354 Dr. H. Debus on the Genesis of 



oxygen must be greater than the specific gravity of an atom 

 of nitrogen. 



This view, in connexion with the general theory of gases 

 then in vogue, led to the conclusion that, in a mixture of 

 oxygen and nitrogen, the heavier atoms of oxygen ought to 

 form a layer at the bottom of the vessel and the lighter 

 nitrogen atoms a separate layer at the top. 



Experiments, on the other hand, proved that air, taken 

 from different heights, was of uniform composition. Dalton 

 also found that a lighter gas, placed above a heavier gas, will 

 gradually diffuse downwards (N.S. 151). To explain these 

 observations a weak chemical affinity was assumed between 

 the molecules of different gases. The globular molecules 

 Dalton considered to be arranged like a pile of shot (N.S. 

 145, 147), and as all the molecules are subject to the same 

 pressure and exert the same counter-pressure, they must be 

 all, independent of their nature, of the same size (N.S. 188), 



; M/S = M7S' = M"/S" = M'"/S'". 



The air is composed of 77*88 vols, of nitrogen, 21*2 vols, of 

 oxygen, and 0*066 vol. of carbonic acid. Hence, according 

 to the above law, for every molecule of carbonic acid there 

 are 31*8 mols. of oxygen and 118 mols. of nitrogen present in 

 our atmosphere. 



Now, if a weak chemical affinity is the cause of diffusion, 

 then compounds of a very complicated nature must result. 

 This appeared to Dalton very improbable, and accordingly he 

 rejected the chemical explanation of the diffusion of gases. 

 Dalton goes on with his notes as follows : — 

 " In the year 1801 I hit upon an hypothesis (N.S. 153) 

 which completely obviated these difficulties. According to 

 this, we were to suppose that the atoms of one kind did not 

 repel the atoms of another kind, but only those of their own 

 kind. This hypothesis most effectually provided for the dif- 

 fusion of any one gas through another, whatever might be 

 their specific gravities, and perfectly reconciled any mixture 

 of gases to the Newtonian theorem (R. 15). Every atom of 

 both or all the gases in the mixture was the centre of repul- 

 sion to the proximate particles of its own kind, disregarding 

 those of the other kind. All the gases united their effort in 

 counteracting the pressure of the atmosphere, or any other 

 pressure that might be opposed to them. This hypothesis, 

 however beautiful might be its application, had some impro- 

 bable features. We were to suppose as many distinct kinds 

 of repulsive power as of gases ; and, moreover, to suppose 



