On the Nature of Heat.
AFTER having made ourselves acquainted with the proAfter having made ourselves acquainted with the pro- pertit s that are usually ascribed to heat, with the effects which it produces, and with the sources whence it is de rived, we shall be more competent to enter upon the inves tigation of its nature. This has been a subject of discus sion from the earliest period of philosophical inquiry, and is yet far from being determined ; for although the most generally received opinion is in favour of its being a sub stance, capable of a separate existence, and possessed of a material, although very subtile nature ; yet there are, on the contrary, many eminent men who regard it as merely a pro perty, necessarily attached to other matter, and arising from some peculiar modification or affection of it.
The illustrious Bacon adopted this latter hypothesis, and conceived that heat depended upon a vibration of the par ticles of matter ; an hypothesis which he endeavoured to substantiate by sheaving, that whatever excited tempera ture, tended to produce a motion in the particles of the heated body. His description of this peculiar action is, that "-heat is an expansive motion, restrained and resisting in the minute parts ;" a phrase which, if expressed in mo dern language, would probably signify a reaction between the expansive power of heat, and the attractive force of the particles of matter towards each other. The idea of Bacon, that heat depends upon a vibratory motion among the parti cles of matter, received the powerful sanction of Boyle and Newton. As, however, observations on the phenomena of nature were multiplied, and especially as chemical science advanced, the hypothesis which considered heat as merely consisting in motion of the particles of matter, appeared less easy to reconcile with the new discoveries, and conse quently a different doctrine was advanced, in which the ef fects of heat were attributed to a species of subtile fluid, of a proper material nature, although differing, in many im portant particulars, from any other kind of matter. The first writer who distinctly maintained this doctrine, and ap plied it in a philosophical manner to the explanation of facts, was Boerhaave ; and it was, for the most part, embraced by the French. The general impression that was produced by Dr Black's discovery, was much in favour of the ma teriality of heat ; and indeed it seems very difficult to ima gine, how a mere property can be so exactly measured, and can be transferred from one body to another, at one time rendered latent, and again coming into action, without its quantity being either increased or diminished. The suc cessive discoveries of Crawford and Irvine, together with the whole fabric of the Lavoisierian chemistry, strongly fa vour the same opinion ; so that, in the present day, it must be regarded as the hypothesis which is by far the most ge nerally received. It has, however, been zealously opposed by Rumford ; and the experiments on the heat excited by friction, of which we have given an account, were brought forward as an unanswerable objection against it. Our li mits will not permit us to take a very full view of all the arguments that have been urged on both sides of the ques tion; but we must endeavour to give a sketch of some of the principal points that have been adduced by the advo cates of each of the opinions. It will scarcely be denied, that if we admit the existence of a subtile elastic fluid, the particles of which are endowed with a repulsive power, which tends to unite itself to all kinds of matter, to insinu ate itself into their pores, to produce their expansion, and, if added in sufficient quantity, to impart to them its own elastic nature, we are in possession of an agent, which very conveniently explains a great variety of phenomena ; for, excepting the experiments of Rumford on friction, and others of a similar nature, we do not know of any facts which are averse to the supposition, or which are not better explained on this than on any other that has been adduced.
It must, however, be admitted, that it is merely an hypo thesis ; and as there is no direct experiment which proves the existence of this subtile elastic fluid, it must be aban doned, if there be any single phenomenon which is abso lutely irreconcilable to it. But before we can allow this to be the case, in the present instance, it is necessary to spew, that our acquaintance with the phenomenon in question is complete ; that we thoroughly understand all its relations, and arc competent to decide upon the connexion which it has with all the other actions to which it may be referred. Now we believe that few persons will assert that this is the ease ; on the other hand, we conceive every one will admit, that any attempt to explain the intimate nature of the mo tion supposed to produce heat, and the manner in which vibrations can excite a sensation such as that which we re fer to this principle, must be a most arduous task. Still more so will it be to explain, how it can be the immediate cause of liquidity and of elastic fluidity ; how it can be trans ferred in definite quantities from one body to another ; how this can be done even while it is lying dormant, or while we have no proof that the motion actually exists. In short, we shall find, that the hypothesis of vibrations is far from being unencumbered with difficulties ; and that we have only removed one, to become involved in a much greater number. Besides, although we have admitted that there is no direct experiment to prove the independent existence of heat, or at least none against which some exception has not been taken ; yet there are facts brought forward, per haps as decisive on this side of the question, as those re specting friction are on the other. We refer to the trans mission of heat through a vacuum. Pictet proved that this takes place in the vacuum of the air-pump ; and Rumford, the great advocate for the immateriality of heat, has shewn that it is capable of passing even through the Torricellian vacuum. There seems no method of reconciling this fact with the hypothesis, except by taking for granted the ex istence of some kind of vapour or elastic fluid, along which it is propagated ; a supposition equally gratuitous, and equally unsupported by direct and independent facts as that for which it is substituted. Indeed, the same remark may be applied to all the phenomena in which the radiation of heat is concerned ; it seems extremely improbable, if not impossible, that these rays are carried along by the air, even when near the surface of the earth, and they must necessa rily traverse an immense track totally devoid of air. If we, in this case, suppose the existence of a subtile medium, in vented for the purpose of carrying them through space, we are here, as in the former instance, creating an agent at /east as hypothetical as the matter of heat. Upon the whole, we are strongly inclined to the opinion which re gards heat as an elastic fluid, of a proper immaterial na ture, although of extreme subtilty, the particles of which are repulsive with respect to each other, but arc attracted by other bodies, with different degrees of force, according to their respective nature.