1 8 Biological Chemistry. 



temperature throughout the whole process of distillation. 

 The temperature indicated will be the boiling point of the 

 substance at the particular atmospheric pressure under 

 which the liquid is distilled. When, however, the liquid 

 consists of a mixture of substances of different boiling 

 points, this constancy of temperature will not be main- 

 tained. If the fractions of the liquid which come over 

 within the different intervals indicated by the thermo- 

 meter are collected separately in different receiving vessels, 

 a partial separation of substances by the process of 

 " fractional distillation " can be accomplished. By a 

 systematic fractionation of these fractions, samples of con- 

 stant boiling point may be obtained. It is not, however, 

 always possible to obtain a pure product even when a 

 sample of constant boiling point has been fractionated out ; 

 and it is seldom possible to separate, even in approximately 

 quantitative manner, two volatile substances when the 

 boiling points are not separated by a relatively large 

 interval. When a separation of two such substances has 

 to be effected, some other process, such as the conversion 

 into easily separable derivatives, has to be discovered. 



The process of purification by fractional distillation has 

 proved, however, to be of great value in a large number of 

 cases, especially when a given substance has only small 

 amounts of admixed impurities, such as the brown colour- 

 ing matters which are so often formed during chemical 

 operations. 



It is, of course, essential, when the process of frac- 

 tional distillation is employed for purification of a sub- 

 stance, that the latter should distil without decomposition. 

 Many substances, however, which decompose on distillation 

 under ordinary atmospheric pressure, can be distilled at 

 lower temperatures and without change under diminished 

 pressure. The process of distillation in vacuo is often 



