SPECIAL REACTIONS OF THE ALBUMINS. 35 



of these may be used in moderately concentrated solution, and are 

 added directly to the albuminous fluid. 



Especially important are the salts of iron, copper, and lead. If 

 ferric chloride is added to an albuminous solution containing an 

 excess of sodium acetate, until a distinct red color results, the albu- 

 mins are completely precipitated on boiling. An excess of the iron 

 must be avoided, as the precipitated albumins will otherwise dis- 

 solve. 



Acetate of copper precipitates all true albumins and serves to 

 separate the primary from the secondary albumoses ; an excess 

 should be avoided. 



On boiling albuminous solutions with hydroxide of lead in the 

 presence of acetate of lead complete precipitation occurs. 



Bichloride of mercury precipitates not only the true albumins 

 and albumoses, but also the peptones. 



The Alkaloidal Reagents. The most important of these are phos- 

 photungstic and phosphomolybdenic acid, mercuropotassic iodide, 

 bismuthopotassic iodide, and cadmium-potassic iodide, all of which 

 precipitate albumins in the presence of a mineral acid. Further, 

 tannic acid, picric acid, and potassium ferrocyanide as well as ferri- 

 cyanide, in the presence of acetic acid, trichloracetic acid, etc. 

 These various reagents are used in from 5 to 10 per cent, solutions, 

 after acidifying the albuminous solution with a mineral acid, or 

 acetic acid of moderate strength as indicated. 



The histons and protamins, which are more markedly basic than 

 the other classes of albumins, are precipitated even with a neutral 

 or slightly alkaline reaction. But all precipitates dissolve when 

 the reaction is markedly alkaline. In an excess of the alkaloidal 

 reagent only the peptones and some of the albumoses dissolve. 



The precipitation by the alkaloidal reagents is attributed to the 

 diamino-complexes of the albuminous molecule, viz., the more 

 markedly basic radicles. 



Color Reactions. The color reactions to be described are indi- 

 vidually not peculiar of the albumins, but merely indicate the 

 presence of certain atomic complexes which in themselves are 

 capable of producing the reactions. Collectively they are charac- 

 teristic, however, and are of special interest as they permit a partial 

 insight into the structure of the albuminous molecule. 



1. The Biuret Reaction. The test is conducted as follows : A 

 few c.c. of the solution to be examined are treated with an excess 

 of a concentrated solution of sodium or potassium hydrate, and then 

 drop by drop with a 2 per cent, solution of copper sulphate. In the 

 presence of native albumins a bluish or reddish-violet color results, 

 while with albumoses, peptones, histons, and certain vitellins the 

 color is a pure red. With larger amounts of albumin the reaction 

 is obtained without difficulty ; if traces only are present, great care 

 must be had not to add too much of the copper solution as other- 

 wise the blue color of the reagent may obscure the reaction. In 



