PHYSICAL THEOEIES OF HEAT. 185 



It will be recollected that the main laws of phenomena for which we 

 have to account, by means of such an hypothesis, are the following : 



(1.) The law of Boyle and Mariotte, that the elasticity of an aii 

 varies as its density. See Chap, iii., Sect. 1 of this Book. 



(-2.) The Law of Gay-Lussac andDalton, that all airs expand equally 

 by heat. See Chap. ii. Sect. 1. 



(3.) The production of heat by sudden compression. See Chap. ii. 

 Sect. 2. 



(4.) Dalton's principle of the mechanical mixture of airs. See Chap, 

 iii. Sect. 3. 



(5.) The Law of expansion of solids' and fluids by heat. See Chap. 

 ii. Sect. 1. 



(6.) Changes of consistence by heat, and the doctrine of latent heat. 

 See Chap. ii. Sect. 3. 



(T.) The Law of the expansive force of steam. See Chap. iii. 

 Sect. 4. 



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. (See Chap. iii. 

 Sect. 5.) 



Laplace's hypothesis 4 is this : that bodies consist of particles, each 

 of which gathers round it, by its attraction, a quantity of caloric : 

 that the particles 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 con- 

 stantly 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 suppositions 

 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 con- 

 sistence (6), are explained by supposing 6 that in solids, the mutual 

 attraction of the particles of the body is the greatest force ; in liquids,, 

 the attraction of the particles for the caloric ; in airs, the repulsion of 



' Mic. <:H. (, v \\ 89. 5 Ib. p. 110. 6 Ib. p. 92. 



