PRESENT FUNDAMENTAL CONCEPTIONS OF PHYSICS. 489 



sained the existence either of an excessively rare elastic heat substance 

 (the element " fire"), which, by penetrating into the body according to 

 its amount, heated it more or less, or they tried to explain heat phenom- 

 ena by the assumption of a continuous motion of greater or smaller 

 amount of the ultimate corpuscles of bodies. Their idea was however 

 too general, too indefinite, and confused in both directions. Black and 

 his adherents in principle shared the first-named opinion, and believed 

 that the important fact of specific heat (discovered by Black) could only 

 be explained by the hypothesis of an extremely rare, elastic, all-pene- 

 trating, and imponderable " caloric." It was supposed that this caloric 

 repelled its own molecules while it attracted those of foreign bodies. If 

 this caloric, by virtue of the attraction exercised by it and the mole- 

 cules of these bodies, penetrated the pores of substances, it expanded 

 the molecules by means of its elasticity, thus increasing the volume of 

 bodies by the supply of heat, and in this manner the expansion of a 

 body by heat was explained. Black and his scientific friends thus de- 

 fined specific heat: Caloric is present in smaller quantities in cold 

 bodies than in warm ones ; it possesses a certain degree of elasticity 

 dependent on its accumulation and the degree of attraction existing 

 between it and the molecules of matter. The greater the latter the 

 feebler the elasticity of the caloric between the molecules will appear, 

 consequently the thermometrical degree of the body will prove the 

 smaller, although the same amount of heat be present, and will there- 

 fore require the more heat to raise its temperature one degree; in other 

 words, its specific heat or heat capacity will be the more considerable. 



The fact that with the condensation of a body its specific heat dimin- 

 ishes, harmonized with this explanation. The caloric hypothesis made 

 the following assumption : If a body is condensed the elasticity of the 

 caloric is thereby increased ; a part of this becomes free and raises the 

 temperature of the body. In the same measure as the heat capacity of 

 a body is diminished or increased the heat will become free or latent. 



The opinion in regard to the existence of a caloric was confirmed, in 

 addition, by the fact of radiation of heat, as far as it was known at the 

 time; the caloric rays (like the rays of light) being regarded as exceed- 

 ingly rare material emanations from heated bodies, moving with infinite 

 rapidity, penetrating the bodies with which they came in contact, in 

 part combining with their substance (as previously indicated) and 

 remaining latent, or in part free and acting as thermometrical heat in 

 the bodies. 



In Black's time all the then known heat phenomena harmonized with 

 this heat hypothesis, or at least they were not in opposition to it; there- 

 fore at that time the caloric theory was almost universally accepted, 

 and this all the more as the terrestrial sources of heat, formerly so dif- 

 ficult to explain (including the development of heat by percussion or 

 compression or by chemical processes) after Black's discovery of specific 

 heat, might be appropriately reduced to a diminution of the heat capac- 



