xv THE MOLECULAR THEORY 337 



constituents, which did not therefore contract in combina- 

 tion, and 



(2) That whilst the density of the gas was a little greater 

 than that of nitrogen it was actually less than that of the 

 oxygen which formed one of its constituents. Davy's actual 

 figures ( Works, III. 9-10) were : 



Nitrogen 3'45 grains per 100 cubic inches. 

 Nitric oxide 34*3 

 Oxygen 35*09 



Dalton argues as follows : 



" If equal measures of azotic and oxygenous gases were 

 mixed, and could be instantly united chemically, they would 

 form nearly two measures of nitrous gas, having the same 

 weight as the two original measures ; but the number of 

 ultimate particles could at most be one half of that before 

 the union. No two elastic fluids, probably, therefore, have 

 the same number of particles, either in the same volume or 

 the same weight" (A.C.R. IV. 5). 



When Gay-Lussac, in 1809, put forward his Law of Vol- 

 umes, the simple hypothesis to which it seemed to point had 

 therefore already been tested by Dalton, and found to be 

 untenable. It is not surprising then that Dalton should 

 become its foremost opponent, his repudiation extending 

 even to a refusal to accept the experimental facts to which 

 Gay-Lussac had called attention. In the second part of 

 his "New System." published in 1810, he sums up his 

 criticism as follows : 



" The truth is, I believe, that gases do not unite in equal 

 or exact measures in any one instance ; when they appear 

 to do so, it is owing to the inaccuracy of our experiments. 

 In no case, perhaps, is there a nearer approach to mathe- 

 matical exactness, than in that of i measure of oxygen to 

 2 of hydrogen ; but here, the most exact experiments I 



z 



