Miuic/iesier Meviohs, Vol. Iv. (191 1), No. 5- 15 



" This led the way to the combination of gases." 

 Undoubtedly the combination of gases was the basis of 

 Dalton's chemical theory, and the gist of his narrative is, 

 that YiQ first concluded the particles of different gases to 

 be different in size, and subscquoitly arrived at his 

 chemical theory. 



iSoj or iSoj ? 



Roscoe and Harden, instead of taking this narrative 

 as a document requiring interpretation in the light of the 

 available information, and above all, in the light of 

 Dalton's habit of mind, have accepted it at its face value. 

 Even then they are compelled to admit there is something 

 wrong. The note-books shew that the chemical theory 

 was formed in 1803, and if the amended diffusion hypo- 

 thesis was formed previously, then the date, 1805, which 

 Dalton gives, must be wrong. Roscoe and Harden con- 

 clude that the date of the amended hypothesis is 1803.-* 



The date is iSo^, 

 There are two grave objections to the supposition that 

 the theories were formed in the order given by Dalton. 

 One of these is based on the nature of the theories, and 

 will be considered in the next paper. The other has to 

 do with the genesis of the diffusion hypothesis. Roscoe 

 and Harden have failed to quote from the note-book the 

 passage which deals with this. It is as follows : — 



" On the iiltimate atoms of elastic fluids. 



"There are but three positions that are any way likely 

 to be true on this head. 



" I. The ult. atoms of all gases are of the same 

 weight. 



2* op. cil,, p. 25. 



