36 THE PROTEINS. 



acid. The concentration of the solution also may make 

 it vary. Through coagulation the nature of albuminous 

 substances is altered and they acquire other properties. By 

 the action of alcohol albuminous compounds are precipi- 

 tated, at first in an unaltered form; but if the alcohol is 

 strong and acts for some time they are coagulated, and are 

 then insoluble in water. 



Coagulation, when spoken of with respect to the 

 protein compounds, must be distinguished from precipita- 

 tion, which it resembles. When albumin is coagulated 

 e.g., by boiling, by mineral acids, or by the continued 

 action of strong alcohol it becomes insoluble in water. 

 It may be precipitated by ammonium sulphate without be- 

 ing coagulated or by not too large an amount of alcohol 

 and still retain its original properties, being soluble again 

 upon the addition of water. 



Some of the albuminous compounds are coagulated by 

 the action of ferments; for example, the fibrin, which is 

 so formed from the blood or lymph. 



Albuminous substances are easily decomposed by the 

 action of the putrefactive bacteria, the nitrogen and sul- 

 phur uniting with hydrogen to form hydrogen sulphid, 

 and ammonia, or, these two together, ammonium sulphid. 

 Other nitrogen compounds are also formed, like the amido 

 acids which contain the amido group, KH 2 , such as leucin 

 and tyrosin. Indol is also one of the nitrogenous putre- 

 factive products. 



Many of the albuminous substances are precipitated 

 by the mineral acids, but upon standing with an excess of 

 the acid, or more quickly by heating, they are dissolved, 

 going into acid albumins. Many will also form insoluble 

 compounds with salts of the heavy metals, such as mercury, 

 copper, and lead. With copper in an alkaline solution they 



