Distillation of Binary Mixtures. 535 



measuring 250 c. c. was of density 1*016 and of strength 

 •1100. From these data we find 



log ff/6,)=-1634, log H/«0=-5995, 



whence 



l-* = -27, a: = '73. 



The number denoted by k represents the ratio of strengths of 

 vapour and liquid when weak mixtures are distilled. 



A new Apparatus with uniform Regime. 



In the theory and experiments so far considered the distil- 

 lation has always been supposed to be simple, that is, the 

 vapour rising from the boiling liquid is supposed to be 

 removed and to be condensed as a whole, so that the distillate 

 has the same composition as the vapour leaving the boiling 

 liquid. In practice, as is well known, this condition is often 

 and advantageously violated. A preliminary partial con- 

 densation of the vapour in the still-head frees it from some 

 of the less volatile ingredient ; and, when the residue is con- 

 densed and collected, the more volatile ingredient is obtained 

 in a nearer approach to purity. Prof. S. Young has shown 

 that the principle is more effectively carried out if the still- 

 head be maintained at a suitable temperature. 



Even with a preliminary partial condensation in the still- 

 head, the " fractionation " of a mixture is usually regarded as 

 a very tedious operation. The stock of mixture in the retort 

 is constantly changing its composition as the distillation and 

 partial condensation proceed, and no uniform regime can be 

 established. Although theoretical simplicity and practical 

 convenience are not always conjoined, a uniform regime seems 

 very desirable, and it excludes the usual arrangement in 

 which the whole supply of mixture is charged into the 

 retort. The return into the retort of the liquid first condensed 

 from the original vapour is also objectionable. 



The problem of distillation may be stated to be the separa- 

 tion from a binary mixture of the whole of the two components 

 in, as nearly as may be, a state of purity. There is no 

 theoretical reason why this should not be effected at one 

 operation ; but for this purpose the mixture must be fed in 

 continuously and not at the. place of highest or lowest tem- 

 perature. A description of the procedure followed in some 

 illustrative experiments will make the nature of the process 

 plain. 



The mixtures actually employed were of water and common 

 alcohol. The choice was perhaps not a happy one, as in 

 consequence of the peculiar properties of strong alcohol it 



