THE COLLOIDAL STATE 115 



ultramicroscopic structure, protoplasm is in part a system of 

 the second class of colloids, the gluelike ones and in part a true 

 solution of salts, sugars, etc. 



Classification. — We have devoted our attention in the present 

 chapter primarily to colloidal suspensions of solid matter, with 

 only a brief reference to liquid suspensions (emulsions) and glue- 

 like colloids. In order satisfactorily to classify, we must have 

 an intimate knowledge of all types of colloidal systems. Still, a 

 premature attempt at classification here will help toward a 

 better understanding of what is to follow. 



Those colloids that resemble glue have several properties 

 which sharply distinguish them from suspensions of solid and 

 liquid matter. One of these is the capacity to form gels, or 

 jellies. Gelatin is the classical example among the gel-forming 

 colloids, but there are many others of great variety. They 

 include albumin, hemoglobin, casein, rubber, vegetable gums, 

 agar, soap, glue, cellulose, silica gel, and such extreme types as 

 glass and mineral gels. The distinguishing feature of nearly 

 all of these is their ability to take up water. In doing so, most 

 of them swell and are therefore said to imbibe water. Graham 

 called these systems gels (the first syllable of "gelatin") when 

 they are in the firm condition, and sols (the first letters of 

 "solution") when they are in the liquid condition. The latter 

 term also applies to suspensions of solids, e.g., a gold sol. Gels 

 are usually glutinous (sticky) and elastic, though there are some 

 nonglutinous and nonelastic ones. They coagulate readily, as 

 do blood, casein (in milk), and albumin (in egg). They are not 

 very sensitive to electrolytes and are therefore precipitated much 

 less readily by salts than are the suspensions. Among the very 

 few properties that they have in common with the suspension 

 colloids are their failure to dialyze, and the exhibition of the 

 Tyndall cone. It is quite evident, therefore, that there are two 

 major groups of colloids which differ greatly — the suspensions 

 and the gel-forming ones. On the basis of their differences, they 

 can be named. It is typical of gels to take up water. The 

 suspension colloids lack this ability and have, therefore, been 

 called hydro-phobic ("water hating"), while the gel-forming 

 colloids are hydrophilic ("water loving"). 



The German colloid chemist Herbert Freundlich, perhaps the 

 best known of all workers in the colloidal field, thought that since 



