Chemical Combination of Gases. tbb 



number of oxygen and hydrogen molecules respectively, we 

 may put 



n = |(M-2r), 



So that the last equation becomes 



,, 2 = ig (M -2r)\N-r) (15) 



It is evident from this equation that the quantity of the 

 compound formed will depend upon the density, for if we 

 make M = 2N we see that the equation can bo expressed as 

 an equation in r/M alone ; but the coefficients will involve M, 

 so that the value of r/M will depend upon the density. If 

 we suppose that r/M is small we see from equation (15) that 

 r/M increases as M increases, so that a larger proportion w t ouU 

 combine at great pressures than at small; or if we look upon 

 it as a case of dissociation we may express the same thing by 

 saying that the dissociation increases as the pressure dimi- 

 nishes. Thus the dissociation of water differs in this respect 

 from the dissociation of hydrochloric acid. 



Let us now take the other alternative, and suppose that a 

 water molecule is formed by the union of two separate hydro- 

 gen atoms wdth an oxygen atom, and that when a water 

 molecule is decomposed it splits up into two free hydrogen 

 atoms and an oxygen atom ; in this case t 3 and r 2 are both 

 infinite, and when things have attained a steady state the 

 equations (13) become 



(16) 











n m 2 " 















>, 











r m 2 p 





so that 

 or putting, 



as 



before, 



n=i(M-2r), 



Ave get 









p=?KN-i 



), 



(17) 



^=i^Vg(M-2,f(N-r), . . , (18) 



