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rium must also be affected by the state of aggregation. "These conclusions," he says, "not only afford us an insight into the hidden nature of heat-vibrations, but they also appear to cast some light on the physical constitution of the atom itself. They seem to lead to the conclusion that the ultimate atom itself is essentially elastic. For if heat-vibrations do not consist in excursions of the atom, then they must consist in alternate expansions and contractions of the atom itself. This again is opposed to the ordinary idea that the atom is essentially solid and impenetrable. But it favors the modern idea, that matter consists of a force of resistance acting from a center."
There are still several other hypotheses possible, based, however, upon the idea that atoms are absolutely solid and extended. An atom may rotate on its axis, it may describe an orbit, or several atoms may revolve about each other, or about a common center of equilibrium, or, as in case of the planets, several of these motions may be performed at once. The only one of these suppositions, however, that is not inconsistent with the fact mentioned above, as established by Professor Tyndall's experiments, is that of the rotation of the atom about its axis. This hypothesis is also rendered more probable, by the fact that it furnishes a more plausible explanation of the phenomena of polarity than any of the others. Thus we see, that if we assume the correctness of the position that the period of the vibration of the atoms is not affected by the state of aggregation, we have left but two theories of heat-vibrations capable of being reconciled to it: first, that of the rotation of the atom on its axis, based on the idea that the atom is essentially solid and extended; second, that of Mr. Croll, that they "consist in alternate expansions and contractions of the atom itself," founded on the theory of Boscovich, that atoms are only centers of forces, or as he expresses it above, "a force of resistance acting from a center." It is plain, therefore, that until a correct theory of the ultimate constitution of matter is established, it will be impossible to determine the exact nature of the atomic motions, by which, according to this theory, heat is produced.
Let us, therefore, leave this undetermined, and apparently undeterminable question, as to the nature of the motion, and
return to the main idea, which our author endeavors to establish; namely, that heat is produced by some sort of motion of the ultimate particles of matter. He says that "from the direct contemplation of some of the phenomena of heat, a profound mind is led almost instinctively to conclude that heat is a kind of motion," and corroborates his assertion by abundant quotations from the writings of Bacon, Locke, and others. These extracts make up the appendices of several of the lectures. We have room for but a small portion of them. Bacon, in the twentieth aphorism of the second book of the "Novum Organum," says, "When I say of motion that it is the genus of which heat is the species, I would be understood to mean, not that heat generates motion, or that motion generates heat, (though both are true in certain cases,) but that heat itself, its essence and quiddity, is motion, and nothing else." Locke expresses the same opinion: “Heat is a very brisk agitation of the insensible parts of the object, which produce 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." In an essay read before the Royal Society, January 25, 1778, entitled "An inquiry concerning the source of the heat which is excited by friction," Count Rumford, after giving an account of his well-. known experiment of boiling water by boring a cannon, discusses the question after this fashion: "By meditating on the results of all these experiments we are naturally brought to that great question which has so often been the subject of speculation among philosophers, namely, What is heat-is there any such thing as an igneous fluid? Is there anything that, with propriety, can be called caloric? . . .
"In reasoning on this subject we must not forget that most remarkable circumstance, that the source of the heat generated by friction in these experiments appeared evidently to be inexhaustible. It is hardly necessary to add, that anything which any insulated body or system of bodies can continue to furnish without limitation cannot possibly be a material substance, and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in these experiments, except it be MOTION."
This is the first and perhaps the best argument on this point, based upon a series of sufficient and carefully-conducted experiments. To Count Rumford, an American by birth, therefore, really belongs the honor of originating the theory that heat is some kind of motion. Sir Humphrey Davy, the associate and successor of Count Rumford, at the Royal Institution, also, in the early part of the present century, instituted an important series of experiments upon the production of heat by friction. In his Chemical Philosophy, page 95, he thus states his opinion on this point: "It seems possible to account for all the phenomena of heat if it be supposed that in solids the particles are in a constant state of vibratory motion, the particles of the hottest bodies moving with the greatest velocity, and through the greatest space; that in fluids and elastic fluids, besides the vibratory motion, which must be conceived the greatest in the last, the particles have a motion round their own axis, with different velocities, the particles of elastic fluids moving with the greatest quickness; and that in ethereal substances the particles move round their own axes, and separate from each other, penetrating in right lines through space. Temperature may be conceived to depend upon the velocities of the vibrations; increase of capacity, on the motion being performed in greater space, and the diminution of temperature during the conversion of solids into fluids or gases, may be explained on the idea of the loss of vibratory motion, in consequence of the revolution of particles round their axes, at the moment when the body becomes fluid or aëriform, or from the loss of rapidity or vibration in consequence of the motion of the particles through greater space."
When we consider the source from which this doctrine originally came, and the clearness and force with which it was stated, it seems somewhat surprising that it should have made so little impression. Yet we must bear in mind that scientific as well as social and political revolutions require time. A few eminent men may at once grasp a new truth, but it is hard to overcome the prejudices of the majority of even scientific men, and persuade them to give up old familiar theories for new ones. The corpuscular theory of heat is so simple, and its terms so easily comprehended, that it was no easy matter to induce men to exchange its substantial doctrines for this new and apparently FOURTH SERIES, VOL. XVII.—4
more unreal and fanciful hypothesis as an explanation of the familiar phenomena. The dynamical theory, it is true, had advocates, and able ones, in the early part of the present century, yet it was not until the publication of Mayer's calculation in 1842, and Joule's experiments in 1843, to determine the mechanical equivalent of heat, that it began to acquire prominence, and attract the attention of the scientific world. Since then, through the persevering labors of Thomson, Rankine, Faraday, Grove, Clausius, Helmholtz, Holtzman, and others, a revolution in scientific thought has been slowly and silently accomplished, and now Professor Tyndall comes with his facility of expression and wealth of illustration, at a time when the world is prepared to hear and adopt the "New Philosophy."
His main object in these lectures is "to bring the rudiments. of a new philosophy within the reach of a person of ordinary intelligence." His success in doing this can only be fully appreciated by reading his book; otherwise it is impossible to get a good idea of the aptness and clearness with which every point is illustrated by the most delicate and striking experiments. Though all the apparatus used on this occasion was of the most perfect and costly character, the success of the lectures was in great part due to the use of the thermo-electric pile and galvanometer, by which the slightest variation of temperature was at once made apparent to a large audience.
By a series of experiments on the friction and percussion of solids, liquids, and gases, he shows that the heat can only be accounted for by the supposition of motion communicated to the atoms of bodies, and not by forcing out the heat already stored up in them, The quantity of heat produced in all these cases seems inexhaustible, and incapable of an explanation by supposing that it was contained in the small quantity of matter acted upon. Count Rumford, as we have seen, came to the same conclusion from his celebrated experiment of boiling water by boring a cannon, described in the paper quoted above. But the subsequent experiment of Sir Humphrey Davy is perhaps the most decided proof of the immateriality of heat; on this point it is an experimentum crucis. He took two pieces of ice in a room whose temperature was below 32°, and carefully excluding all external heat, he rubbed them together until both
⚫were melted to water, whose temperature was found to be 35°. Now this could not be accounted for by supposing the capacity of the ice for heat to be diminished by the friction, for the capacity of ice for heat is only half that of water at the same temperature. The capacity for heat has, therefore, been doubled by the melting. That is, the water contains twice as much heat as existed in the ice. It follows, then, that there must have been a generation, not a mere transference of heat. This can only be accounted for on the theory that a motion or vibration of the particles of ice is produced by friction. "Therefore," Davy says, "we may reasonably conclude that this motion or vibration is heat or the repulsive power." Thus we see that speculation and experiment both lead us inevitably to the conclusion that heat is not material, but is caused by some kind of motion of the atoms of bodies.
We have abundant application of this principle in all cases where heat is developed by percussion or friction. When a leaden bullet is heated by the descent of a cold sledge hammer, this theory affords a ready explanation of the phenomena. Instead of the force of the descending hammer being destroyed by the bullet and the anvil, as was formerly supposed, we say that the motion of the mass of the sledge is transferred to the atoms of the lead, and manifests itself as heat. The heat generated by chemical combination may be explained in the same way. The motion of translation, by which the atoms approach each other in consequence of their mutual attraction, is changed into a vibratory motion of the atoms, and is converted into heat. Thus when the atoms of hydrogen and oxygen fall together to form water, heat is produced by a transferrence of motion, just as in the case of the hammer and the bullet. Likewise in ordinary combustion, the heat is caused by the clashing of the atoms of the oxygen of the atmosphere upon the atoms of the carbon of the combustible. An attempt has also been made to account for the heat of the sun by supposing it to be caused by the showering down of meteoric matter upon its surface. This is known as the Meteoric Theory of the Sun's Heat, and was first advanced by Meyer in his "Beiträge zur Dynamik des Himmels." It has since had some able advocates; and, when we consider that no other theory of the sun's heat yet advanced is even as satisfactory as