4 Meldrum, Development of the Atomic Theory. 



impede the flow of water. Hence, if two gases were 

 brought together, they were found, sooner or later, to be 

 uniformly mixed. 



Dalton and the diffusion of gases. 



After the theory had been explained, Dalton deemed 

 it necessary to make new experiments on the diffusion of 

 gases. Priestley, who originally drew attention to this 

 phenomenon, was inclined to think it accidental in its 

 nature. He thought that if " two kinds of air were put into 

 the same vessel with very great care, without the least 

 agitation that might mix or blend them together, they 

 might continue separate."* Dalton's experiments, made 

 with the simplest of apparatus, proved, in his own words, 

 " the remarkable fact, tJiat a lighter elastic fluid ca)inot ?-est 

 upon a heavier''^ The importance of this work, by which 

 he established diffusion as a genuine property of gases, 

 was recognised by Berthollet, who carefully repeated it." 



Dalton was evidently much gratified by the agree- 

 ment between his theory and the facts of diffusion. He 

 concludes his memoir on diffusion with a note of 

 triumph : — " The facts, stated above, taken together, 

 appear to me to form as decisive evidence for that theory 

 of elastic fluids which I maintain, and against the one 

 commonly received, as any physical principle which has 

 ever been deemed a subject of dispute, can adduce."^ 



Dalton^ s theory and the vapour of ivater. 



Obviously, a special case of the mixed gases question 

 is that of the water vapour in the atmosphere. The 



* "Experiments and Observations, &c.," abridged, vol. 2, p. 441. 



^ Manchestei- Memoirs, [2], vol. I, p. 260, 1805, 



M^m. cCArceuil, vol. 2, p. 463, 1809. 



' Manchester Memoirs, [2], vol. i, p. 270, 1805. 



