temperature of 30. 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 59, 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 to the common opi- 

 nion of cold being only relative, and only 

 a negative term implying the abstract- 

 tion of heat, than the facts which shew 

 the apparent radiation, absorption, and 

 reflexion of cold ; the evidence of which 

 stands on the skme 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 



advanced by M. Prevost, -which 

 goes a considerable way to reconcile ihe 

 appurent 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 SOOlb. 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 tm 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 

 mattrass 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. 



Mr. Leslie also found, not only the 

 same effect in this experiment, but that 

 the action of a cold radiating surface upon 

 the tin reflector produced exactly the 

 same proportional effect upon the differ- 

 ential thermometer as the hot radiating 

 surface, only in the opposite direction of 

 the scale. The differential thermome- 

 ter, which is always at zero when both 

 bulbs are equally heated, is beatifully 

 calculated to shew this striking experi- 



