PROTEINS 87 



but the emulsoid colloids are precipitated much less readily, 

 that is, only by adding much more of the electrolyte. It has 

 been observed that the effective part of the precipitating salt or 

 substance is the ion bearing the opposite charge to that on the 

 colloid. If the precipitating part of the salt is the metal, then 

 in general colloids bearing negative charges will be precipitated. 

 In this connection it has been observed that trivalent metals are 

 better preciptation reagents than divalent metals, and divalent 

 metals in general are better precipitation reagents than mono- 

 valent metals. Thus to precipitate a given colloid from solu- 

 tion a ferric salt is better than a mercuric salt, and a mercuric 

 salt better than a sodium salt. That is, a smaller concentration 

 of ferric chloride than of mercuric chloride is required, etc. But 

 all ions of the same valency do not have equal precipitation 

 powers. They vary according to their solution tension. 



When a colloid is precipitated by an electrolyte the precipi- 

 tate contains some of the precipitating ion, so the precipitate is 

 believed to be a compound of the colloid and the precipitating 

 ion. The precipitation of colloids, however, is undoubtedly de- 

 pendent on other and more complicated factors than the mere 

 formation of salts or similar compounds of the colloids. For 

 further discussion of this subject the student is referred to 

 larger or more specialiezd works. 



A suspensoid colloid, as has been stated, is easily precipitated 

 by the addition of an electrolyte. If a small amount of an emul- 

 soid colloid is added to a suspensoid colloid solution, the latter is 

 much less easily precipitated. The suspensoid colloid is "pro- 

 tected" by the emulsoid colloid (albumin, for example). This 

 phenomenon is called the protective action of colloids. 



The properties of colloidal solutions are being investigated 

 at present by many scientific men. 



Returning to the methods of precipitating proteins in particu- 

 lar, we find a variety of methods available. 



Heat. Many proteins are precipitated by heat. A slightly 

 acid reaction, and the presence of salts is desirable if precipi- 

 tation is to be complete. Alkaline solutions, of proteins do not 



