S Meldrum, Development of the Atomic Theory. 



literally from my journal of that date.'"* Then comes an 

 account of the atomic theory, and on that there follows 

 the passage already quoted, connecting" marsh-gas and 

 olefiant gas with the genesis of the theory. Here the 

 question arises, is all this taken from the journal, both the 

 sketch of the theory and of how the theory arose? Only 

 an examination of the journal can settle this point, but I 

 have not succeeded in ascertaining where it is kept, if, 

 indeed, it is still in existence. 



It must be admitted also that Thomson seems to 

 become more and more positive regarding the genesis of 

 the theory as time goes on. The account which I have 

 been considering was published in 1831. Six years 

 earlier he had advanced the same account in a more hesi- 

 tating way : — " Unless my recollection fails me, Mr. Dal- 

 ton's theory was originally deduced from his experiments 

 on olefiant gas and carburetted hydrogen.'"'' Yet there is 

 no intrinsic improbability that Thomson's recollection is 

 correct. One cannot doubt that during the interview 

 Dalton was much less interested in the question of the 

 origin than in the theory itself If Thomson inquired 

 about the origin, Dalton may have made the inquiry an 

 opportunity of expounding the theory in terms of its 

 latest triumph, namely, the composition of marsh-gas and 

 olefiant gas. 



3. The Amended Theory of^' Mixed Gases" 



There remains for consideration the account which 

 Dalton gave in a lecture (the 17th ofa series) at the Royal 

 Institution of London, on the 27th January, 18 10. The 



'" Thomas Thomson, " liisloiy of Chcmisliy," vol. 2, p. 287. 

 '■'Thomas Thomson, " An Attempt to Estabhsh the First Principles 

 of Chemistry by Experiment," vol. i, p. 11, 1825. 



