CASEINE. 97 



II. No pink colour is developed. 



a. But the nitric acid solution turns yellow as it 

 is evaporated, and leaves a residue insoluble in 

 potassium carbonate ; Xanthine. 



b. The nitric acid solution turns dark brown 

 and leaves a residue soluble in ammonia; 

 Cystine. 



Caseine. 1. Add to skimmed milk a little hydro- 

 chloric acid, wash the curd with water, then with water 

 acidified with hydrochloric acid, and finally with plain 

 water. Dissolve the jelly (caseine hydrochlorate) in a 

 large quantity of water, filter, add ammonium car- 

 bonate cautiously, wash the precipitate (free caseine), 

 exhaust with alcohol and ether, and wash again with 

 water. 



2. Dissolve the moist caseine thus obtained in dilute 

 hydrochloric acid, filter. The liquid will rotate 

 polarised light to the left, and will possess the pro- 

 perties of a solution of albumen. 



3. To a weak solution of caustic potash add moist 

 jaseine as long as dissolved. The alkali will be neu- 

 tralized. Filter and use for the following ex- 

 periments : 



4. Add an acid the caseine will be precipitated, but 

 will redis solve in excess of the acid. 



5. Boil the solution. It will not coagulate, but an 

 insoluble pellicle will form on the surface. 



6. Add solution of a calcium salt, or of magnesium 

 sulphate. A precipitate will appear on boiling. 



7. Add solution of lead acetate, cupric sulphate, 



7 



