Several experiments, however, have been adduced, which seem to afford the same evidence for the existence of a frigorific principle. that we have of caloric. These x pet iments arc all connected with radiation or absorp tion ; after describing them, we shall state the hypothe ses which have been proposed to explain them, in con formity with the opinion that cold is merely the abstrac tion of caloric.
NN'e have already mentioned, that when a plate of po lished metal is exposed to a heated body, almost all the rays of caloric are reflected. If the rays strike the sur face of the plate perpendicularly, they are reflected di rectly backwards ; but if they strike it obliquely, they are also reflected obliquely ; and the angle of reflection, as in the case of light, is equal to the angle of incidence. Such a figure, therefore, might be given to the metallic surface, that when the heated body to which it is ex posed is placed in a certain position, all the rays incident upon it might be reflected in a direction parallel to each other. This will he the case if the metallic reflector have the surface of a parabolic conoid, and the hot body be placed in the focus of the generating parabola. Ac cordingly, if another mirror of the same form be placed directly opposite, the foci being turned towards each other, and the axes coincident, the emanations from the hot body in the one focus will meet, after two reflec tions, in the other, and produce there an elevation of temperature corresponding to that of the hot body. If the surface of the reflectors be coated with lamp-black, the effect is scarcely perceptible, because almost all the rays of caloric are then absorbed. In short, the great est effect is produced, when the surface of the hot body is most favourable to radiation, the temperature being supposed the same ; and when the reflectors are most highly polished, and formed of some metal which has the least disposition to absorb caloric.
It was ascertained by Pictet, with an apparatus simi lar to the one we have described, that when a matrass filled with snow was placed in the one focus, and a deli cate air thermometer in the other, the reflectors being at the distance of 10! feet, the thermometer immediate ly indicated a reduction of temperature of several de grees, and rose again when the matrass was withdrawn. On replacing the matrass, and pouring nitrous acid upon the snow, the temperature of the thermometer sunk or 6° lower than before. In this experiment, cold seems
to have been radiated by the snow, and after being re flected from the surface of each mirror, to have been absorbed by the thermometer ; and hence the reduction of temperature which it sustained.
In order to ascertain the nature of this radiant cold, Mr Leslie, who has particular!) investigated the subject by experiment, took a tin cubical vessel, having one side painted with lamp black, another coated with writing paper, a third covered with a pane of glass, and the fourth bright and polished : he filled the vessel thus pre pared m ith ice, and having opposed it to the thermome ter in the focus of one of the reflectors, he found the reduction of temperature sustained by the thermometer to be least from the metallic surface, greater from the pane of glass, and greatest of all from the side coated with lamp-black. From a variety of other experiments, he ascertained, that the powers of different substances in radiating, reflecting, and absorbing cold, are precise ly the same ass their powers of radiating, reflecting, and absorbing heat. Hence, it has been concluded, that we have the same proofs for the existence of cold as a dis tine• and independent principle, that we have for the ex:'r -net of heat.
T. explanation of these facts, in conformity with the doctrine that cold is merely a negation of heat, has given greater difficulty to chemists than we conceive belongs naturally to the subject. According to the hypothesiA of Prevost, caloric is radiated from all bodies at all tem peratures, the quantity radiated being proportional to the difference of temperatures. When the body placed in the focus of one of the reflectors has a more elevated tem perature than the thermometer in the focus of the op posite reflector, the temperature of the thermometer is increased, because it receives more caloric by absorption, than it communicates by radiation. When both the body and the thermometer are at the same temperature, no change takes place, because each communicates the same quantity which it receives ; hut if the temperature of the body be lower than that of the thermometer, though it still radiates caloric, it radiates a smaller quantity than the thermometer, and therefore the temperature of tLe latter is reduced.