138 PROTOPLASM 



coagulation takes place, and coagula are formed. Certain 

 coagula have special names, e.g., a soap curd. Another property 

 which is rather generally characteristic of jellies but not neces- 

 sarily true of coagula is reversibility. Firm, hydrated gelatin can 

 be made liquid simply by heating, and the sol thus formed wUl 

 again set to a gel. Such simple reversibility does not take place 

 in true coagula. Heated albumin forms an irreversible coagulum. 



The divergent meanings that have been given the term gel and 

 related expressions are best harmonized by defining gel so that it 

 will include all types of firm lyophilic (and some lyophobic) 

 colloidal systems, with jellies and coagula as subtypes. In this 

 way will these terms be used on the following pages. Jellies, 

 then, are reversible gels; they set by stiffening, by the process of 

 gelatinization ; they are elastic and swell in water. Coagula are 

 (strictly) irreversible gels; they become firm by the process of 

 coagulation; they are usually relatively inelastic {i.e., of very low 

 extensibility), and they take up but do not swell in water. 



There are a number of other terms which describe processes 

 similar to if not identical with gelation or coagulation. These 

 are precipitation, agglutination, agglomeration, and salting out. 

 Such terms were first used when nothing was known of the 

 mechanism of the processes; the end products simply looked 

 different, or the original substances were of a different kind. 

 Different names were therefore given to the processes; thus, 

 blood coagulates, salts and colloidal suspensions precipitate 

 (though coagulate is also used here), and bacteria agglutinate. 

 The result in each case may be the same — a clumping together of 

 particles into aggregates which cannot usually be readily redis- 

 persed. Various attempts have been made to restrict these 

 terms {e.g., coagulate to organic systems such as blood and milk), 

 but no success has come of it, primarily because the more that is 

 known about them the more does it appear that they are funda- 

 mentally alike. We may, however, distinguish between the 

 gelatinization of a reversible system (gelatin) and the coagulation 

 of an irreversible one (albumin). 



Attempts have been made arbitrarily to state when a gel ceases 

 to be a gel and becomes a sol. All such attempts lead to confu- 

 sion. One can say that a gel firm enough to maintain its shape 

 unsupported is a true gel but if it collapses, it is a sol. How- 

 ever, this is purely arbitrary, and a collapsed gel is not properly 



