Manchester Memoirs, Vol. Iv. (191 0. ^^0. 0. 



VI. The Development of the Atomic Theory : 

 (5) Dalton's Chemical Theory. 



By Andrew Nor^ian Meldrum, D.Sc. 



( Carnegie A'esearr/i Fclloiv. ) 



(Commiiiikated by Prof. H. B. Dixon, M.A., F.R.S.) 



Received October, igio. Read January 2^th. igi i. 



Introduction. 



In the year 1801 Dalton's physical atomic theory 

 (described in the fourth paper of this series) was devised as 

 an explanation of the diffusion of gases. Since the pre- 

 vailing tendency of the time had been to regard diffusion 

 as due to chemical affinity between the gases concerned, 

 Dalton was forced to consider carefully the nature of 

 physical and chemical changes, and to draw a distinction 

 between them. His own theory of diffusion turned on 

 this distinction. Thus, in the course of his argument 

 against the supposition that diffusion is due to chemical 

 affinity, he asks the question, " Why do not oxygenous 

 and azotic gases, taken in due proportion and mixed, 

 constitute nitric acid gas, another elastic fluid, totally 

 distinct in its properties, from either of the ingredients."^ 

 Obviously, therefore, whilst Dalton's attention was being 

 d\rec\.&d principally to physical phenomena, he had in his 

 mind a distinct conception of chemical change. 



The object of this paper is to consider how Dalton 

 passed from the physical atomic theory, which was formed 

 first, to the chemical one, which was formed afterwards. 

 The author has already shown, in the second paper of 

 this series, that the various narratives we possess of the 

 origin of the chemical theory, can be traced back to 



^ Manchester Memoirs, vol. 5, pp. 538-539, 1802. 

 March ytli, igii. 



