inality; but he has collected what was scattered, united what was incoherent, spoken aloud what many secretly think, and that in a short, lively, well-written book. He does us a real service by giving us a real adversary to combat, instead of those intangible phantoms which, floating incessantly between materialism and spiritualism, allow no attack at any point.

[To be continued.]

ART. III.-TYNDALL'S

"HEAT AS A MODE OF MOTION."

It

Heat Considered as a Mode of Motion: Being a Course of Twelve Lectures delivered at the Royal Institution of Great Britain, in the Season of 1862, by JOHN TYNDALL, F. R. S., Professor of Natural Philosophy in the Royal Institution. New York: D. Appleton. PROFESSOR TYNDALL calls this a "New Philosophy." may be well to inquire in what sense a doctrine held and advocated by leading scientific men for the last century or more is now to be called "new;" for there is very little in the philosophy of this book that cannot be found in the writings of eminent physicists from the time of Bacon to the present. Yet these statements of it have been for the most part fragmentary, and in such a form as not to attract the attention of the masses of the lovers of science. It remained for Professor Tyndall to make it popular. With his charming style and brilliant experiments he has embodied the whole philosophy in such a form as to bring it within the comprehension of persons of ordinary intelligence, and to make it not only readable, but as fascinating, to even ordinary readers, as a romance. In a popular sense, therefore, he is really the expounder of a "New Philosophy." He claims no originality, however, for the doctrines he illustrates so strikingly. Yet we think his merit no less than that of the first discoverers of them; for the faculty of stating a doctrine clearly is as rare as that of originating it, and without the one the other would be of but little use to the world. In the advancement of science, therefore, the mission of the expounder is as important as that of the investigator. The popular applause with which these lectures have been received, both in Europe and in this country, we consider to

the fullest extent deserved, for to them the public owes its first acquaintance with this theory of heat, which before had been known only to men of science.

The old corpuscular theory of heat, which this displaces, held that heat was matter--a subtile fluid, which filled the pores of bodies, and, as it were, enveloped their atoms. It was usually defined to be "that substance whose entrance into our bodies causes the sensation of warmth, and its egress the sensation of cold." The capacity of a body for heat meant something like the capacity of a sponge for water. When a body expanded or became rarer, its capacity was increased, and heat was absorbed and became imperceptible or latent; when compressed again, the heat was forced out and became sensible, as water was squeezed out of a sponge. On this principle the heat produced by friction and percussion was explained. “The dynamical theory, or as it is sometimes called, the mechanical theory of heat, discards the idea of materiality as applied to heat. The supporters of this theory do not believe heat to be matter, but an accident or condition of matter; namely, a motion of its ultimate particles.”—Page 39. With regard to the precise character of this molecular motion, no satisfactory theory has been agreed upon. It is yet an open question, to be settled by future investigations. Still, the generally received opinion among physicists seems to have been, that this motion. consists in excursions or oscillations of the atom across centers of equilibrium external to itself. Later investigations on this point, however, seem to unsettle this opinion. Professor Tyndall himself, in a paper on Radiant Heat, read before the Royal Society in March last, shows conclusively that "the period of heat-vibrations is not affected by the state of aggregation of the molecules or atoms of the heated body. The force of cohesion binding the atoms together exercises no effect on the rapidity of vibration." Mr. James Croll, in discussing these results of Professor Tyndall's experiments, in the Philosophical Magazine for May, shows that they are hostile to the theory that heat-vibrations consist in excursions of the atoms across centers of equilibrium external to themselves. For, he contends, the relation of an atom to its center of equilibrium depends entirely on the state of aggregation, and, therefore, the period of its excursions across this center of equilib

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 wellknown 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