358 Dr. H, Debus on the Genesis of 



theory cannot have been formed before the year 1805. But 

 Roscoe and Harden found in Dalton's note-book a table of 

 atomic weights dated the 6th of September, 1803. This date 

 shows that the theory is at least two years older than it ought 

 to be according to Roscoe and Harden's view (R. 2SJ). In 

 order to get over this difficulty, they assume (R. 25) that 

 Dalton committed a clerical error by writing 1805 instead of 

 1803 in his description of the formation of the theory of 

 diffusion based on an unequal size of the particles. Daltcn 

 does not say that the theory of diffusion led him to the con- 

 ception of the atomic theory. The statement that it did do so 

 is only an inference of Messrs. Roscoe and Harden. Therefore 

 they are not justified in attributing a clerical error to him, and 

 transferring the conception of the theorv of diffusion from 1805 

 to 1803. 



Dalton's paper, " Experimental Enquiry into the Pro- 

 portions of the several Gases or Elastic Fluids constituting 

 the Atmosphere," was printed in the ' Memoirs of the Literary 

 and Philosophical Society of Manchester,' second series, vol. i. 

 1805, under Dalton's supervision as Secretary of the Society 

 (R. 31). We have in this paper the following remarks on the 

 principle that the elastic or repulsive power of each molecule 

 is confined to its own kind : — " This principle accords with all 

 experience, and, I have no doubt, will soon be perceived and 

 acknowledged by Chemists and Philosophers in general " 

 (Al. 5) . Now this principle is the principle of the theory of 

 diffusion formed in 1801 (pp. 354, 355) ; consequently he must 

 have still held this theory in 1805, at the time when the paper 

 was printed, and the theory of diffusion which superseded it 

 must have been formed at a later period in the year 1805. 

 Therefore this later theory of the diffusion of gases cannot have 

 been the origin of the atomic theory of 1803. The conclusion 

 we arrive at is : " The opinion of Roscoe and Harden that the 

 theory of the diffusion of gases of 1805 led Dalton to the con- 

 ception of his atomic theory is not in accordance with facts/' 



II. The Genesis of Dalton s Atomic Theory, 



The notes published by Roscoe and Harden enable me to 

 render my view of the genesis of this theory, described on 

 p. 356 (D. 58), more accurate and complete. 



In order to explain equilibrium in a mixture of gases, 

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

 But he was not able for several years to test his conception by 

 experiments, as no method was known for the determination 

 of M, the molecular weight. The desire to discover such a 

 method made him, probably, very observant of all circum- 

 stances which could promote his wish (N.S. 187-188). 



