38 MEMOIR OF DR. DALTON, AND 



lified, that there would be a diminishing quantity of oxygen, 

 which is the heaviest gas in the atmosphere, according as 

 the height increased. This was Dalton's opinion, but it has 

 not turned out to be the case. This law was much assailed, 

 and at the same time much misunderstood. The objection 

 that vapour did not rise so rapidly in air as in a vacuum 

 seemed to him a strong one, which he did not quite get over, 

 but considered it as presenting the same difficulty to all 

 theories of the solution of water in air. 



The law was stated too broadly, it did not even allow room 

 for the impenetrability of matter to have its due place, and 

 many persons supposed it to mean that a space filled with one 

 gas, might be filled with an equal quantity of another. 



He subsequently stated these two propositions in the 

 following form, which he published in the second edition of 

 his " New system of chemistry," when, after many years, he 

 reviewed himself and his reviewers, p. 191, Part I., 1842. 



"1. The diffusion of gases through each other is effected 

 by means of the repulsion belonging to the homogeneous 

 particles; or to that principle which is always energetic to 

 produce the dilatation of the gas. 



" 2. When any two or more mixed gases acquire an equi- 

 librium, the elastic energy of each against the surface of the 

 vessel or of any liquid, is precisely the same as if it were the 

 only gas present occupying the whole space, and all the rest 

 were withdrawn." 



There is no doubt that the law had been hastily expressed : 

 explaining some points, it contradicted others. The pheno- 

 menon of the mixing of gases is easily explained, if we admit 

 the constant intestine motion of the particles to be a necessary 

 condition of the existence of a body in a gaseous state. (See 

 a paper " On the changes of temperature produced by the 

 rarefaction and condensation of air," by J. P. Joule. Phil. 

 Magaz,, May, 1845.) 



The second essay is on the force of steam or vapour. He 



