382 Royal Society : — 



vious to the application of tension, the heat existing in the mo- 

 lecules is unable to produce any expansion against the force of 

 cohesion. But when the influence of cohesion is partly counter- 

 acted by the tension applied, the heat then becomes enabled to 

 perform work of expansion, and a cooling effect is the result. 



LXII. Proceedings of Learned Societies. 



ROYAL SOCIETY. 

 [Continued from p. 236.] 

 April 23, 1863. — Major-General Sabine, President, in the Chair. 

 nPHE following communication was read : — 



-*- " On the Distillation of Mixtures : a Contribution to the Theory 

 of Fractional Distillation." By J. A. Wanklyn, Esq. 



There are many points in the boiling of mixtures which are ob- 

 scure. The tension of the vapours at the temperature whereat the 

 mixture boils, and the proportions in which the constituents of the 

 mixture are present, are not the only factors which determine the 

 relative rates at which the constituents distil. There have, for in- 

 stance, to be taken into account the adhesion of the liquids to one 

 another, and the vapour-densities of these liquids. On the present 

 occasion I have to call attention to the influence of this latter ele- 

 ment, which influence seems to have been lost sight of by most of 

 those who have applied themselves to this subject. 



Leaving out of account for a moment the influence of adhesion, 

 and simplifying the influence of the proportion in which the ingre- 

 dients are present by taking equal weights of two liquids of different 

 boiling-points, we may set down the rates at which these ingredients 

 will distil as determined by the tensions of the liquids and the den- 

 sities of the vapours. In the first instant of time the quantity of 

 each ingredient which distils will be found by multiplying its ten- 

 sion at the boiling-point of the mixture by its vapour-density. It 

 thus appears that the liquid with the highest tension will not of 

 necessity distil the quickest, for what the other liquids want in ten- 

 sion they may make up by the greater density of the vapours which 

 they give off. And so when we mix a more volatile with a less 

 volatile liquid and proceed to distil the mixture, we shall now and 

 then find that the less volatile liquid distils faster than the more 

 volatile one. I will here bring forward an experiment to illustrate 

 this point. 



Vapour-density. 



Methyl-alcohol boils at 66° C 1-107 



Iodide of ethyle boils at 72° C 5*397 



I took 18 grammes of methyl-alcohol and 17 grammes of iodide of 

 ethyle, mixed them, and distilled off rather more than one-third of 

 the mixture. The distillate consisted of 



6*0 grammes methyl-alcohol, 

 8-7 grammes iodide of ethyle, 

 1^7 



