Before we conclude this discussion concerning the im materiality of heat, it will be proper to notice the experi ments which have been made, in order to ascertain whe ther it be actually possessed of gravity, or rather, whether its weight can be measured by a balance. The best con trived experiments of this description were those of Fordyce. He very carefully weighed a quantity of water, froze the water, and then again weighed it. Now he argued, that in this process, the water must have parted with the latent heat which maintains it in the liquid form ; so that if heat be a ponderable substance, it might be expected that the ice would exhibit a diminution in its weight,, equivalent to that of the caloric which had escaped. The result, how ever, did not correspond with this idea ; and indeed, in some of the most accurate trials, it seemed as if the body that had parted with its heat had even acquired a slight addi tion of weight. It is, however, generally admitted, that no decisive conclusion can be drawn from such experiments ; and that, from the conception that we have of the extreme tenuity of the particles of heat, it is not probable, that any portion which we can have it in our power to impart to a body, could be detected by the instruments that we employ in ascertaining the weight of bodies. Our preference, as we stated above, is certainly in favour of the hypothesis of the materiality of heat; because we think it explains the phenomena in general with greater facility, and is encum bered with less difficulties than the immaterial hypothesis. Vet we must remember, that it is not decisively proved by any direct and unexceptionable experiments ; and it must also be acknowledged, that it has not received the sanction of some eminent philosophers, both at the revival of letters and in our own times. We have already mentioned the opinion of Rumford ; and Professor Leslie, to whom science is so peculiarly indebted for his numerous experiments and discoveries on the subject of heat, also adopts the hypothe sis, which ascribes its effects to a certain motion among the particles of bodies. He conceives that the propagation and transmission of heat, is very similar to that of sound, and that it in fact consists in Get tain aerial undulations. A hot body communicates a portion of its heat to the contiguous stratum of air ; this immediately expands, and by this ex pansion, a vibration is excited in the adjoining stratum ; this is propagated to the next, and so on until the equilibrium is produced. The passage of heat is therefore of the same velocity with the undulation of the air, or rather is identi cal with it ; and according as the surfaces of bodies act up on the air in contact with them, so will they radiate heat with greater or less readiness. The facility of radiation is supposed to be principally owing to the approximation of the air to the surface ; those bodies radiating the best, to which the air comes into the closest contact. This view of the subject explains why the best radiators should like wise be the best absorbers of heat ; and it is also supposed to afford a reason for the effect of roughening the surface, or covering it with substances which destroy its polish. Professor Leslie seems to have advanced his hypothesis, merely as a convenient manner of accounting for some of his own experiments ; he has not stated it in such a way as to apply to all the phenomena of heat, nor has he attempt ed to reconcile it with the experiments of Herschel, and others which appear decidedly adverse to it. Indeed, we much doubt whether it can be adapted even to some of his own experiments on the radiation and reflection of heat; for it must be supposed, that the undulations of the air are propelled from the surface of bodies, impinge upon others, and are again reflected, exactly as they radiate heat, with the same velocity, and in the same direction ; a circum stance which we apprehend it would be extremely difficult to prove. It may be farther remarked, that although Pro fessor Leslie's hypothesis supposes that the undulations of the air are the cause of heat, or that which produces the phenomena in question ; yet there are many expressions which would seem to imply, that this heat is something dis tinct from the undulations themselves, and that the heat is transferred or transmitted from one wave to the other, not that the waves are themselves the actual cause of the ef fect. The farther consideration of this question would in volve us in a discussion that would exceed the limits to which we are necessarily restricted.
In the former part of this article we alluded to the chemical properties of heat, but we proposed to defer the consideration of them until we had made ourselves more fully acquainted with its nature and its effects. On this subject there are two points to be determined : first, Does heat possess any properties, or any relation to other bodies, which can strictly be called chemical ? And secondly, What are these relations ? When we speak of a substance being possessed of chemical properties, we mean, that by its being separated from or added to another sub stance, the two substances, in the case of their decomposi tion, or in the other case, the compound of the two, manifest different chemical relations from those which they possess ed before the operation. By some philosophers it has heen
supposed that this change takes place with respect to heat, that when it enters into combination with a body, as for ex ample with oxygen, and forms oxygen gas, a compound is produced, possessed of new properties that are not merely mechanical, while at the same time the heat itself has en tirely lost its former qualities by its union with the oxygen, and like other bodies, after a chemical composition, has im parted to the compound properties quite different from its own. If we again decompose the oxygen gas, as by unit ing its base to carbon, the heat is precipitated, and resumes all its former properties, which had been entirely destroy ed, or rendered latent, while it remained in the combined state. But although there may appear to be a considerable degree of plausibility in this method of reasoning, it has not been generally acquiesced in. The great objection which has been urged against the opinion, that heat unites chemi cally to bodies, is, that it may, in all cases, be separated from them, merely by a reduction of temperature, without the intervention of a more powerful affinity ; a circumstance which is conceived to be essential to a proper chemical de IN the preceding article, we have given an account of the experiments of Dr Herschel, Sir H. Englefield, and M. Berard, from which it follows, that there exists beyond the red extremity of the spectrum a set of invisible rays, which affect the thermometer more powerfully than even the red light of the spectrum.
These experiments were repeated, under the most fa urable circumstances, by our celebrated countryman Sir d. Davy, who favoured us with an account of them when we had the pleasure of seeing him at Geneva. Availing himse!f of the fine climate and the serene sky of Italy, he resolved to inquire into the cause of the difference between the results obtained by Dr Herschel and Berard; the former having placed the point of maximum intensity beyond the red rays, while the latter found the heat greatest within the extremity of the red space. It occurred to Sir Humphry, that thermometers with circular balls of the size used by Berard must necessarily shew the maximum of heat to be within the red space, as a cooling agency must always be exerted by that part of the bulb which is out of the limits of the calorific rays ; and hence the diminution of tempera ture which this occasioned would compensate for the greater effect produced by the invisible rays. In order to remove this source of error, Sir H. Davy employed ex tremely slender thermometers, not more than.Ath of an inch in diameter, with very long bulbs filled with air con fined by a coloured fluid ; and in this way the bulb was af fected only by the invisible rays. The general results of these experiments, which were repeated also at Geneva, confirmed those of Dr Herschel, and may be considered as removing the objection of Professor Leslie, that, in our cli mate particularly, there must always be a concentration of extraneous light beyond the red extremity of the spec trum.
We regard it, therefore, as a point established in physics, that there is a distinct effect produced upon the thermome ter beyond the red extremity of the spectrum ; but we cannot admit for a moment the conclusion which has been universally drawn from it, that it is produced by a separate set of heat-making rays, refracted by the prism, and hav ing a greater refrangibility than red light. Dr Herschel has attempted to prove that invisible culinary heat is also sus ceptible of refraction like the invisible rays, and M. Prevost and M. Delaroche have likewise endeavoured to shew that this heat is capable of permeating glass. We have no doubt that this opinion will be found to be erroneous, and we think it is demonstrated,* that invisible culinary heat is incapable of refraction, or of permeating glass like light, and hence we cannot suppose that the invisible rays of the sun are capable of being refracted by glass. If this shall be found to be the case, it will follow that the invisi ble heat of the sun is a substance possessing properties essentially different from those of invisible culinary heat. As radiant heat, therefore, possesses very different pro perties from light, we think it will be found, that light, like all other matter, whether gaseous, fluid, or solid, is capable of being heated, and that the solar rays are nothing more than heated light. Hence the least refrangible rays, having the greatest velocity, or momentum, will produce the most powerful calorific effect. 'We shall have occasion to resume this subject in another part of our work. See OPTICS. ED.