CH. IV.] 



AMINO-ACIDS. 



71 



II. Fractional precipitation, i.e. by adding a reagent 

 to the mixture which forms an insoluble com- 

 pound with only one or a few of the substances 

 present. In this way, by the use of mercuric 

 sulphate, tryptophane was first isolated, and 

 cystine and tyrosine can also be obtained ; by 

 the use of mercuric chloride histidine is separ- 

 ated. 



III. Fractional distillation of the esters. This method, 

 introduced by Emil Fischer, led to the discovery 

 of several of the amino-acids, and serves for the 

 quantitative estimation of some of them. 



Percentage amounts of some amino-acids in certain 

 proteins : 



The figures in heavy type draw attention to the reason why 

 certain proteins are used for tl e preparation of particular amino-acids. 



