1863.] 535 



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 ethyl boils at 72 C 5'397 



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

 ethyl, 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 ethyl, 

 7^7 



which shows that in this case the less volatile constituent had boiled 

 the faster, the less volatile iodide of ethyl having a very much 

 higher vapour-density than methyl-alcohol. 



It will be obvious that when the vapour-densities and tensions 

 are inversely proportional, the mixture must distil over unchanged. 

 This influence of vapour-density goes a great way to explain why 

 homologous bodies are so difficult of separation by means of frac- 

 tional distillation. The more complex the formula the higher the 

 boiling-point, but also the higher the vapour-density, and therefore 

 the greater the value of the vapour. Why oils, &c. distil so readily 

 in steam is also explained ; for aqueous vapour is one of the lightest, 

 while oily vapours are generally heavy. 



2d2 



