PHYSICAL THEORIES OF HEAT. 589 



(7). The Law of the expansive force of steam. 

 See p. 574 and note (TA). 



Besides these, there are laws of which it is 

 doubtful whether they are or are not included in 

 the preceding, as the low temperature of the air in 

 the higher parts of the atmosphere, (p. 580.) 



Laplace's hypothesis 4 is this: that bodies con- 

 sist of particles, each of which gathers round it, by 

 its attraction, a quantity of caloric : that the par- 

 ticles of the bodies attract each other, besides 

 attracting the caloric, and that the particles of the 

 caloric repel each other. In gases, the particles of 

 the bodies are so far removed, that their mutual 

 attraction is insensible, and the matter tends to 

 expand by the mutual repulsion of the caloric. He 

 conceives this caloric to be constantly radiating 

 among the particles; the density of this internal 

 radiation is the temperature, and he proves that, on 

 this supposition, the elasticity of the air will be as 

 the density, and as this temperature. Hence follow 

 the three first rules above stated. The same sup- 

 positions lead to Dalton's principle of mixtures (4), 

 though without involving his mode of conception ; 

 for Laplace says that whatever the mutual action of 

 two gases be, the whole pressure will be equal to 

 the sum of the separate pressures 5 . Expansion (5), 

 and the changes of consistence (6), are explained by 

 supposing 6 that in solids, the mutual attraction of 

 the particles of the body is the greatest force, in 



4 Mec. Ctl. t. v. p. 89. 5 Ib. p. 110. 8 Ib. p. 92. 



