72 PHYSICAL CHEMISTRY 



electrolyte is added suddenly some colloidal particles have their 

 charges reversed and attract others, forming larger aggregates, 

 which are more difficult to keep in suspension. If the electrolyte 

 be added slowly, all of the particles have the same charge, which 

 is not reduced enough to cause precipitation. Evidently the 

 effect of increasing the valence of the ions is the same as the 

 effect of adding them suddenly. 



Some precipitating electrolytes partially hydrolyze, forming 

 colloidal solutions of opposite sign to the one to be precipitated. 

 We therefore get mixed effects of electrolytic and colloidal pre- 

 cipitation. Thus, a negative mastix solution is precipitated by 

 ferric chloride and also by ferric hydroxide. If a more con- 

 centrated solution of ferric chloride be added to the mastix 

 we get precipitation by the ferric ions. If a very dilute solution 

 of ferric chloride is added, colloidal ferric hydrate is formed by 

 hydrolysis, but precipitation does not occur unless the sum of 

 the charges on the hydrate exactly neutralizes the sum of the 

 charges on the mastix. Hence the precipitating concentration 

 is very exactly defined and varies with the concentration of the 

 mastix sol. Since there are two concentrations of the precipi- 

 tant that are effective this is called an irregular series. 



Low concentrations of non-electrolytes do not precipitate. 



The suspensoids are similar to suspensions in that they are 

 precipitated by small amounts of electrolytes and the precipitate 

 is irreversible. They do not affect the viscosity or surface ten- 

 sion of water, or exert osmotic pressure. This is true of dilute 

 sols, but when very concentrated they form gels, the viscosity 

 of which is necessarily high. 



True suspensoids are probably not important in cell chemistry, 

 but emulsoids may be changed by heat or reagents so as to assume 

 more or less the character of suspensoids. Emulsoids so treated 

 are said to be denatured. Thus, if a very dilute solution of egg 

 albumin is boiled it is converted into an opalescent solution in 

 which the particles may be seen with the ultramicroscope, and 

 irreversibly precipitated by electrolytes. The denatured albumin 

 particles differ from true suspensoid particles in that they con- 

 tain water, but not as much as emulsoid particles. Hydroxides 

 (such as dialyzed iron) contain some water. 



Living cells may be so changed as to resemble the particles 



