336 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



Volumes as giving support to his theory, refused to recognise 

 its validity, and even proceeded to question the evidence on 

 which it was based. The argument which influenced him 

 so strongly is summarised in the following paragraphs. 



The simplest way of applying the atomic theory to gases 

 was to assume that the space occupied by the atom (or 

 molecule) was the same for all gases. This hypothesis was 

 worked out by Dalton in the first volume of his "New 

 System," published in 1808, where he writes : 



"At the time I formed the theory of mixed gases, I had 

 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 " (A.C.R. IV. 6-7). 



" By the size or volume of an ultimate particle, I mean 

 in this place, .the space it occupies in the state of a pure 

 elastic fluid" (A. C.R. IV. 6). 



But the logical consequences of the hypothesis were such 

 that Dalton " became convinced that different gases have 

 not their particles of the same size," and concluded : 



" That every species of pure elastic fluid has its particles 

 globular and all of a size; but that no two species agree in the 

 size of their particles, the pressure and temperature being 

 the same " (A.C.R. IV. 7). 



This conclusion was reached as the result of an argument 

 which showed that, if the atoms or molecules of all gases 

 occupied the same volume, there must be a decrease of 'volume 

 and an increase of density, whenever the atoms of two simple 

 gases united together to form the molecules of a compound 

 gas. But in the case of nitric oxide (nitrous air), Davy's, 

 observations showed : 



(i) That the volume of the gas was equal to that of its 



