224 Biological Chemistry. 



converting them into their ethyl esters, and submitting 

 the latter to the process of fractional distillation under a 

 very low pressure produced by a mechanically driven 

 Fleuss pump (the apparatus for which is figured on p. 19), 

 or by means of charcoal soaked in liquid air. A series of 

 fractions is thereby obtained, the lower of which contain 

 the esters of the simpler products, such as glycine and 

 alanine, whereas the intermediate ones contain chiefly the 

 valine and leucine esters, and the highest ones the esters of 

 glutamic and aspartic acids, phenylalanine and serine. 

 The processes for the separation of the mixtures in the 

 various fractions are somewhat complex ; the esters are 

 first hydrolyzed, and in some cases the individual acids are 

 separated by fractional crystallization, in others by taking 

 advantage of the differences in the properties of the copper 

 and lead salts ; in other cases, again, a partial separation 

 of the esters themselves can be brought about by making 

 use of the differences of their solubilities in organic 

 solvents. For the complete methods and the various 

 modifications proposed of Fischer's original method, refer- 

 ence must be made to more complete text-books. The 

 esters themselves are prepared directly from the crude 

 hydrolysis products. When hydrochloric acid has been 

 employed the hydrolysis mixture is evaporated to a small 

 bulk in vacuo, the syrupy mixture thus obtained is dis- 

 solved in alcohol, and the solution thereby produced is satu- 

 rated with dry hydrochloric acid gas. The alcohol is then 

 evaporated off in vacuo, the residue is redissolved in 

 alcohol, and again saturated with hydrochloric acid. This 

 process is generally repeated twice more. When sulphuric 

 acid has been employed for hydrolysis, the same method of 

 esterification is used after the separation of the sulphuric 

 acid by baryta. By this means the ammo-acids are con- 

 verted into the hydrochlorides of the esters. Glycine 



