THE NITROGENOUS SUBSTANCES 



69 



FIG. 38. Crystals of serum-albumin, 

 after Wichmann. 



Proteids which are dissolved by means of the above-named indifferent 

 solvents can be isolated from the tissues and fluids of the body probably 

 unchanged, and are therefore designated as native proteids. 



The solubility of these substances is intimately related to their acidic or 

 basic character. Proteids react with both acids and bases forming salts, and 

 themselves therefore partake of the nature of both bases and acids. The acidic 

 and basic characters, however, are not equally developed in all proteids. Those 

 in which the two are approximately equal have a. neutral reaction and are 

 soluble in water and in solutions of neutral salts. Others react as weak acids, 

 are insoluble in neutral fluids, but are solu- 

 ble in weak alkaline solutions, and are pre- 

 cipitated from the latter by weak acids. 

 Others again react as weak bases, are solu- 

 ble in weak acids and are precipitated by 

 weak bases. 



The proteids are precipitated from their 

 solutions by various reagents, the reactions 

 being for the most part traceable to their 

 double character as acids and bases. As 

 acids they form with the salts of the heavy 

 metals precipitates of insoluble proteid- 

 metal salts. As bases they form insoluble salts with numerous weak acids, 

 such as tannic acid, phospho-tungstic acid, hydroferrocyanic acid (the so- 

 called alkaloid reagents). Proteids cannot be recovered unchanged from these 

 precipitates ; they have been modified by the reactions. 



Proteids are precipitated and at the same time modified by strong mineral 

 acids (e. g., Heller's test with HN0 3 ) and by alcohol. They are modified 

 also by heating their solutions. If a proteid in solution is treated with a 

 concentrated solution of certain neutral salts of the alkalies or metallic earths 

 particularly ammonium, magnesium or sodium sulphate or with these salts 

 in substance, it separates out unmodified i. e., is " salted out." The con- 

 centration of the neutral salt necessary for salting out varies greatly for 

 different proteids, and we have in this circumstance a method of separating 

 different proteids in the same solution from one another. 



The chemical elements characteristic of simple proteids are C, N, S, H 

 and 0. A compound in which no S is found ought not to be described as a 

 true proteid. 



The percentage composition of the proteids, which consist of these five 

 elements only, varies within rather narrow limits: 



C 50.6-55.0 per cent average, 52 per cent. 



H 6.5- 7.7 " " 7 " 



N 15.0-18.5 " " 16 



S 0.3- 2.2 " ........ " 2 



20.5-23.5 " " 23 



On burning proteid, various mineral constituents remain as the ash. These 

 it appears cannot be completely removed from proteid without changing its 



