CHEMICAL CONSTITUENTS OF ABNORMAL SECRETIONS. 79 



acid becomes red. If the mixture be again evaporated 

 to dryness, it yields a blackish residue ; reactions which 

 are thought to be quite characteristic of Tyrosine. 



Tyrosine, however, as met with in urine, is generally 

 so mixed up with Leucine and other abnormal matters, 

 that, before applying the test, it is necessary to purify it. 

 This may be done by precipitating all the colouring 

 matter by means of a solution of basic acetate of lead ; 

 filter, and then free the liquid from the excess of lead 

 by means of a current of sulphuretted hydrogen ; again 

 filter, and evaporate the clear liquid to nearly dryness, 

 when the Tyrosine will crystallize in the white stellate 

 groups of small prisms, or thickly-set groups of needles 

 or spiculated balls already described. 



Another mode of testing for impure tyrosine is that 

 recommended by Hoffman namely, add to the sus- 

 pected solution a little nitrate of the protoxide of 

 mercury, which will throw down a red precipitate, and 

 turn the supernatant liquid rose-coloured if Tyrosine be 

 present. 



Frerichs gives the following : The suspected substance 

 is put into a watch-glass along with sulphuric acid, 

 and after standing halt' an hour the mixture is diluted 

 with distilled water. It is next neutralized with carbo- 

 nate of soda, filtered, and to the clear filtrate a few drops 

 rrlilnride of iron, devoid of acid, is added, when this 

 nee of Tyrosine is recognzied by the formation of a 

 dark purple colour. 



Should these tests, however, fail in yielding sufficient 

 evidence of the presence of either Leucine or Tyrosine, 

 we must then resort to the use of the microscope, which 

 fortunately is amply sufficient for all practical purposes. 



