the Nature of Liquids. 135 



to this limit. Although the curves representing the density 

 of the saturated vapour in figs. 1 and 2 apparently point to a 

 vapour-density greater than 60, yet a trend in the curve is 

 not impossible ; and it is conceivable that at lower tempera- 

 tures than those represented, the density might remain normal 

 forC 4 H 8 4 . 



If there is a definite limit to the vapour-density of acetic 

 acid, then the following considerations will hold. It has been 

 pointed out in our paper on acetic acid, that condensation 

 took place before pressure ceased to rise ; and the same phe- 

 nomenon was observed with chloral ethyl-alcoholate, where 

 dissociation is known to occur. Now with alcohol and with 

 ether absolutely no sign of this behaviour was observable ; 

 condensation occurred the moment the vapour-pressure was 

 reached, but not till then. This behaviour corresponds to that 

 of a mixture. If an indifferent gas, to take an extreme 

 instance, is compressed along with the vapour of a conden- 

 sable liquid, pressure continues to rise after condensation has 

 commenced, until the gas, if possible, has been dissolved, or 

 has itself condensed. On the other hand, if a small quantity 

 of liquid of high boiling-point be present along with a large 

 quantity of liquid of low boiling-point, the liquid of higher 

 boiling-point separates out first, on reduction of volume, 

 while pressure continues to rise. This was indeed noticed 

 with an impure sample of ether; and the absence of this 

 behaviour affords proof of the homogeneity of a liquid. 



Supposing the vapour of acetic acid to consist of molecules 

 of two different degrees of complexity, it is probable that the 

 more complex would be first condensed, and that pressure 

 would rise until the less complex molecules had also con- 

 densed. This was in fact observed. But below a certain 

 temperature the substance would consist almost wholly of 

 more complex molecules, and the phenomenon would then be 

 less visible. This is indeed the case with the isothermals at 

 50° and at 78°*4. At higher temperatures the phenomenon 

 becomes evident. That this behaviour is not the effect of im- 

 purity has been proved by the fact that the vapour-pressures 

 at low temperatures, measured by the statical and by the 

 dynamical methods, were identical. 



Formulas representing the dependence of dissociation on 

 pressure and temperature have been proposed from thermo- 

 dynamical considerations by Prof. Willard Gibbs*. The 

 formula is for acetic acid 



, 2-073(D-2-073) 3520 • , --„.<> 



lQ g (4-146-D )^ = r+273 +lQ ^~ 11349 - 



* American Journal of Science and Arts, 1879, p. 277. 



