PHYSICAL THEORIES OF HEAT. 591 



to explain the tendency of neighbouring bodies to 

 equality of temperature; and this leads to the 

 higher generalization, that heat is radiant from 

 points below the surface. But in the doctrine of the 

 relation of heat to gases, as delivered by Laplace, 

 there is none of this unexpected confirmation ; and 

 though he explains some of the leading laws, his 

 assumptions bear a large proportion to the laws 

 explained. Thus, from the assumption that the 

 repulsion of gases arises from the mutual repulsion 

 of the particles of caloric, he finds that the pressure 

 in any gas is as the square of the density and of the 

 quantity of caloric 8 ; and from the assumption that 

 the temperature is the internal radiation, he finds 

 that this temperature is as the density and the 

 square of the caloric 9 . Hence he obtains the law of 

 Boyle and Mariotte, and that of Dalton and Gay- 

 Lussac. But this view of the subject requires other 

 assumptions when we come to latent heat; and 

 accordingly, he introduces, to express the latent 

 heat, a new quantity 10 . Yet this quantity produces 

 no effect on his calculations, nor does he apply his 

 reasoning to any problem in which latent heat is 

 concerned. 



Without,, then, deciding upon this theory, we 

 may venture to say that it is wanting in all the pro- 

 minent and striking characteristics which we have 



8 P=2 HKpV(l) p. 107- f q'n (fl)=pc (2) p. 108. 

 10 The quantity i, p. 113. 



