* NITROGENOUS ORGANIC CELL-CONSTITUENTS. 91 



The first and third of these tests may be used for separating albu- 

 minous bodies from other substances in solution, a process which is 

 often necessary in the examination of organic fluids. So also if the fluid 

 is acidulated with acetic acid, and then added to a large bulk of strong 

 alcohol, albuminous bodies may thus be coagulated, and their separation 

 from other ingredients of the solution rendered possible. 



A precipitate produced by boiling alone is not a sufficient proof 

 of the presence of albumen, since certain substances, such as calcium 

 phosphate in human urine and calcium carbonate in the urine of her- 

 bivora, will be thrown down by boiling. If the precipitate is permanent 

 on the addition of nitric acid after boiling, albumen is present, since the 

 salts above mentioned will be redissolved by the acid. Alkali may also 

 hinder the coagulation of albumen by heat, and an acid reaction is there- 

 fore essential for the emplo3^ment of this test. 



Occasionally albumen is present in solution in amount too small to 

 be detected by any of the preceding tests. The detection of traces of 

 albumen is then rendered possible by various color reactions. 



The Biuret Reaction. When a small amount of caustic potash 

 solution is added to a dilute solution of cupric sulphate a precipitate of 

 cupric hydrate will be thrown down. If an excess of potash is now 

 added, the precipitate will be redissolved and the fluid take on a light- 

 blue color. If, however, albuminous bodies be present in solution, and 

 this procedure be carried out, on solution of the precipitate of cupric 

 hydrate the fluid will take on a violet color instead of u blue. This test 

 may be used to detect the presence of albuminous bodies in extremely 

 small amount in solution. It may be also used for the recognition of 

 the albuminous nature of solids. If a solid body which is supposed to 

 contain albuminous bodies be touched first with a drop of cupric sul- 

 phate solution, then with a drop of potash solution, and then washed 

 with water, the spot so treated will be found to have a violet color. 

 This test is also used for the recognition of peptone. A solution of 

 peptone so treated will become red instead of violet. 



Xantho-proteic Reaction. When albuminous bodies in solution are 

 boiled with nitric acid, the solution and coagulum, if one be present, 

 take on a yellow color. If the solution be then allowed to cool and 

 strong ammonia added, the upper layers of the solution, or the coagu- 

 lum, if any be present, will become orange colored. 



Millon's Reaction. If a little Millon's reagent* be added to a solu- 

 tion which contains albumen, if the albumen be present in considerable 



* Millon's reagent is prepared by dissolving mercury in its own weight of nitric 

 acid by the aid of gentle heat. The solution is then poured into a glass vessel, and twice 

 its volume of water added ; a crystalline precipitate will separate in a few hours, and the 

 yellowish supernatant fluid, which may be readily decanted off, is Millon's reagent. 



