480 PROTOPLASM 



phenomena. But there are other processes which are eitlicr 

 actually or superficially very similar to coagulation. One of 

 these is precipitation. If hydrochloric acid is added to a solution 

 of silver nitrate, silver chloride is precipitated. When the muddy 

 waters of the Mississippi River meet the salt water of the sea, 

 the mud is precipitated. These two events appear to be identical, 

 but actually they are due to quite distinct happenings. The 

 precipitation of the silver salt is due to a chemical exchange of 

 ions. The precipitation of the suspended clay particles in the 

 river is due to surface phenomena involving no interchange in 

 a chemical sense. Only the latter is true coagulation. 



Other processes which are similar to coagulation, in that they 

 involve the production of solid from dispersed matter, are gelation, 

 gelatinization (setting), coalescence, fiocculation, agglutination, and 

 salting out. 



Gelation is the change that a colloidal suspension undergoes in 

 forming a reversible or irreversible gel. It will be recalled 

 (page 138) that we decided to use the term "gel" to indicate both 

 jellies (reversible, elastic, nonporous gels of the type of gelatin) 

 and coagula (irreversible, relatively inelastic, porous gels of the 

 type of silica gel). Thus does gelation become the solidification 

 of the sols of both jellies and coagula. Gelatinization is the set- 

 ting, or gelation, of a reversible gel or jelly. A solution 

 of gelatin gelatinizes into a readily reversible jelly, while one of 

 silicic acid coagulates into an irreversible, inseparable mass of 

 silica. Both gelate or set. Coagulation may be defined as the 

 irreversible aggregation of particles into an inseparable mass. 

 But such a definition eliminates those processes where irreversible 

 but separate masses are formed and also all reversible forms of 

 aggregation. (By reversibility is usually meant a return to the 

 original state, liquid or solid, by a reversal of the same process 

 that produced the new, solid or liquid, state; thus, gelatin is 

 reversed by merely reversing the process of heating or cooling 

 which caused solation, or gelation, but a boiled egg cannot be so 

 reversed.) 



To restrict coagulation to irreversible gelation is arbitrary 

 though often done. Coagulated sols of iron oxide, vanadium 

 pentoxide, and arsenous sulphide may be reversed when weak 

 electrolytes are used and not too long a time allowed for the 

 coagulation, yet the process is distinctly coagulation. We may 



