COLD. When we leave a room at the temperature of 60°, and go into the air in a frosty day at the temperature of 30°, we say it is cold; or when the hand is held in water at the temperature of 100° for a few minutes, and then sudden ly plunged into water at the tempera ture of 40°, the latter is said to be cold. This, however, is merely an expression of the sensation excited in the body, which depends solely' on the abstraction of its heat. This may be proved by the following experiment. If three quanti ties of water are taken, the first at the temperature of 30°, the second at the temperature of 50°, and the third at the temperature of 98°. Immerse the right hand into the water at the temperature of 98°, and the left into the water at the temperature of Let them both re main for a minute, and then suddenly plunge both hands into the water at the intermediate temperature of 50°, to the right hand'it will feel cold, and to the left warm : thus different sensations are produced by the same body at the same time, and at the same temperature. But this depends entirely on the previous state of the hands, and on the absorption or abstraction of the caloric. The right, which was placed in the water at the temperature of 98°, absorbed caloric, be cause the temperature of the water is above that of the body. This excites the sensation of heat : but when the same hand is placed in the water at the temperature of 50°, it is deprived of ca loric, because the surrounding medium is far below its temperature, and thus the sensation of cold is produced. But from the left, placed in the water at 30°, ca loric is abstracted, which gives the sen sation of cold, and the same hand placed in the water at 58°, receives caloric, and this entering the body, excites the sensa tion of heat Thus the term cold is ex pressive of the relative temperature of two bodies. There have, however, been persons who'would account for the phe nomena of cold by the existence of frigorific particles, supposed to be float ing in the air, and by mixing with liquid bodies convert them to solids, and there are facts which seem to support this doc trine.
Nothing appears at first sight more di rectly contradictory t o the common opi nion of cold being only relative, and only a negative term implying the abstract lion of heat, than the facts which shew the apparent radiation, absorption, and reflexion of cold ; the evidence of which stands on the same ground as the corres ponding motions of heat, namely, on the rise or fall of the thermometer. If the rise of the liquor on the scale of a ther mometer, whose bulb is placed in the focus of a mirror, be considered as a proof of the propulsion of certain calorific rays from a distant heated surface, and their subsequent reflexion according to the laws of catoptrics, the sinking of the same thermometer liquor under similar circumstances of position, when the sur face, which before was sensibly hotter than the atmosphere, is now sensibly colder, would seem, from a parity of rea soning, to indicate the propulsion and re flexion of frigorific rays. Nor can we
consider this question as at all determin ed, though an ingenious hypothesis has theft): advanced by M. Prevost, which goes a considerable way to reconcile the apparent contradiction of the doctrine of the unity of heat and cold.
It is singular, that the reflection of cold should have been accidentally discover ed, and decidedly announced about the year 1667, by the members of the Flo rentine Academy del Cimento, without any further prosecution of so curious a fact. The experiment is the following : a mass of ice of about 50016. was set some distance before a concave glass mirror, and the bulb of a spirit thermometer put in the focus, to try whether cold would be reflected. Immediately the spirit of the thermometer began to sink, and fell several degrees. To prove that this was not merely owing to the contiguity of the ice, the surface of the mirror was cover ed with a cloth, to prevent the reflexion, and the thermometer again rose. No further inference is drawn from this ex periment, and the author of it seemed even to doubt of the reality of the re flexion, and to be disposed to impute it to some other unknown cause. This ex periment was repeated in a much more accurate way by M. Pictet. The appara tus which he used was the same as that before described, as employed for the reflection of heat ; that is, two tin mir rors placed directly opposite each other at some distance, in the focus of one of which was placed the bulb of a very sen sible thermometer, and in the other, the vessel intended to produce the heat or cold. In this instance, this latter was a tnattrass full of snow : the mirrors were separated to the distance of 10 feet. At the instant the mattrass was placed in one focus, the thermometer in the oppo site focus began to sink, and descended several degrees. When stationary, ni trous acid was poured on the snow, which produced a cold of much greater intensity, and the thermometer in conse quence immediately descended several degrees lower. When taken out of the focus, it again rose to the common tem perature.