But the splendid exterior of the amphitheatre was not in harmony with the bare and naked walls of the interior; there were none of those durable and grand seats of marble, such as adorn the amphitheatre of Verona, from which it is probable that the whole of the arena and conveniences for the spectators had been constructed of wood. Their total disappearance led us to reflect upon the causes of the destruction of so many of the works of the older nations. I said, in our metaphysical abstractions, we refer the changes, the destruction of material forms, to time, but there must be physical laws in Nature by which they are produced; and I begged our new friend to give us some ideas on this subject in his character of chemical philosopher. If human science, I said, has discovered the principle of the decay of things, it is possible that human art may supply means of conservation, and bestow immortality on some of the works which appear destined by their perfection for future ages.

_The Unknown_.--I shall willingly communicate to you my views of the operation of time, philosophically considered. A great philosopher has said, man can in no other way command Nature but in obeying her laws; and, in these laws, the principle of change is a principle of life; without decay, there can be no reproduction; and everything belonging to the earth, whether in its primitive state, or modified by human hands, is submitted to certain and immutable laws of destruction, as permanent and universal as those which produce the planetary motions. The property which, as far as our experience extends, universally belongs to matter, gravitation, is the first and most general cause of change in our terrestrial system; and, whilst it preserves the great mass of the globe in a uniform state, its influence is continually producing alterations upon the surface. The water, raised in vapour by the solar heat, is precipitated by the cool air in the atmosphere; it is carried down by gravitation to the surface, and gains its mechanical force from this law. Whatever is elevated above the superfices by the powers of vegetation or animal life, or by the efforts of man, by gravitation constantly tends to the common centre of attraction; and the great reason of the duration of the pyramid above all other forms is, that it is most fitted to resist the force of gravitation. The arch, the pillar, and all perpendicular constructions, are liable to fall when a degradation from chemical or mechanical causes takes place in their inferior parts. The forms upon the surface of the globe are preserved from the influence of gravitation by the attraction of cohesion, or by chemical attraction; but if their parts had freedom of motion, they would all be levelled by this power, gravitation, and the globe would appear as a plane and smooth oblate spheroid, flattened at the poles. The attraction of cohesion or chemical attraction, in its most energetic state, is not liable to be destroyed by gravitation; this power only assists the agencies of other causes of degradation. Attraction, of whatever kind, tends, as it were, to produce rest--a sort of eternal sleep in Nature. The great antagonist power is heat. By the influence of the sun the globe is exposed to great varieties of temperature; an addition of heat expands bodies, and an abstraction of heat causes them to contract; by variation of heat, certain kinds of matter are rendered fluid, or elastic, and changes from fluids into solids, or from solids or fluids into elastic substances, and _vice versa_, are produced; and all these phenomena are connected with alterations tending to the decay or destruction of bodies. It is not probable that the mere contraction or expansion of a solid, from the subtraction or addition of heat, tends to loosen its parts; but if water exists in these parts, then its expansion, either in becoming vapour or ice, tends not only to diminish their cohesion, but to break them into fragments. There is, you know, a very remarkable property of water--its expansion by cooling, and at the time of becoming ice--and this is a great cause of destruction in the northern climates; for where ice forms in the crevices or cavities of stones, or when water which has penetrated into cement freezes, its expansion acts with the force of the lever or the screw in destroying or separating the parts of bodies. The mechanical powers of water, as rain, hail, or snow, in descending from the atmosphere, are not entirely without effect; for in acting upon the projections of solids, drops of water or particles of snow, and still more of hail, have a power of abrasion, and a very soft substance, from its mass assisting gravitation, may break a much harder one. The glacier, by its motion, grinds into powder the surface of the granite rock; and the Alpine torrents, that have their origin under glaciers, are always turbid, from the destruction of the rocks on which the glacier is formed. The effect of a torrent in deepening its bed will explain the mechanical agency of fluid-water, though this effect is infinitely increased, and sometimes almost entirely dependent, upon the solid matters which are carried down by it. An angular fragment of stone in the course of ages moved in the cavity of a rock makes a deep round excavation, and is worn itself into a spherical form. A torrent of rain flowing down the side of a building carries with it the silicious dust, or sand, or matter which the wind has deposited there, and acts upon a scale infinitely more minute, but according to the same law. The buildings of ancient Rome have not only been liable to the constant operation of the rain-courses, or minute torrents produced by rains, but even the Tiber, swollen with floods of the Sabine mountains and the Apennines, has often entered into the city, and a winter seldom passes away in which the area of the Pantheon has not been filled with water, and the reflection of the cupola seen in a smooth lake below. The monuments of Egypt are perhaps the most ancient and permanent of those belonging to the earth, and in that country rain is almost unknown. And all the causes of degradation connected with the agency of water act more in the temperate climates than in the hot ones, and most of all in those countries where the inequalities of temperature are greatest. The mechanical effects of air are principally in the action of winds in assisting the operation of gravitation, and in abrading by dust, sand, stones, and atmospheric water. These effects, unless it be in the case of a building blown down by a tempest, are imperceptible in days, or even years; yet a gentle current of air carrying the silicious sand of the desert, or the dust of a road for ages against the face of a structure, must ultimately tend to injure it, for with infinite or unlimited duration, an extremely small cause will produce a very great effect. The mechanical agency of electricity is very limited; the effects of lightning have, however, been witnessed, even in some of the great monuments of antiquity, the Colosaeum at Rome, for instance; and only last year, in a violent thunderstorm, some of the marble, I have been informed, was struck from the top of one of the arches in this building, and a perpendicular rent made, of some feet in diameter. But the chemical effects of electricity, though excessively slow and gradual, yet are much more efficient in the great work of destruction. It is to the general chemical doctrines of the changes produced by this powerful agent that I must now direct your especial attention.

_Eub_.--Would not the consideration of the subject have been more distinct, and your explanations of the phenomena more simple, had you commenced by dividing the causes of change into mechanical and chemical; if you had first considered them separately, and then their joint effects?

_The Unknown_.--The order I have adopted is not very remote from this. But I was perhaps wrong in treating first of the agency of gravitation, which owes almost all its powers to the operation of other causes. In consequence of your hint, I shall alter my plan a little, and consider first the chemical agency of water, then that of air, and lastly that of electricity. In every species of chemical change, temperature is concerned. But unless the results of volcanoes and earthquakes be directly referred to this power, it has no chemical effect in relation to the changes ascribed to time simply considered as heat, but its operations, which are the most important belonging to the terrestrial cycle of changes, are blended with, or bring into activity, those of other agents. One of the most distinct and destructive agencies of water depends upon its solvent powers, which are usually greatest when its temperature is highest. Water is capable of dissolving, in larger or smaller proportions, most compound bodies, and the calcareous and alkaline elements of stones are particularly liable to this kind of operation. When water holds in solution carbonic acid, which is always the case when it is precipitated from the atmosphere, its power of dissolving carbonate of lime is very much increased, and in the neighbourhood of great cities, where the atmosphere contains a large proportion of this principle, the solvent powers of rain upon the marble exposed to it must be greatest. Whoever examines the marble statues in the British Museum, which have been removed from the exterior of the Parthenon, will be convinced that they have suffered from this agency; and an effect distinct in the pure atmosphere and temperate climate of Athens, must be upon a higher scale in the vicinity of other great European cities, where the consumption of fuel produces carbonic acid in large quantities. Metallic substances, such as iron, copper, bronze, brass, tin, and lead, whether they exist in stones, or are used for support or connection in buildings, are liable to be corroded by water holding in solution the principles of the atmosphere; and the rust and corrosion, which are made, poetically, qualities of time, depend upon the oxidating powers of water, which by supplying oxygen in a dissolved or condensed state enables the metals to form new combinations. All the vegetable substances, exposed to water and air, are liable to decay, and even the vapour in the air, attracted by wood, gradually reacts upon its fibres and assists decomposition, or enables its elements to take new arrangements. Hence it is that none of the roofs of ancient buildings more than a thousand years old remain, unless it be such as are constructed of stone, as those of the Pantheon of Rome and the tomb of Theodoric at Ravenna, the cupola of which is composed of a single block of marble. The pictures of the Greek masters, which were painted on the wood of the abies, or pine of the Mediterranean, likewise, as we are informed by Pliny, owed their destruction not to a change in the colours, not to the alteration of the calcareous ground on which they were painted, but to the decay of the tablets of wood on which the intonaco or stucco was laid. Amongst the substances employed in building, wood, iron, tin, and lead, are most liable to decay from the operation of water, then marble, when exposed to its influence in the fluid form; brass, copper, granite, sienite, and porphyry are more durable. But in stones, much depends upon the peculiar nature of their constituent parts; when the feldspar of the granite rocks contains little alkali or calcareous earth, it is a very permanent stone; but, when in granite, porphyry, or sienite, either the feldspar contains much alkaline matter, or the mica, schorl, or hornblende much protoxide of iron, the action of water containing oxygen and carbonic acid on the ferruginous elements tends to produce the disintegration of the stone. The red granite, black sienite, and red porphyry of Egypt, which are seen at Rome in obelisks, columns, and sarcophagi, are amongst the most durable compound stones; but the grey granites of Corsica and Elba are extremely liable to undergo alteration: the feldspar contains much alkaline matter; and the mica and schorl, much protoxide of iron. A remarkable instance of the decay of granite may be seen in the Hanging Tower of Pisa; whilst the marble pillars in the basement remain scarcely altered, the granite ones have lost a considerable portion of their surface, which falls off continually in scales, and exhibits everywhere stains from the formation of peroxide of iron. The kaolin, or clay, used in most countries for the manufacture of fine porcelain or china, is generally produced from the feldspar of decomposing granite, in which the cause of decay is the dissolution and separation of the alkaline ingredients.

_Eub_.--I have seen serpentines, basalts, and lavas which internally were dark, and which from their weight, I should suppose, must contain oxide of iron, superficially brown or red, and decomposing. Undoubtedly this was from the action of water impregnated with air upon their ferruginous elements.