Reilly and Hickinbottom — Method of Distillation in Steam. 133 



HausbrancP states that tlie composition of vapours (from liquids which 

 mix with water) depends, " according to certain laws, upon the composition 

 of the boiling mixture of liquids, but, unfortunately, is not accurately known 

 for most mixtures of liquids, although this property is utilized on the largest 

 scale in the industries for the distillation of such liquids." Even in the case 

 of a liquid which does not mix with water, part of the liquid may be 

 mechanically " taken away " witli the steam, and for some substances the pro- 

 cess of " blowing over " with saturated steam has wide industrial applications. 

 Hausbrand gives the following approximate data for the amount of steam 

 required to "carry over" 100 kilograms of various substances: — 



(1 kilo of steam at atmospheric pressure represents 637 calories.) 



No differentiation is made between the cases in which mixtures of constant 

 boiling-point, or of mechanical carrying over of the liquid in the current of 

 steam, occur. Many other substances are purified by distillation in steam, 

 such as various "intermediates" (e.g., aniline, orthonitrophenol, chloropicrin, 

 etc.), petroleum products, essential oils, various esters, lactic acid, etc. 



It should be noted that even among compounds closely related, the vapour 

 pressures at 100° may differ widely, and no a priori reasoning, without 

 actual experiment, can decide which compound will distil the more readily 

 in steam. The results recorded above indicate in a broad manner the effect 

 on the distillation constant of introducing various groups into the molecule. 

 A phenyl group reduces tlie distillation constant, while an hydroxyl group 

 replacing a carboxyl group causes an increase. When, however, carboxyl 

 and hydroxyl groups are present in the molecule, the distillation constant is 

 generally less than that of a compound containing only one of these radicles. 

 The distillation constant is affected in a striking manner by constitutive 

 differences in isomeric substances, and, as far as can be judged from the limited 

 number of observations available, the influence of constitution on the 

 distillation constant appears to be general. 



The constants of acids and alcohols containing a branched chain are higher 



' Evaporating, Condensing, and Cooling Apparatus, 2nd ed., pp. 19-20. 



S2 



