CALORIC. 



must consist of a mixture of rays of light 

 and rays of caloric. That this is the 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 shews 

 us that glass only intercepts the rays of 

 caloric till it be saturated with them, was 

 discovered by Dr. Robinson. These facts 

 are sufficient to convince us, that the rays 

 of light and of caloric are different, and 

 that the coloured rays derive their heat- 

 ing power from the rays of caloric w"hich 

 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 just 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, that 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 com- 

 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 et of very ingenious 



experiments on this subject, about the 

 year 1790, which led to the same con- 

 clusion. 



All the phenomena concur to skew, 

 that the rays of caloric move with a very 

 considerable velocity, though the rate 

 has not been ascertained in a satisfactory 

 manner. Some experiments of Mr. Leslie 

 would lead us to conclude, that they move 

 with the same velocity as sound. The 

 following experiment of M. Pictet indi- 

 cates a very considerable velocity. He 

 placed two concave mirrors at the dis- 

 tance of 69 feet from each other ; the one 

 of tin, the other of plaster gilt, and 18 

 inches in diameter. Into the focus of this 

 last mirror he put an air thermometer, 

 and a hot bullet of iron into that of the 

 other. A few inches from the face of the 

 tin mirror there was placed a thick screen, 

 which was removed as soon as the bullet 

 reached the focus. The thermometer 

 rose the instant the screen was removed 

 without any perceptible interval, conse- 

 quently the time which caloric takes in 

 moving 69 feet is too minute to be mea- 

 sured. The velocity of caloric, if it is 

 equal to that of light, would prove that 

 its particles must be equally minute. 

 Therefore/ neither the addition of caloric, 

 nor its abstraction, can sensibly affect the 

 weight of bodies. 



Caloric agrees with light in another pro- 

 perty no less peculiar: its particles are 

 never found cohering together in masses; 

 and whenever they are forcibly accumu- 

 lated, they fly off in all directions, and se- 

 parate from each other with inconceiv- 

 able rapidity. This property necessarily 

 supposes the existence of a mutual repul- 

 sion between the particles of caloric. 

 Thus it appeal's that caloric and light re- 

 semble each other in a great number of 

 properties. Both are emitted from the 

 sun in rays, with the velocity of 200,000 

 miles in a second; both of them are re- 

 fracted by transparent bodies, and re- 

 flected by polished surfaces; both of 

 them consist of particles which mutually 

 repel each other, and which produced no 

 sensible effect upon the weight of other 

 bodies. They differ, however, in this 

 particular: light produces in us the sen- 

 sation of vision; caloric, on the contrary, 

 the sensation of heat. Upon the whole, 

 we are authorized, by the above statement 

 of facts, to conclude, that the solar light 

 is composed of three distinct substances, 

 in some measure separable by the prism, 

 on account of the difference of their re- 

 frangibility. The colorific rays are the 

 least refrangible, the deoxidizing rays 

 are most refrangible, and the calorific 



