14 Biological Chemistry. 



of a purer product, and the process must be repeated until 

 a sample is obtained which exhibits all the properties of a 

 homogeneous substance. By a process of fractional crystal- 

 lization it is often possible to separate two substances 

 which are soluble in the same solvent. The less soluble 

 substance will separate first, whilst the more soluble will 

 remain in the mother liquors ; these on evaporation will 

 yield a further crop of crystals, which again, on re- 

 crystallization, can be either partially or wholly separated 

 into two products. By systematically carrying out a pro- 

 cess of " fractional crystallization " it is often possible to 

 separate a mixture into its various components. Such a 

 separation is, however, seldom complete and quantitative, 

 although it is often possible to obtain by this method 

 samples of the substances sufficiently pure for analysis and 

 complete identification. A more satisfactory method of 

 separation of pure substances from a mixture consists in 

 treating the latter with a solvent in which only one of the 

 constituents is soluble. Thus, for example, if a mixture of 

 common salt and malonic acid is treated with ether, only 

 the acid will dissolve. On evaporating off the ether, the 

 malonic acid will be obtained free from admixture with 

 the salt. By such means a quantitative separation can be 

 accomplished. 



The process of separating one substance from another 

 by the employment of a liquid in which only one is 

 soluble, can also be applied to a solution of a mixture. 

 Thus, if the above mixture of sodium chloride and malonic 

 acid is dissolved in water, and the aqueous solution is 

 shaken with ether, the aqueous and ethereal layers will, 

 on standing, separate into two phases, the upper one con- 

 sisting of moist ether and the lower one of water in which 

 a small amount of ether is dissolved. If, now, these layers 

 be separated in a " separating funnel " (see illustration, 



