DIGESTION. 209 



the presence of free mineral acids and by the addition of too much 

 ferric chloride. 



HOFMANN'S Test. If some water is poured on a small quantity 

 of tyrosin in a test-tube and a few drops of MILLON'S reagent 

 added and then the mixture boiled for a certain time, the liquid 

 becomes a beautiful red and then yields a red precipitate. Or 

 mercuric nitrate may first be added, then, after this has boiled, 

 nitric acid which contains some nitrous acid. 



SCHERER'S Test. If tyrosin is carefully evaporated to dryness 

 with nitric acid on platinum-foil, a beautiful yellow residue (nitro- 

 tyrosin nitrate) is obtained, which gives a deep reddish-yellow color 

 with caustic soda. This test is not characteristic, as other bodies 

 give a similar reaction. 



Leucin and tyrosin may be prepared in large quantities by 

 boiling albuminous bodies or albuminoids with dilute mineral 

 acids. Ordinarily we boil hoof-shavings (2 parts) with dilute sul- 

 phuric acid (5 parts concentrated acid and 13 parts water) for 24 

 hours. After boiling the solution it is diluted with water and neu- 

 tralized while still warm with milk of lime and then filtered. The 

 calcium sulphate is repeatedly boiled with water, and the several 

 filtrates are united and concentrated. The lime is precipitated 

 from the concentrated liquid by oxalic acid and the precipitate 

 filtered off, repeatedly boiled with water, all filtrates united and 

 evaporated to crystallization. What first crystallizes consists 

 chiefly of tyrosin with only a little leucin. By concentration a new 

 crystallization may be produced in the mother-liquor, which consists 

 of leucin with some tyrosin. To separate leucin and tyrosin from 

 each other their different solubilities in water may be taken advan- 

 tage of in preparing them on a large scale, but surer and better 

 results are obtained by the following method of HLASIWETZ and 

 HABERMANIST. The crystalline mass is boiled with a large quantity 

 of water and enough ammonia to dissolve it. To this boiling-hot 

 solution enough basic lead acetate is added until the precipitate 

 formed is nearly white; now filter, heat the light yellow filtrate to 

 boiling, neutralize with sulphuric acid, and filter while boiling hot. 

 After cooling, nearly all the tyrosin is precipitated, while the leuciu 

 remains in the solution. The tyrosin may be purified by recrys- 

 tallizing from boiling water or from ammoniacal water. The 

 above-mentioned mother-liquor rich in leucin is treated with HgS, 

 the filtrate concentrated and boiled with an excess of freshly-pre- 

 cipitated copper oxyhydrate. A part of the leucin is precipitated, 

 and the residue remains in the solution and partly crystallizes as a 

 cuprous compound on cooling. The copper is removed from the 



