36 THE ALBUMINS. 



Precipitation of the Albumins. It has been pointed out that 

 with the exception of the peptones practically all albumins can be 

 precipitated from their neutral or feebly acid solutions by certain 

 neutral salts. During this process they apparently undergo no 

 alteration in structure or in their properties, and remain soluble 

 in the usual neutral media. There is a large number of other sub- 

 stances, however, which while they also precipitate the albumins, 

 cither cause their coagulation or combine with them to form com- 

 ix muds which are insoluble in water. Some of these reagents are 

 extensively used in the chemical laboratory for the purpose of test- 

 ing for albumins in various solutions. The most important ones are 

 here briefly considered : 



1. The mineral acids, viz., nitric, hydrochloric, sulphuric, and 

 metaphosphoric acid. The most important of these is nitric acid, 

 for the reason that it does not redissolve the precipitated albumins 

 in the presence of neutral salts, even if an excess has been added 

 and the mixture is boiled. The test is conducted by allowing a cer- 

 tain amount of the acid to flow beneath the solution to be tested, 

 when in the presence of albumin a white ring of coagulated albumin 

 appears at the zone of contact (Heller's test). 



Orthophosphoric acid can be employed only in very concentrated 

 form. 



2. The salts of the heavy metals. The salts which are usually 

 employed are copper sulphate, ferric chloride, neutral and basic 

 acetate of lead, platinum chloride, mercuric chloride, silver nitrate, 

 uranium acetate, and others. In combining with these the albumins 

 act as weak organic acids ; they thus set free the corresponding 

 acids of the salts and combine with the metallic oxides. 



Especially important are the salts of iron and lead. If ferric 

 chloride is added to an albuminous solution containing an excess of 

 sodium acetate until a distinct red color is obtained, the albumins 

 are completely precipitated on boiling. The same result is reached 

 on boiling albuminous solutions with hydroxide of lead in the 

 presence of lead acetate. 



Other reagents which may be employed for the purpose of testing 

 for albumins are the following, but in combining with these the 

 albumins act the part of a base : 



3. Tannic acid, or picric acid after acidifying with acetic acid. 



4. Mercuropotassic iodide, bismuthopotassic iodide, phosphotung- 

 stic acid, and phosphomolybdenic acid, all cause the complete pre- 

 cipitation of albumins in the presence of a mineral acid. These 

 reagents are further utilized very extensively as precipitants of 

 organic bases in general, and notably of the vegetable and animal 

 alkaloids. 



5. Hydriodic acid in the presence of mercuropotassic iodide ; 

 the albuminous solution is previously acidified with hydrochloric 

 acid. 



6. Potassium ferrocyanide, as well as the ferricyanide, in the 



