THE COLLOIDAL STATE 



117 



helpful, it is not colloidal, for colloid chemistry has grown up 

 on a physical and not on a chemical basis. 

 The following is Zsigmondy's classification: 



B. 

 C. 

 D. 



I. Inorganic 

 Metals 



1. Pure 



2. With protective colloids 

 Nonmetals (colloidal sulphur) 

 Oxides (iron oxide, silica gel) 

 Sulphides (arsenic trisulphide) 



II. Organic 



A. Organic salts 



1. Soaps 



2. Dyes 



B. Proteins 



E. Salts (silver salts of photography) 



Zsigmondy's groups of colloidal systems lack the emulsions 

 and such organic gel-forming colloids as agar, starch, and cellu- 

 lose. These latter might be included under "carbohydrates," 

 between the organic salts and proteins. 



Victor Cofman has devised a classification in which both phy- 

 sical and chemical properties are considered. The plan has the 

 good feature of indicating the overlapping of types, by taking 

 into consideration the imperceptible graduations which occur 

 between one kind of system and another. Cofman's classifica- 

 tion is the following : 



One might well ask, is there, among all these diverse types of 

 systems, one property which definitely stamps the colloidal state? 

 The answer is, yes, the proportion of surface to volume. Col- 

 loidal systems owe their distinctly colloidal properties to the 

 extraordinary amount of surface exposed. When the exposed 

 surface of a dispersed substance is very great in proportion to the 

 total amount of dispersed substance present, i.e., when a sub- 

 stance is in the colloidal state, then surface phenomena determine 

 the behavior of the system. Spongy (colloidal) platinum is very 



