36 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



and which consists of the silver salts of arginine and histidine, is 

 filtered off and stirred up together with the filter paper in a mortar 

 with baryta, when it is again filtered off and washed with baryta 

 water. The lysine in the filtrate is separated according to VI. 



The precipitate of the silver salts of arginine and histidine is 

 suspended in water containing sulphuric acid, and decomposed with 

 hydrogen sulphide. The filtrate from the silver sulphide and barium 

 sulphate, which is thoroughly extracted in the usual manner with 

 boiling water, is evaporated down to remove the hydrogen sulphide 

 and again made up to I litre ; a Kjeldahl nitrogen determination in 

 20 c.c. now gives the amount of nitrogen in the substances precipi- 

 tated by silver and baryta. 



IV. Estimation and Isolation of Histidine. 



(a) The greater portion of the histidine is removed by precipita- 

 tion with mercuric sulphate. The solution is concentrated to about 

 250 c.c., sulphuric acid is added till the solution contains 5 per cent, 

 of this acid, and it is treated with a slight excess of mercuric sulphate 

 solution. The precipitate of histidine mercury sulphate is allowed to 

 stand for twelve to twenty-four hours when it is filtered off, washed 

 with 5 per cent, sulphuric acid, suspended in water and decomposed 

 with hydrogen sulphide. The filtrate and washings from the mercuric 

 sulphide which contain the histidine are neutralised with baryta and 

 barium nitrate added until barium sulphate is no longer precipitated. 

 The barium sulphate is filtered off and thoroughly washed. The 

 histidine is then thrown down as silver compound and estimated as 

 under (). 



(b) The filtrate from the histidine mercuric sulphate is freed from 

 mercury by hydrogen sulphide and from sulphuric acid by neutralising 

 to litmus with baryta and adding barium nitrate as long as a precipi- 

 tate is formed. Both precipitates are filtered off and washed. 



The solution is then concentrated to 300 c.c., acidified with nitric 

 acid, if necessary, and treated with silver nitrate, as before, till a test 

 drop gives a yellow colour with baryta ; when this occurs it is exactly 

 neutralised to litmus with baryta and 5 c.c. of a cold saturated solu- 

 tion of baryta are added. If 10 c.c. of the filtered solution when tested 

 with a drop of baryta give a precipitate which indicates that the silver 

 salt of histidine is not completely thrown down, 2 c.c. of saturated 

 baryta solution are added to the main bulk, and this test is repeated 



