356 Dr. H. Debus on the Genesis of 



tides, of which, he was firmly convinced, all gases were made 

 up, he had recourse to the results of chemical analysis. 

 Assisted by the assumption that combination always takes 

 place, in the simplest possible way, he thus arrived at the idea 

 that combination takes place between particles of different 

 weights, and this it was which differentiated his theory from 

 the historic speculations of the Greeks. The extension of 

 this idea to substances in general necessarily led him to the 

 law of combination in multiple proportions, and the com- 

 parison with experiment brilliantly confirmed the truth of his 

 deduction " (R. 49-51)., 



The problem of ascertaining the relative diameters, or rather 

 the relative volumes, of the molecules of different gases, led 

 Dalton, according to Roscoe and Harden, to the invention of 

 the atomic theory. In the pamphlet mentioned before (p. 350) , 

 I have stated that Dalton wished to verify the hypothesis 

 M/S = C, viz. to ascertain whether the molecular volumes of 

 different gases are of equal or of different magnitudes. For 

 this purpose he required to know the respective molecular 

 and atomic weights. This necessity led him to the formation 

 of the atomic theory (D. 58). 



I am much pleased to observe that, with regard to the 

 problem which originated the atomic theory, Roscoe and 

 Harden, after a careful study of Dalton's note-books, arrive 

 at the same view which I published two years ago, but I 

 regret very much that I cannot approve the reasons which 

 have guided their judgment. 



The first assertion of Roscoe and Harden, '"'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," I have not met, nor do I 

 remember to have seen in any of Dalton's writings, nor am I 

 able to deduce such an assertion from Dalton's notes, quoted 

 before. These notes treat of two distinct subjects, the 

 theories of gaseous diffusion and the problems of the atomic 

 theory. The description passes abruptly from the one to the 

 other without any connecting link. How did Dalton get 

 over the chasm which separates the two ? We want to know 

 the genesis of Dalton's methods of determining atomic weights. 

 This, the essence of the matter, Messrs. Roscoe and Harden 

 pass over in silence ! 



Dalton explained the diffusion of gases on the assumption 

 that the molecules of different gases are not of the same size. 

 The question, therefore, was, Are the molecules of different 

 gases really of unequal volume ? "I soon found/' says 



