136 Drs. Ramsay and Young on 



The numbers 2*073 and 4*146 are the densities referred to 

 air of the molecules C 2 H 4 2 and C 4 H 8 4 respectively ; D is 

 the observed density; and 3520 and 11*349 are constants 

 deduced from the determinations of Cahours and Bineau. 

 This formula, of which the constants in its author's opinion 

 can claim only approximate correctness, is quite inadequate 

 to represent actual facts at high temperatures and high 

 pressures where cohesion becomes marked. For example, it 

 gives at a temperature of 280° for the density of the saturated 

 vapour 35*13 instead of the observed number 62*62. 



If our opinion be correct, and if the abnormal density of 

 saturated vapours and of vapours near their saturation-points 

 and also above their critical points, at high pressures, of stable 

 substances, be due to mere molecular proximity, and not to 

 any form of molecular combination • then a dissociating sub- 

 stance must exhibit a vapour-density which may be partly 

 due to this cause. With such a substance as ammonium 

 chloride, which, we have shown, is almost completely disso- 

 ciated at 280°, the products of dissociation (hydrogen chloride 

 and ammonia) are under such conditions of temperature and 

 pressure that they would probably behave as perfect gases ; 

 the relatively few molecules of ammonium chloride which 

 remain undecomposed in the gaseous state are under such low 

 pressure, that their density is probably normal for the formula 

 NH 4 01 ; and in this case it is probable that the chemical 

 factor alone determines the vapour-density. But with acetic 

 acid the increase of density above 150° is evidently wholly 

 due to the physical cause ; while the abnormality is partly 

 due to a physical, partly to a chemical, cause. It is, how- 

 ever, impossible in this case to ascertain at what temperature 

 the physical cause begins to operate. It is evidently to be 

 wished that, from a study of the behaviour of stable substances, 

 some general law could be discovered which would embrace 

 all instances of physical abnormality ; and many attempts 

 have been made in this direction, but as yet with only partial 

 success. Willard Gibbs, on the other hand, has attacked the 

 problem from the chemical side ; and we have shown that his 

 formula ceases to apply when the physical change becomes 

 predominant. 



Messrs. E. and L. Natanson* have recently published a 

 research on the vapour-densities of nitric peroxide (N 2 4 or 

 N0 2 ), which, taken in conjunction with experiments of ours 

 on the vapour-pressures of that body (Phil. Trans. 1886, 

 Part L), affords a striking confirmation of the correctness of 

 our views. They give an isolated observation at — 12°*6; 

 * Wiedemann's Annalen, 1886, p. 606. 



