the Genesis of Dalton s Atomic Theory. 155 



the formula M/S = C, which Debus employs throughout his 

 paper, is moreover misleading, since it implies that the idea 

 of relative molecular weight was at that time present to 

 Dalton's mind, whereas we have no evidence whatever that 

 this was the case. 



On this point therefore we retain our original position 

 (R. and H. p. 47) that Dalton " never appears to have believed 

 in the ' law of equal volumes/ and this only occurred to him 

 as a possible alternative, at once shown to be inconsistent 

 with fact, to the statement which he recognized as the true 

 one, viz., ' that no two elastic fluids agree in the size of their 

 particles.-''" 



Our view of the origin of the theory, on the other hand, is 

 based upon a passage of Dalton's MSS. (1810) in which he 

 professedly gives " a brief historical sketch of the train of 

 thought and experience which led me to the conclusions 

 about to be detailed." 



Dissatisfied with the explanation of diffusion which he had 

 given in 1801, he, at a later date, came to the conclusion that 

 the phenomenon might be explained on the supposition that 

 the atoms of the different gases were of different sizes. We 

 here quote the passage in full (R. and H. pp. 16, 17) : — 



" 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 *. By size I mean the hard particle 

 at the centre and the atmosphere of heat taken together. If, 

 for instance, there be not exactly the same number of atoms 

 of oxygen in a given volume of air, as of azote in the same 

 volume, then the sizes of the particles of oxygen must be 

 different from those of azote. 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 nitrous gas, and those two 

 could not have more atoms of nitrous gas than the one measure 

 had of azote or oxygen. Hence the suggestion that all gases 

 of different kinds have a difference in the size of their 

 atoms ; and thus we arrive at the reason for that diffusion of 



* Debus in quoting this passage (p. 355) here inserts the words " or 

 when the expression M/S = C is of different value for different gases." 

 These words are not present in the original, and their insertion is a pro- 

 cedure which might lead the reader to suppose that the formula was used 

 by Dalton. 



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