34 THE ALBUM I Mi. 



Color-reactions. The color-reactions to be described arc not 

 exclusively characteristic of the albumins, and in examinations in 

 this direction it is always necessary to employ a number of these 

 tests before drawing conclusions as to the presence or absence of 

 an albuminous substance. 



1. The Xanthoproteic Reaction. This reaction depends upon the 

 formation of certain nitro-derivatives when albumins are treated 

 with concentrated nitric acid. 'Hie test is conducted as follows: A 

 lew c.c. of the solution in question are treated with a few drops of 

 concentrated nitric acid, when in the presence of certain albumins a 

 white flaky precipitate develops, which turns yellow on boiling. 

 With other forms the solution remains clear, but also turns yellow 

 on boiling. If in either case ammonia is then added in excess, a 

 deep-orange color results which is very characteristic. 



The reaction is supposedly dependent upon the existence in the 

 albuminous molecule of a certain aromatic radicle or radicles belong- 

 ing to the phenol or phenyl group. It is therefore also obtained 

 with tyrosin, phenol, cresol, phenyl-acetic and phenyl-propionic 

 acid, as also with leucin. 



2. Millon's Reaction. This is apparently referable to the pres- 

 ence of the oxyben/oic acid radicle in the albuminous molecule, 

 and is accordingly obtained with all those albumins which on tryplic 

 digestion yield tyrosin. 



The reagent is prepared as follows : A few grammes of mercuric 

 nitrate are treated with an amount of water that is just sufficient for 

 their solution. Any basic salt that may have been formed is dis- 

 solved with fuming nitric acid, when a solution of sodium acetate 

 is added, drop by drop, until the mixture reacts with a dilute 

 solution of phenol, as described below. 



The test is conducted by adding a few drops of the solution to be 

 examined to a few c.c. of the reagent, when in the presence of albu- 

 mins a white precipitate is formed, which turns a brick red on the 

 application of heat. If nndissolved albumins are examined in this 

 manner, they are transformed into brownish-red flakes. 



3. The Reaction of Adamkiewicz. This reaction is now thought 

 to be referable to the simultaneous presence in the albuminous mole- 

 cule of a carbohydrate radicle, together with the aromatic groups 

 which give Millon's reaction. 



The test is conducted as follows : A particle of the dry albumi- 

 nous substance is dissolved in a small amount of glacial acetic acid 

 bv the aid of heat, and then treated with one-half its volume of 

 concentrated sulphuric acid. Immediately, or on boiling, a violet 

 color develops, and the fluid at the same time becomes slightly fluo- 

 rescent. The test, however, is not altogether reliable, and with 

 albumoses and peptones irives a positive reaction only when these 

 are present in concentrated form. 



4. The Biuret Reaction. It is thought that this reaction is depen- 

 dent upon the presence of a area-forming radicle in the albuminous 



