Mauckesier ATonoirs, Vol. Iv. (191 1), No. 5. 11 



though usually dated October, 1801, was, as a matter of 

 fact, not read then as the first three were before this 

 Society. 



Later developments. 



Between the paper of April, 1800, and the " Experi- 

 mental Essays" of October, 1801, Dalton took up the 

 study of only two additional topics. One of these, the 

 vapour pressure of other liquids than water, was a natural 

 outcome of previous work, and calls for no special com- 

 ment here. The other was that of the explanation of 

 the phenomena of mixed gases. This, a large topic and 

 not an experimental one, is discussed in the last section 

 of this paper. But here is the place to point out that 

 Dalton's reflections on this subject led to two experi- 

 mental inquiries of the greatest consequence. One of 

 these, already mentioned in this paper, was the study of 

 the diffusion of gases. The other was the determination 

 by Dalton of the composition of the atmosphere, the 

 outcome of which, as will be shown in the next paper, 

 was the formation of the chemical theory. 



III. The two forms of the physical theory 



AND the dates OF THEH^ ORIGIN. 



The date of the first diffusion hypothesis. 



Dalton, in the Introduction to his set of four "Experi- 

 mental Essays" of October, 1801, explains that this 

 theory of mixed gases was arrived at after his other 

 results. " The first law \_i.e., the mixed gases theory] 

 which is as a mirror in which all the experiments are best 

 viewed, was last detected, and after all the particular 

 facts had been previously ascertained." ^^ 



^* Manthcstcr Memoirs, [i], vol. 5, p. 536. 



