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renders it not only useless, but destructive, to the progress of the Natural Philosopher to endeavour to explore the beginnings of his science. While he gropes about, seeking the source, his contemporaries are borne, with ever-increasing swiftness, along the broadening and deepening current of the river, to the "great ocean of truth which lies unexplored before them.”

In the physical world we are cognisant of but four elementary or primordial ideas besides the inevitable Time and Space. They are Matter, Force, Position, and Motion. Of these, motion is simple change of position; and force is recognised as the agent in every change of motion. Till we know what the ultimate nature of matter is, it will be premature to speculate as to the ultimate nature of force; though we have reason to believe that it depends upon the diffusion of highly attenuated matter throughout space. But, keeping to the four elementary ideas above, it is evident that to one or other of these every distinct physical conception must be referred. To which does Heat belong? The old notions of heat were that it was Matter; or, according to some philosophers, Force. It is only within about a century that proofs have been gradually arrived at that sensible, or thermometric, heat consists of Motion; while the so-called “ Latent Heat” of Black may possibly not be heat at all, but may consist of Position. These are startling statements, as we have made them, but they will be fully explained, and to some extent developed, in the course of the Article.

Thus it appears, that of the four available hypotheses as to the nature of Heat the two necessarily erroneous ones have, till lately, been almost universally adopted. So much for the trustworthiness of the metaphysical treatment of a physical question ! Such a lesson should never be lost sight of; so deserved and so complete a refutation of the sophistical nonsense of the schoolmen, and so valuable a warning to the Natural Philosopher who is disposed to à priori argument as more dignified and less laborious than experiment, can scarcely occur again. Even the despised perpetual-motionist has more reason on his side than the metaphysical pretender to discovery of the laws of nature; he, to his cost—but to his credit also—appeals to experiment to test the validity of his principle; but the mighty intellect of his rival scorns such peddling with apparatus, to it all truth is intuitive; nay more, what it cannot comprehend cannot be truth. But the days of its authority have nearly expired—luckily for human progress.

When heat was considered to be matter, under the name of Caloric, it was regarded of course as uncreatable and indestructible by any process at the command of man. And we cheerfully allow that many very plausible explanations of curious physical

phenomena were arrived at by the labour and ingenuity of the partisans of this theory. Thus it was natural to suppose, that when caloric entered a body, or rather combined with it, the body. should in general expand; and even when heating produced contraction there were analogies, quite sufficient to bear out the theory, supplied by such mixtures or alloys as alcohol and water, or copper and tin; where the bulk of the compound is considerably less than the sum of the bulks of the components. Conduction of heat, or transference of caloric from one body to another, or from part to part of the same body, also presented no difficulty. So it was with the experiments which led to what was called (from the principles of this theory) the specific heat of bodies ; it had merely to be assumed that different bodies required different proportions of caloric to be mixed with them to produce equal effects in the form of change of temperature. Thus, the specific heat of water being called i, that of mercury is .033, i.e., a pound of water requires 30 times more caloric to be mixed with it to produce a given change of temperature (measured by the thermometer), than a pound of mercury. The fact that in heating ice no rise of temperature is observed, however much heat may have been applied, until the whole of the ice is melted-and similar phenomena observed in every case of melting or liquefaction, as well as in boiling or vaporization-led Black to propound the doctrine of Latent Heat. The fundamental ideas of this doctrine, that water differs from ice at the same temperature simply by the admixture of a definite equivalent of caloric; that the steam which escapes from boiling water, though showing the same temperature to the thermometer, contains a vastly greater amount of caloric; and similar ideas for all similar cases, were thus easily and directly reduced to the caloric theory. The additional quantity of caloric in such cases was supposed simply to change the molecular state of the body, without altering its temperature: hence the name. In all this there need be no hesitation, so far as we can see, in pronouncing the explanations given by the material theory of heat quite satisfactory, although in many cases they are certainly cumbrous, and difficult of application.

But another class of common phenomena afforded no such easy application of the theory, namely, the development of heat by friction or concussion; and it must be allowed that many of the warmest supporters of the caloric hypothesis frankly admitted that their explanations of these effects were not quite satisfactory. The general tendency of these explanations was towards assuming a change in the capacity for caloric to be produced by the disintegration caused by friction or by the compression caused by impact—though it was excessively diffi

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cult to see how two such opposite processes could each produce a diminution of the capacity. And although the difficulty is lessened by considering a change in both capacity and latent heat to be produced by attrition or condensation, it is by no means removed.

The mischievous consequences of long persistence in a false theory were perhaps never better exemplified than in the case of this supposed materiality of heat; for so completely were the scientific men of last century imbued with it, that when Davy gave a conclusive proof of the actual creation of heat in a very simple experiment, his consequent argument against the materiality of heat (or the existence of caloric) attracted little attention, and was treated by many of those who condescended to notice it as a wild and extravagant speculation. It is certain that even Davy himself was led astray in his argument, by using the hypothesis of change of capacity as the basis of his reasoning, and that he might have been met successfully by any able Calorist who, though maintaining the materiality of heat, might have been willing to throw overboard one or two of the less essential tenets of his school of philosophers.

But Davy's experiment, rightly viewed, is completely decisive of the question; and, in spite of the imperfection of his reasoning from it (due entirely to the prevailing sophisms of the Calorists), was perfectly satisfactory to himself. He developed, in a singularly brief and lucid form, the fundamental principles of the true theory, in a tract, forming part of the Contributions to Physical and Medical Knowledge, principally from the West of England, collected by Thomas Beddoes, M.D., published at Bristol in 1799.

Davy commenced by causing two pieces of ice to rub against each other, until both were almost entirely melted by the friction. Here water considerably above the freezing point was produced, and as the capacity of ice for heat was known to be less than that of water, it followed at once from this experiment, that the ice contained more caloric after being melted than before, because--(1.) Its temperature was raised, and its capacity for heat increased ; (2.) It had in addition the latent heat of fusion. Unless, then, it had drawn caloric from surrounding bodies there must have been creation of caloric, a result perfectly inadmissible to supporters of the material theory. To show that no heat was abstracted from surrounding bodies, he proceeded to cause two pieces of metal to rub against each other by means of clockwork, the whole apparatus being placed on a block of ice, which had some unfrozen water in a canal on its surface, and enclosed in a very perfect vacuum, produced by the now well-known application of carbonic-acid gas and caustic

potash. Here again heat was developed by the friction, but it did not come from the ice (for the water in contact with it was not frozen), nor from surrounding bodies (for in this case it must have passed through, and melted, the ice, but the ice remained unaltered). From these perfectly conclusive experiments, Davy proceeds thus

“ Heat, then, or that power which prevents the actual contact of the corpuscles of bodies, and which is the cause of our peculiar sensations of heat and cold, may be defined a peculiar motion, probably a vibration, of the corpuscles of bodies, tending to separate them. It may with propriety be called the repulsive motion."

“Bodies exist in different states, and these states depend on the differences of the action of attraction, and of the repulsive power, on their corpuscles, or in other words, on their different quantities of attraction and repulsion."

Let us here remark, incidentally, what an immense simplification is at once introduced into our conception of the laws which regulate the intermolecular forces in bodies. Davy, by a single sentence or two, thus demolished for ever the ingeniously unnatural speculations of Boscovich and his school, who represented the law of the force exerted by one molecule or particle of a body on another, by a most complex alternation of attractions and repulsions, succeeding each other as the distance between the two was gradually diminished, a law so inconsistent with the simplicity of that of gravitation, as to lead us to wonder that it was ever seriously propounded.

Davy, in fact, makes this very application, and illustrates the effect of the repulsive motion in balancing the attraction of cohesion in bodies by the very apt comparison of the orbital motion of a planet preventing its being drawn nearer to the sun. We shall not attempt to follow his further development of this discovery, where he falls into an ingenious mistake in consequence of his belief in the corpuscular theory of light. It has nothing to do with our subject; yet, though now known to be erroneous, it is worthy of its author.

The rest of this short tract, so far as it relates to heat, is concerned with the laws of communication of heat, which he shows to be quite analogous to those of the communication of motion. It was not, however, so far as we know, till 1812 that Davy distinctly laid down, in a perfectly comprehensive form, the law of the phenomenon. In his Chemical Philosophy, published in that year, he enunciates the following perfectly definite and most important proposition :

“The immediate cause of the phenomenon of heat, then, is motion, and the laws of its communication are precisely the same as the laws of the communication of motion.” The im

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mense consequences of this statement we shall presently consider, after we have briefly described the labours of a contemporary of Davy, who almost succeeded in 1798, in demonstrating the immateriality of heat; but whose work is especially valuable as containing the first recorded approximation to the measurement of heat in terms of ordinary mechanical units, which, singularly enough, does not appear to have been attempted by Davy.

In the Philosophical Transactions for the last-named year, there is a most instructive paper by Count Rumford, entitled, An Inquiry concerning the Source of the Heat which is excited by Friction. The author's experiments were made at Munich while he superintended the boring of cannon in the Arsenal; indeed, he remarks, that “very interesting philosophical experiments may often be made, almost without trouble or expense, by means of machinery contrived for the mere mechanical purposes of the arts and manufactures.” He was struck with the very great heat developed by the friction or attrition of the steel borer on the brass casting; and especially, in comparing it with the very small quantity of chips or powder removed from the metal, justly observing that it was inconceivable that a mere change of the capacity for heat in so small a relative quantity of brass, could develop heat sufficient in some cases to boil a large quantity of water.

"In reasoning on this subject,” he says, “we must not forget to consider 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 the manner that heat was excited and communicated in these experiments, except it be MOTION.”

We shall have occasion again, more than once, to make valuable extracts from this extremely lucid and philosophical paper; meanwhile we may merely observe, that Rumford has pointed out other methods to be employed in determining the amount of heat produced by the expenditure of mechanical power, instancing particularly the agitation of water or other liquids, as in churning.

It may be well to pause for a moment at this stage, and carefully consider to what extent the true theory of heat had really been advanced about the commencement of the present century. And it is easy to see from the preceding pages that the

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