360 Dr. H. Debus on the Genesis of 



the idea that the atoms combine chemically only according to 

 very small numbers — one atom of A with one, two, or three atoms 

 of B? There are strong reasons in support of this conclusion. 

 Dalton, who for some years had held the hypothesis M/S = C 

 (N.S. 188), who was accustomed to explain expansion, diffu- 

 sion, solution, &c, in a mechanical way by means of the old 

 atomic theory, as Boyle did 120 years before Dalton's time, 

 was naturally led by the observation of the combining pro- 

 portions of oxygen and nitric oxide to the view that in simple 

 compounds one atom of A is united to one or tw r o atoms of 

 B. His previous speculations and observations (p. 354) had 

 prepared him for this conception, and once the idea was 

 formed the transition to the tenets described on p. 353 was 

 easy. Proust and others had made observations like Dalton, 

 but to none had this explanation occurred. 



My present view of the origin of Dalton's atomic theory 

 is : — 



Dalton wished to verify the hypothesis M/S = C; for this 

 purpose he endeavoured to find methods for the determination 

 of the atomic and molecular weights. The observations of the 

 combining proportions of nitric oxide and oxygen suggested 

 to him a principle which enabled him to form such methods. 



In judging this view, one has to remember that the obser- 

 vations on the combining proportions of oxygen and nitric 

 oxide noted on August 4th, 1803, were followed within a 

 month by calculations of atomic weights and the diameters of 

 atoms and molecules. (R. 38 and 41.) 



III. Roscoe and Harden's Critique. 



The first part of my view of the genesis of Dalton's atomic 

 theory, described above, is, according to Roscoe and Harden, 

 not correct. They say : — 



" The view expressed by Debus, that the use of the term 

 specific gravities by Thomson in his account of the atomic 

 theory implied the acceptance of the law M/S= C by Dalton in 

 1804, is also seen to be incorrect. Dalton expressly states in 

 1803 (R. 27) that the specific gravities of different gases and 

 the specific gravities of their particles are not the same thing. 

 He never appears to have believed in the law M/S = C, 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 gases agree in 

 the size of their particles " (R. 47) . 



In order to form a correct opinion of the assertions contained 

 in this quotation, we must go back and consider the statements 

 of Dalton on which these assertions are founded. 



