34 THE ALBUMINS. 



heim, and others. These compounds originate only in the presence 

 of free acids as such. 



Behavior toward Alcohol. While some of the albumins 

 (albumoses) dissolve in dilute alcohol with comparative ease, strong 

 alcohol acts in much the same manner as the neutral salts. But 

 it is to be noted that after prolonged exposure, and especially in 

 the presence of salts, the albumins are coagulated, and then remain 

 refractory to all neutral solvents. 



Special Reactions of the Albumins. 



Precipitation. 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. As 

 a result of this process they apparently undergo no change in 

 structure or in their general properties, and remain soluble in the 

 usual neutral media. There is a large number of substances, how- 

 ever, which also precipitate the albumins, but which either cause 

 their coagulation or combine with them to form compounds which 

 are insoluble in water. Many of these reagents are extensively used 

 in the chemical laboratory in testing for albumins ; they furnish reac- 

 tions which individually are not absolutely characteristic, but the 

 albuminous nature of a substance can usually be regarded as estab- 

 lished when a positive result is obtained with all or at least the 

 larger number of the reagents. The reactions are common to the 

 true albumins, the proteids, and the majority of the albumoses. 



The most important reagents are the following : 



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

 metaphosphoric acid. These are employed in concentrated form. 

 The one most commonly in use is nitric acid. The test is conducted 

 by allowing a small amount of the acid to flow beneath the fluid to 

 be tested, when a white ring of coagulated albumin appears at the 

 zone of contact (Heller's test). In the case of the true albumins, 

 all of which give the reaction, the precipitate is insoluble in an 

 excess of the acid, even on heating. But with the albumoses, which 

 in part are also precipitated, the precipitate dissolves on boiling 

 and reappears on cooling. 



2. The Salts of the Heavy Metals. In combining with these the 

 albumins play the part of weak organic acids. They set free the 

 corresponding acids of the salts and combine with the metallic 

 oxides to form compounds which are insoluble in neutral, alkaline, 

 and acid solutions. With the exception of myogen, haemoglobin, 

 and certain albumoses, the resulting precipitates are as a rule insolu- 

 ble in an excess of the reagent. 



The salts which are usually employed are the sulphate and ace- 

 tate of copper, the chloride and acetate of iron, the neutral and 

 basic acetate of lead, the bichloride of mercury, nitrate of silver, 

 acetate of uranium, acetate of zinc, chloride of platinum, etc. All 



