CALORIC, in chemistry, a word used to signify that substance or property, by which the phenomena of heat are pro duced. Concerning the nature of caloric there are two opinions, which have divid ed philosophers ever since they turned their attention to the subject. Some sup. pose, that caloric, like gravity, is merely a property of matter, and that it consists in a peculiar vibration of its particles ; others, on the contrary, think that it is a distinct substance. Each of these opinions has been supported by the greatest phi. losophers ; and till lately the obscurity of the subject has been such, that both sides have been able to produce exceed ingly plausible and forcible arguments. The recent improvements, however, in this branch of chemistry, have gradually rendered the latter opinion much more probable than the former : and a recent discovery made by Dr. Herschel], has at last nearly put an end to the dispute, by demonstrating that caloric is not a pro perty, but a peculiar substance ; or at least that we have the same reason for considering it to be a substance, as we have for believing that light is material. Dr. Herschel] had been employed in mak ing observations on the sun, by means of telescopes. To prevent the inconve nience arising from the heat, be used co loured glasses : but these glasses, when they were deep enough coloured to in tercept the light, very soon cracked, and broke in pieces. This circumstance in duced him to examine the heating power of the different coloured rays. He made each of them in its turn fall upon the bulb of a thermometer, near which two other thermometers were placed, to serve as a standard. The number of degrees which the thermometer exposed to the coloured ray rose above the other two thermometers indicated the heating pow er of that ray. He found that the most refrangible rays have the least heating power, and that the heating power gra dually increases as the refrangibility di. minishes. The violet ray therefore has the smallest heating power, and the red ray the greatest. Dr. Herschel found, that the heating power of the violet, green, and red rays, are to each other as the following numbers : Violet . . . . = 16 Green . . .. =_- 22.4 Red — 55.
It struck' Dr. Herschel as remarkable, that the illuminating power and the ing power of the rays folknv such differ ent laws. The first exists in greatest perfection in the middle of the spectrum, and diminishes as we approach either ex tremity; but the second increases con stantly from the violet end, and is greatest at the red end. This led him to suspect, that perhaps the heating power does not stop at the end of the visible spectrum, but is continued beyond it. He placed the thermometer completely beyond the boundary of the red ray, but still in the line of the spectrum, and it rose still higher than it had done when exposed to the red ray. On shifting the thermome ter still farther,it continued to rise, and the rise did not reach its maximum till the thermometer was half an inch beyond the utmost extremity of the red ray. When shifted still farther, it sunk a little, but the power of heating was sensible at the distance of 1i inch from the red ray. These important experiments have been lately repeated andfully confirmed by Sir Henry Englefield, in the presence of some very good judges.
From these it follows, that there are rays emitted from the sun which produce heat, but have not the power of illumi nating; and that these are the rays which produce the greatest quantity of heat. Consequently caloric is emitted from the sun in rays, and the rays of ca loric are not the same with the rays of light. On examining the other extremity of the spectrum, Dr. Herschel ascertain ed that no rays of caloric can be traced beyond the violet ray. He had found,
however, that all the coloured rays of the spectrum have the power of heating ; it may be questioned, therefore, whether there be any rays which do not warm. The coloured rays must either have the property of exciting heat as rays of light, or they must derive that property from a mixture of rays of caloric. If the first of these suppositions were true, light ought to excite heat in all cases ; but it has been long known to philosophers, that the light of the moon does not the least sensible heat, even when con centrated so strongly as to surpass in point of illumination the brightest can dles or lamps, and yet these produce a very sensible heat. Here then are rays of. light which do not produce heat : rays, too, composed of all the seven prismatic coloured rays. We must conclude from this well known fact, that rays of light do not excite heat; and consequently that the coloured rays from the sun and com bustible bodies, since they excite heat, must consist of a mixture of rays of light and rays of caloric. That this isthe case was demonstrated long ago by Dr. Hooke, and afterwards by Scheele, who separat ed the two species from each other by a very simple method. If a glass mirror be held before a fire, it reflects the rays of light, but not the rays of caloric ; a me tallic mirror, on the other hand, reflects both. The glass mirror becomes hot ; the metallic mirror does not alter its tem perature. If a plate of glass be suddenly interposed between a glowing fire and the face, it intercepts completely the warming power of the fire, without caus ing any sensible diminution of its brillian cy; consequently it intercepts the rays of caloric, but allows the rays of light to pass. If the glass be allowed to remain, in its station till its temperature has reached its maximum, in that situation it ceases to intercept the rays of caloric, but allows them to pass as freely as the rays of light. This curious fact, which spews us that glass only intercepts the rays of caloric till it be saturated with them, was discovered by Dr. Robinson. Thesefacts are sufficient to convince us, that the rays of light and of caloric are different, and that the coloured ray s derive their heat ing power from the rays of caloric which they contain. Thus it appears that solar light is composed of three sets of rays, the colorific, the calorific, and the deoxi dizing. The rays of caloric are refract ed by transparent bodies as the rays of light. We see, too, that, like the rays of light, they differ in their refrangibili ty ; that some of them are as refrangible as the violet rays ; but that the greater number of them are less refrangible than the red rays. Whether they are trans mitted through all transparent bodies has not been ascertained ; neither has the difference of their refraction in different mediums been examined. We are cer tain, however,lhat they are transmitted and refracted by all transparent bodies which have been employed as burning glasses. Dr. Herschell has also proved, by experiment, that it is not only the caloric emitted by the sun which is refrangible, but likewise the rays emitted by mon fires, by candles, by hot iron, and even by hot water. The rays of caloric are reflected by polished surfaces in the same manner as the rays of light. This was lately proved by Herschel : but it had been demonstrated long before by Scheele, who had even ascertained that the angle of their reflection is equal to the angle of their incidence. M. Pictet also had made a set of very ingenious experiments on this subject, about tue year 1790, which led to the same con clusion.