THE PROBLEMS OF SOLUTION 115 



at once to be, in reality, tests for the individual 

 ions of those salts. At one time it seemed likely 

 that all cases of rapid chemical action might be 

 reduced to reactions between electrolytic ions, 

 but experiments by Kahlenberg and others seem 

 to show that in non-aqueous solvents rapid 

 reactions may occur not in any way correlated 

 with electrolytic conductivity. However this 

 may be, in water many chemical actions are 

 certainly connected in a very intimate way with 

 the electrical properties, and the dissociation 

 theory gives a satisfactory method of co-ordinating 

 the two sets of properties. In some reactions the 

 actual electric charges on the ions seem to be the 

 determining factors of the whole process. 



There is a marked difference in chemical and 

 physical properties between bodies of definite crys- 

 talline form, such as most inorganic salts, and soft 

 or amorphous substances, such as albumen and the 

 various kinds of jelly. Long ago Graham distin- 

 guished the two groups as crystalloids and colloids 

 respectively, and particularly examined them with 

 regard to their relative powers of diffusion through 

 water. He found that, while crystalloids diffuse 

 comparatively rapidly, the motion of colloids is 

 so slow that it is often almost inappreciable. 



Many different kinds of chemical compounds 

 show colloidal properties. Besides a vast number 

 of animxal and vegetable substances, some of which 

 are of fundamental importance in the phenomena 

 of living matter, many of the precipitates which 

 are formed in the course of inorganic chemical 

 reactions appear in an amorphous or colloidal 

 state. The sulphides of such metals as antimony 



