154 Sir. H. E. Eoscoe and Mr. A. Harden 



on 



empirical rules of chemical synthesis, that we have en- 

 deavoured to trace. 



In doing so we were led to the conclusion that, in contra- 

 diction to the usually received view, Dalton' s atomic theory 

 was suggested by certain physical phenomena, especially the 

 diffusion of gases, and not by the results of chemical analysis. 

 Debus objects (p. 350) that this does not constitute a new 

 view, inasmuch as he had himself previously arrived at a 

 similar general conclusion, to which, indeed, we referred in 

 our original work (R. and H. p. 6). We differ, however, 

 from Debus as to the nature of the train of reasoning which 

 led Dalton to the foundation of his theory, and it is our view 

 of the actual genesis of the theory, taken together with the 

 conclusion stated above, which justifies the wording of our 

 title, " A New View of the Origin of Dalton's Atomic Theory." 



Turning now to the evidence at our disposal, Debus and 

 ourselves are in agreement up to a certain point. There 

 seems to be no question that it was the study of the diffusion 

 of gases which gave the primary impulse to Dalton's mind. 

 At first he endeavoured (1801) to account for this phenomenon 

 by supposing " that the particles of one gas are not elastic or 

 repulsive in regard to the particles of another gas, but only 

 to the particles of their own kind" (N. S. p. 154), so that 

 each gas acted as a vacuum to the other. Moreover, as he 

 states in the N. S. p. 188, he had in 1801 "a confused 

 idea, as many have I suppose, at this time, that the particles 

 of elastic fluids are all of the same size ; that a given volume 

 of oxygenous gas contains just as many particles as the same 

 volume of hydrogenous ; or if not, that we had no data from 

 which the question could be solved"*. 



Relying solely on this u confused idea," Debus, and here 

 our differences begin, makes the following statement (p. 358): 

 J c In order to explain equilibrium in a mixture of gases, Dalton 

 had adopted, in the year 1801, the hypothesis M/S = C" 

 (M = molecular weight, S = specific gravity: the symbols 

 M/S = C represent the empirical law that equal volumes of 

 different gases contain, at normal temperature and pressure, 

 an equal number of molecules) . 



As a matter of fact, and as can at once be seen from the 

 quotation given above, " the hypothesis M/S = C" is not in any 

 way involved in the explanation of the phenomenon given by 

 Dalton, and could not therefore be adopted in order to explain 

 it. The precise expression of Dalton's " confused idea " by 



* It may "be here noticed that Debus in quoting this passage (pp. 361, 

 362) invariably omits the last clause, which materially affects the sense 

 of the whole passage. 



