568 HISTORY OF THERMOTICS. 



in the sequel of other generalizations concerning 

 gases, he was led to the persuasion, that when air 

 and steam are mixed together, each follows its 

 separate laws of equilibrium, the particles of each 

 being elastic with regard to those of their own kind 

 only: so that steam may be conceived as flowing 

 among the particles of air 10 "like a stream of water 

 among pebbles ;" and the resistance which air offers 

 to evaporation arises, not from its weight, but from 

 the inertia of its particles. 



It will be found that the theory of independent 

 vapour, understood with these conditions, will in- 

 clude all the facts of the case ; gradual evapora- 

 tion in air ; sudden evaporation in a vacuum ; the 

 increase of the air's elasticity by vapour ; condensa- 

 tion by its various causes ; and other phenomena. 



But Mr. Dalton also made experiments to prove 

 his fundamental principle, that if two different gases 

 communicate, they will diffuse themselves through 

 each other 11 ; slowly, if the opening of communica- 

 tion be small. He observes also, that all the gases 

 had equal solvent powers for vapour, which could 

 hardly have happened, had chemical affinity been 

 concerned. Nor does the density of the air make 

 any difference. 



Taking all these circumstances into the account, 

 Mr. Dalton abandoned the idea of solution. "In 

 the autumn of 1801," he says, " I hit upon an idea 



10 Manchester Memoirs, vol. v. p. 581 , 



" New System of Chemical Philosophy, vol. i. p. 151. 



