VAPOR-DENSITIES. 373 



of molecules of PC1 3 and C1 2 will have the density i(4'98 + 2'22) 

 or 3*60. It follows that, at least so far as the range of the possible 

 values of its density is concerned, we may regard the vapor as a 

 mixture in variable proportions of two kinds of gas having the 

 densities 7*20 and 3'60 respectively. The observed values of the 

 density accord with this supposition. 



These hypotheses respecting the constitution of the vapors are 

 corroborated, in the case of peroxide of nitrogen and perchloride 

 of phosphorus, by other circumstances. The varying color of the 

 first vapor may be accounted for by supposing that the molecules 

 of the type N 2 4 are colorless, while each molecule of the type NO 2 

 has a constant color. This supposition affords a simple relation 

 between the density of the vapor and the depth of its color, which 

 has been verified by experiment.* 



The vapor of the perchloride of phosphorus shows with increasing 

 temperature in an increasing degree the characteristic color of 

 chlorine. The amount of the color appears to be such as is required 

 by the hypothesis respecting the constitution of the vapor on the 

 very probable supposition that the perchloride proper is colorless, 

 but the case hardly admits of such exact numerical determinations 

 as are possible with respect to the peroxide of nitrogen.! But since 

 the products of dissociation 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 dissociation has in this way been proved by direct 



experiment. | 



In the case of acetic and formic acids, we have no other evidence 

 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 peroxide of nitrogen 

 and the perchloride of phosphorus, it will be difficult to refer them 

 to a different cause. 



But however it may be with these acids, the peroxide of nitrogen 

 and the perchloride of phosphorus evidently furnish us with the 

 means of studying the laws of chemical equilibrium in gas-mixtures 

 in which chemical change is possible and does in fact take place 

 reversibly, with varying conditions of temperature and pressure. 

 Or, if from any considerations we can deduce a general law 



* Salet, " Sur la coloration du peroxyde d'azote," Comptes Eendus, t. Ixvii, p. 488. 



fH. Sainte-Claire Deville, "Sur les densites de vapeur," Comptea Rendus, t. Ixii, 

 p. 1157. 



jWanklyn and Robinson, "On Diffusion of Vapours: a means of distinguishing 

 between apparent and real Vapour-densities of Chemical Compounds," Proc. Hoy. Soc., 

 vol. xii, p. 507. 



