Dalton s Atomic Theory. 355 



that beat was not the repulsive power in any one case ; 

 positions certainly not very probable. 



" Upon reconsidering this subject, it occurred to me that I 

 had never contemplated the effect of difference of size in the 

 particles of elastic fluids, or when the expression M/8 = G is 

 of different value for different gases. And if the sizes be 

 different, then on the supposition that the repulsive power is 

 heat, no equilibrium can be established by particles of unequal 

 sizes pressing against each other. 



" This idea occurred to me in 1805. I soon found that the 

 sizes of the particles of elastic fluids must be different. For 

 a measure of azotic gas and one of oxygen, if chemically 

 united, would make nearly two measures of nitric oxide, and 

 those two could not have more molecules of nitric oxide than 

 one measure had of oxygen or nitrogen. 



" Hence the suggestion that all gases of different kinds 

 have a difference in the size of their molecules ; and thus we 

 arrive at the reason for that diffusion of every gas through 

 every other gas, without calling in any other repulsive power 

 than the well-known one of heat. 



" This then is the present view which I have of the consti- 

 tution of a mixture of elastic fluids (year 1810). The different 

 sizes of the particles of elastic fluids under like circumstances 

 of temperature and pressure being once established, it became 

 an object to determine the relative sizes and weights, together 

 with the relative number of atoms in a given volume. This 

 led the way to the combinations of gases, and the number of 

 atoms entering into such combinations, the particulars of 



which will be detailed more at large in the sequel " 



(R. 13-17). 



From these statements of Dalton, Roscoe and Harden 

 deduce the following genesis of the atomic theory : — 



" The balance of evidence is, therefore, strongly in favour of 

 the statement made in London by Dalton himself in 1810, that 

 he was led to the atomic theory of chemistry in the first 

 instance by purely physical considerations in opposition to 

 the view hitherto held by chemists, that the discovery b} r 

 Dalton of the fact of combination in multiple proportions led 

 him to devise the atomic theory as an explanation. It, there- 

 fore, becomes necessary for us to modify our view as to the 

 foundation of the atomic theory. There seems to be no 

 doubt that the idea of atomic structure arose in Dalton' s 

 mind as a purely physical conception, forced upon him by 

 his study of the physical properties of the atmosphere and 

 other gases. Confronted, in the course of his study, with 

 the problem of ascertaining the relative diameters of the par. 



2 C2 



