Distillation of Binary Mixtures. 523 



alcohol be strengthened beyond the point named, and if (Le 

 Bel) we start with still stronger alcohol (prepared by chemical 

 desiccation) the effect of distillation is reversed. The vapour 

 being now weaker (in alcohol) than the liquid, the residue in 

 the retort strengthens until it reaches purity. 



In the case of substances which have no tendency to mix, 

 e. g., water and bisulphide of carbon, the composition of the 

 vapour is, as we have seen, always the same. The repre- 

 sentative curve, reducing to a straight line parallel to the axis 

 of abscissae, or rather to the broken line AEFD (fig. 1), 



Fig. 1. 



H 2 50 '00 



necessarily crosses the diagonal. The point of intersection 

 (H) represents a condition of things in which the compositions 

 of the liquid and vapour are the same. As distillation pro- 

 ceeds, the residue retains its composition, and both ingredients 

 are exhausted together. 



If we commence with a liquid containing CS 2 in excess of 

 the above proportion, the excess gradually increases until 

 nothing but CS 2 remains behind. In the same way, if the 

 water be originally in excess, the excess accentuates itself 

 until the (finite) residue is pure water. The critical condition 

 is thus in a sense unstable, and can only be realized by 

 adjustment beforehand. 



The conclusions drawn above may be generalized. What- 

 ever may be the ingredients of a binary mixture, in the 

 upper triangular half of the square the vapour is stronger (we 

 will say) than the liquid, in the lower half weaker. Hence, 

 as the liquid distills away, progress from a point in the upper 

 half is towards diminishing abscissae, and in the lower half 

 towards increasing abscissae. When, as in fig. 1, the curve 

 in its course from A to D crosses AD from left to right, the 

 condition represented by the point of intersection H is 

 unstable. When, as in the case of hydrochloric acid (fig. 3), 

 the crossing takes place from right to left, i. e. from the lower 



2M2 



