56 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



nitrogen present as ammonia, by distilling portions of 100 c.c. with 

 magnesium oxide. 



The ammonia is removed from the remainder by evaporating with 

 magnesia, or better barium carbonate, on the water-bath. 



The two portions, freed from ammonia, are combined, and made 

 alkaline with baryta, or barium carbonate. 



The separate solutions are now combined, the precipitate of barium 

 carbonate and barium sulphate is filtered off and washed by boiling 

 with water three times ; the excess of barium is removed from the 

 filtrate by dilute sulphuric acid and the precipitate again filtered off 

 and washed out. Filtrate and washings are combined together, 

 evaporated down and made up to I litre and a Kjeldahl nitrogen de- 

 termination again made. Allowing for the nitrogen given off as 

 ammonia, the difference between this and the previous estimation 

 gives the humin nitrogen II. contained in the alkaline barium magnesia 

 precipitate. 



III. Precipitation of Arginine and Histidine. 



The solution, which now contains a small quantity of sulphuric 

 acid, is placed in a 5 litre flask, and treated with a hot saturated solu- 

 tion of silver sulphate, 1 which is slowly added, until the solution con- 

 tains sufficient to give a yellow-brown, not a white or pale yellow 

 precipitate, on removing a drop and testing it with baryta water in a 

 watch-glass. If, during the process, there be any undissolved silver 

 sulphate at the bottom of the flask, it is dissolved by adding more 

 water before a fresh quantity is added, in order that a yellow-brown 

 precipitate be given in the test drop with baryta. As soon as sufficient 

 silver is present to combine with all the arginine and histidine, the 

 solution is allowed to cool to 40 and is saturated with finely 

 powdered baryta, i.e., until some remains undissolved after repeated 

 shaking. The precipitate, which is thus formed 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 



1 Osborne prefers to use silver nitrate. 



