522 Lord Kayleigh or the 



as the number of ingredients remains unchanged, the com- 

 position of the vapour rising from the boiling mixture is a 

 function of the temperature (or total pressure) only. Hence 

 in simple distillation the composition of the distillate 

 remains constant until perhaps one constituent of the liquid 

 (not necessarily the most volatile) is exhausted. At this 

 point the distillate, as well as the boiling-temperature, changes 

 dis continuously and the altered values are preserved until a 

 second constituent is exhausted, and so on. None of the 

 separate distillates thus obtained would be altered by repetition 

 of the process at the same pressure. 



Liquids which form true Mixtures. 

 The above is as far as possible from what happens in the 

 case of miscible liquids, e. g., water and common alcohol. 

 Here the composition of the vapour, as well as the boiling- 

 point under given pressure, depends upon the composition of 

 the liquid, and all three will in general change continuously 

 as the distillation proceeds. But, so long as the total pressure 

 is fixed, to a given composition of the liquid corresponds a 

 definite composition of the vapour ; and it is the function of 

 experiment to determine the relation between the two. The 

 results of such experiments may be exhibited graphically 

 upon a square diagram (e. g. figs. 3 and 4, pp. 530 and 533) 

 in the form of a curve stretching between opposite corners 

 of the square, the abscissa of any point upon the curve re- 

 presenting the composition of the. liquid and the ordinate 

 representing the composition of the vapour in equilibrium 

 with it. For the pure substances at the ends of the scale, 

 represented by opposite corners of the square, the compositions 

 of liquid and vapour are necessarily the same. 



The character of the separation capable of being effected by 

 distillation depends in great measure upon whether or not 

 the curve meets the diagonal at any intermediate point, as 

 well as at the extremities. If there be no such intersection, 

 the curve lies entirely in the upper (or in the lower) triangular 

 half of the square, so that for all mixtures the distillate is 

 richer (or poorer) than the liquid. As the distillation of a 

 limited quantity of mixed liquid proceeds, the composition of 

 the residue moves always in one direction and must finally 

 approach one or other condition of purity. 



If on the other hand the curve crosses the diagonal, the 

 point of intersection represents a state of things in which the 

 liquid and vapour have the same composition, so that distilla- 

 tion ceases to produce any effect. This happens for example 

 with a solution of hydrochloric acid at a strength of 20 per 

 cent. (fig. 3) and with aqueous alcohol at a strength of 96 per 

 cent. By no process of distillation can originally weak 



