CHAPTER II.

THE GENERATION OF COSMICAL HEAT.

In the preceding Chapter we endeavoured to show how the action of gravitation upon the particles of diffused primordial matter would result in the formation, by condensation and aggregation, of a spherical planetary body. We have now to consider another result of the gravitating action, and for this we must call to our aid a branch of scientific enquiry and investigation unrecognized as such at the period of Laplace’s speculations, and which has been developed almost entirely within the past quarter of a century.

The “great philosophical doctrine of the present era of science,” as the subject about to engage our attention has been justly termed, bears the title of the “Conservation of Force,” or—as some ambiguity is likely to attend the definition of the term “Force”—the “Conservation of Energy.” The basis of the doctrine is the broad and comprehensive natural law which teaches us that the quantity of force comprised by the universe, like the quantity of matter contained in it, is a fixed and invariable amount, which can be neither added to nor taken from, but which is for ever undergoing change and transformation from one form to another. That we cannot create force ought to be as obvious a fact as that we cannot create matter; and what we cannot create we cannot destroy. As in the universe we see no new matter created, but the same matter constantly disappearing from one form and reappearing in another, so we can find no new force ever coming into action—no description of force that is not to be referred to some previous manner of existence.

Without entering upon a metaphysical discussion of the term “force,” it will be sufficient for our purpose to consider it as something which produces or resists motion, and hence we may argue that the ultimate effect of force is motion. The force of gravity on the earth results in the motion or tendency of all bodies towards its centre, and, similarly, the action of gravitation upon the atoms or particles of a primeval planet resulted in the motion of those particles towards each other. We cannot conceive force otherwise than by its effects, or the motion it produces.

And force we are taught is indestructible; therefore motion must be indestructible also. But when a falling body strikes the earth, or a gunshot strikes its target, or a hammer delivers a blow upon an anvil, or a brake is pressed against a rotating wheel, motion is arrested, and it would seem natural to infer that it is destroyed. But if we say it is indestructible, what becomes of it? The philosophical answer to the question is this—that the motion of the mass becomes transferred to the particles or molecules composing it, and transformed to molecular motion, and this molecular motion manifests itself to us as heat. The particles or atoms of matter are held together by cohesion, or, in other words, by the action of molecular attraction. When heat is applied to these particles, motion is set up among them, they are set in vibration, and thus, requiring and making wider room, they urge each other apart, and the well-known _expansion by heat_ is the result. If the heat be further continued a more violent molecular motion ensues, every increase of heat tending to urge the atoms further apart, till at length they overcome their cohesive attraction and move about each other, and a _liquid or molten condition_ results. If the heat be still further increased, the atoms break away from their cohesive fetters altogether and leap off the mass in the form of vapour, and the matter thus assumes the _gaseous or vaporous form_. Thus we see that the phenomena of heat are phenomena of motion, and of motion only.

This mutual relation between heat and work presented itself as an embryo idea to the minds of several of the earlier philosophers, by whom it was maintained in opposition to the _material theory_ which held heat to be a kind of matter or subtle fluid stored up in the inter-atomic spaces of all bodies, capable of being separated and procured from them by rubbing them together, but not generated thereby. Bacon, in his “Novum Organum,” says that “heat itself, its essence and quiddity, is motion and nothing else.” Locke defines heat as “a very brisk agitation of the insensible parts of an object, which produces in us that sensation from whence we denominate the object hot; so what in our sensation is _heat_, in the object is nothing but _motion_.” Descartes and his followers upheld a similar opinion. Richard Boyle, two hundred years ago, actually wrote a treatise entitled “The Mechanical Theory of Heat and Cold,” and the ingenious Count Rumford made some highly interesting and significant experiments on the subject, which are described in a paper read before the Royal Society in 1798, entitled “An Inquiry concerning the Source of Heat excited by Friction.” But the conceptions of these authors remained isolated and unfruitful for more than a century, and might have passed, meantime, into the oblivion of barren speculation, but for the impulse which this branch of inquiry has lately received. Now, however, they stand forth as notable instances of truth trying to force itself into recognition while yet men’s minds were unprepared or disinclined to receive it. The key to the beautiful mechanical theory of heat was found by these searching minds, but the unclasping of the lock that should disclose its beauty and value was reserved for the philosophers of the present age.

Simultaneously and independently, and without even the knowledge of each other, three men, far removed from probable intercourse, conceived the same ideas and worked out nearly similar results concerning the mechanical theory of heat. Seeing that motion was convertible into heat, and heat into motion, it became of the utmost importance to determine the exact relation that existed between the two elements. The first who raised the idea to philosophic clearness was Dr. Julius Robert Mayer, a physician of Heilbronn in Germany. In certain observations connected with his medical practice it occurred to him that there must be a necessary equivalent between work and heat, a necessary numerical relation between them. “The variations of the difference of colour of arterial and venous blood directed his attention to the theory of respiration. He soon saw in the respiration of animals the origin of their motive powers, and the comparison of animals to thermic machines afterwards suggested to him the important principle with which his name will remain for ever connected.”

Next in order of publication of his results stands the name of Colding, a Danish engineer, who about the year 1843 presented a series of memoirs on the steam engine to the Royal Society of Copenhagen, in which he put forth views almost identical with those of Mayer.

Last in publication order, but foremost in the importance of his experimental treatment of the subject, was our own countryman, Dr. Joule of Manchester. “Entirely independent of Mayer, with his mind firmly fixed upon a principle, and undismayed by the coolness with which his first labours appear to have been received, he persisted for years in his attempts to prove the invariability of the relation which subsists between heat and ordinary mechanical power.” (We are quoting from Professor Tyndall’s valuable work on “Heat considered as a Mode of Motion.”) “He placed water in a suitable vessel, agitated the water by paddles, and determined both the amount of heat developed by the stirring of the liquid and the amount of labour expended in its production. He did the same with mercury and sperm oil. He also caused discs of cast iron to rub against each other, and measured the heat produced by their friction, and the force expended in overcoming it. He urged water through capillary tubes, and determined the amount of heat generated by the friction of the liquid against the sides of the tubes. And the results of his experiments leave no shadow of doubt upon the mind that, under all circumstances, the quantity of heat generated by the same amount of force is fixed and invariable. A given amount of force, in causing the iron discs to rotate against each other, produced precisely the same amount of heat as when it was applied to agitate water, mercury, or sperm oil. * * * * _The absolute amount of heat_ generated by the same expenditure of power, was in all cases the same.”

“In this way it was found that the quantity of heat which would raise one pound of water one degree Fahrenheit in temperature, is exactly equal to what would be generated if a pound weight, after having fallen through a height of 772 feet, had its moving force destroyed by collision with the earth. Conversely, the amount of heat necessary to raise a pound of water one degree in temperature, would, if all applied mechanically, be competent to raise a pound weight 772 feet high, or it would raise 772 pounds one foot high. The term ‘foot pounds’ has been introduced to express in a convenient way the lifting of one pound to the height of a foot. Thus the quantity of heat necessary to raise the temperature of a pound of water one degree Fahrenheit being taken as a standard, 772 foot-pounds constitute what is called the _mechanical equivalent_ of heat.”

By a process entirely different, and by an independent course of reasoning, Mayer had, a few months previous to Joule, determined this equivalent to be 771·4 foot-pounds. Such a remarkable coincidence arrived at by pursuing different routes gives this value a strong claim to accuracy, and raises the Mechanical Theory of Heat to the dignity of an exact science, and its enunciators to the foremost place in the ranks of physical philosophers.

In linking together the labours of the two remarkable men above alluded to, Prof. Tyndall remarks, that “Mayer’s labours have in some measure the stamp of profound intuition, which rose however to the energy of undoubting conviction in the author’s mind. Joule’s labours, on the contrary, are an experimental demonstration. Mayer _thought_ his theory out, and rose to its grandest applications. Joule _worked_ his theory out, and gave it the solidity of natural truth. True to the speculative instinct of his country, Mayer drew large and mighty conclusions from slender premises; while the Englishman aimed above all things at the firm establishment of facts.... To each belongs a reputation which will not quickly fade, for the share he has had, not only in establishing the dynamical theory of heat, but also in leading the way towards a right appreciation of the general energies of the universe.”

But from these generalities we must pass to the application of the mechanical theory of heat to our special subject. We have learnt that every form of motion is convertible into heat. We know that the falling meteor or shooting star, whose motion is impeded by friction against the earth’s atmosphere, is heated thereby to a temperature of incandescence. Let us then suppose that myriads of such cosmical particles came into collision from the effect of their mutual attraction, or that the component atoms of a vast nebulous mass violently converged under the like influence. What would follow? Obviously the generation of an intense heat by the arrest of converging motion, such a heat as would result in the fusion of the whole into one mass. Mayer, in one of his most remarkable papers (“Celestial Dynamics”) remarks that the “Newtonian theory of gravitation, whilst it enables us to determine, from its present form, the earth’s state of aggregation in ages past, at the same time points out to us a source of heat powerful enough to produce such a state of aggregation—powerful enough to melt worlds: it teaches us to consider the molten state of a planet as the result of the mechanical union of cosmical masses, and to derive the radiation of the sun and the heat in the bowels of the earth from a common origin.”

And the same laws that governed the formation of the earth, governed also the formation of the moon: the variations of Nature’s operations are _quantitative_ only and not _qualitative_. The Divine Will that made the earth made the moon also, and the means and mode of working were the same for both. The geological phenomena of the earth afford unmistakeable evidence of its original fluid or molten condition, and the appearance of the moon is as unmistakeably that of a body once in an igneous or molten state. The enigma of the earth’s primary formation is solved by the application of the dynamical theory of heat. By this theory the generation of cosmical heat is removed from the quicksands of conjecture and established upon the firm ground of direct calculation: for the absolute amount of heat generated by the collision of a given amount of matter is (of course, with some little uncertainty) deducible from a mathematical formula. Mayer has computed the amount of heat that the matter of the earth would have generated, if it had been formed originally of only two parts drawn into collision by their mutual attraction, and has found that it would be from 0 to 32,000 or 47,000[1] Centigrade degrees, according as one part was infinitely small as compared with the other, or as the two parts were of equal size. Professor Helmholtz, another labourer in the same field of science, has computed the amount of heat generated by the condensation of the whole of the matter composing the solar system: this he finds would be equivalent to the heat that would be required to raise the temperature of a mass of water equal to the sum of the masses of all the bodies of the system to 28,000,000 (twenty-eight million) degrees of the Centigrade scale.

These examples afford abundant evidence of sufficient heat having been generated by the aggregation of the matter of the moon to reduce it to a state of fusion, and so to produce, from a nebulous chaos of diffused cosmical matter, a molten body of definite outline and size.

It is requisite here to remark that fusion does not necessarily imply combustion. It has been frequently asked, How can a volcanic theory of the lunar phenomena be upheld consistently with the condition that it possesses no atmosphere to support Fire? To this we would reply that to produce a state of incandescence or a molten condition it is _not_ necessary that the body be surrounded by an atmosphere. The intensely rapid motion of the particles of matter of bodies, which the dynamical theory shows to be the origin of the molten state, exists quite independently of such external matter as an atmosphere. The complex mixture of gases and vapours which we term “air,” has nothing whatever to do with the fusion of substances, whatever it may have to do with their combustion. Combustion is a chemical phenomenon, due to the combination of the oxygen of that air with the heated particles of the combustible matter: oxygen is the sole supporter of combustion, and hence combustion is to be regarded rather as a phenomenon of oxygen than as a phenomenon of the matter with which that oxygen combines. The greatest intensity of heat may exist without oxygen, and consequently without combustion. In support of this argument it will be sufficient to adduce, upon the authority of Dr. Tyndall, the fact that a platinum wire can be raised to a luminous temperature and actually _fused_ in a perfect vacuum.

But while the mass of condensing cosmical matter was thus accumulating and forming the globe of the moon, the heat consequent upon the aggregation of its particles was suffering some diminution from the effect of radiation. So long as the radiated heat lost fell short of the dynamical heat generated, no effect of cooling would be manifest; but when the _vis viva_ of the condensing matter was all converted into its equivalent of heat, or when the accession of heat fell short of that radiated, a necessary cooling must ensue, and this cooling would be accompanied by a solidification of that part of the mass which was most free to radiate its heat into surrounding space: that part would obviously be the outer surface.

With the solidification of this external crust began the “year one” of selenological history.

The phenomena attendant upon the cooling of the mass we will consider in the next Chapter.