160 The Genesis of Dalton s Atomic Theory. 



Thomson, moreover, in his account of the atomic theory 

 invariably used the term relative density or density of an 

 atom, not only for gaseous, but also for liquid and solid 

 bodies. 



So obvious did it appear that Thomson used the expression 

 density of an atom to mean its relative weight that we as- 

 sumed that Debus, in his original pamphlet, had interpreted 

 it in the same manner as ourselves, and it was upon this 

 supposition that our criticism on this point (R. and H. p. 1 1) 

 was founded. As it now appears that Debus considers that 

 the words must be interpreted in their literal sense, our 

 former criticism naturally falls. This interpretation, however, 

 as we have just shown, cannot be maintained, and hence 

 the argument which Debus bases upon it is fallacious. 



As regards the process of thought or experience by which 

 Dalton arrived at his empirical rules, of which Debus u wants 

 to know " the origin, there is little to be added to the views 

 we have already expressed. Dalton, with his " corpuscular 

 turn of mind/' seeking for a method of determining the re- 

 lative sizes of his "atoms," no doubt perceived that two 

 factors were concerned in the problem : the densities of the 

 gases and the relative weights of the atoms of which they 

 are made up. For the determination of the latter Dalton 

 naturally turned to chemical analysis, but again found that 

 two factors were involved : the relative weights of the two 

 substances which combined and the relative number of the 

 atoms of each which took part in the combination. Unable 

 to devise any positive method of determining the latter, he 

 had recourse to a series of empirical rules, which are based 

 on the principle that, in the absence of distinct evidence, it 

 may be assumed that combination always takes place in the 

 simplest possible way. 



The fact of the combination of two elements in several 

 different proportions had long been familiar to chemists, and 

 Dalton, with his profound belief in indivisible atoms of cha- 

 racteristic weight, could hardly fail at once to see that this 

 might be explained by combination between varying numbers 

 of the atoms of the two elements. 



The question of the influence of Dalton's experiments on 

 nitrous gas and oxygen upon his speculations (Debus, p. 360) 

 has already been fully treated by us in our original work 

 (R. and H. pp. 31-38), and we have there given the reasons 

 which render it probable that these experiments did not play 

 any very important part in the development of the atomic 

 theory. 



We may sum up our conclusions as follows :— 



