278 J. W. Gibbs — Vapor- Densities. 
position of the perchloride, there must be as many mole- 
‘cules of the type Cl, as of the type PC], Now a gas-mix- 
ture containing an equal number of molecules of PCI; and Ch 
are in this case dissimilar, they may be partially separated by 
diffusion through a neutral gas, the lighter chlorine diffusing 
more rapidly than the heavier protochloride. The fact of dis- 
dence than the variations of the densities in support. of the 
hypothesis of the compound nature of the vapor, yet if these 
variations shall appear to follow the same law as those of the 
roxide of nitrogen and the perchloride of phosphorus, 1t will 
difficult to refer them to a different cause 
can deduce a general law determining the proportions of an 
* Salet, “Sur la coloration du peroxyde d’azote.” Comptes Rendus, t. Ixvii, 
1 . ir : ille, “Sur les densités de vapeur.” Comptes Rendus, ¢- 
: $ Wanklyn and Robinson. ‘On Diffusion of Vapours: a means of distinguish- 
between apour-densities of Chemical Compounds. Proc. 
