EVOLUTION OF COLLOIDS 149 



determine a movement upwards or downwards 

 in these delicately balanced colloidal com- 

 pounds. 



Another factor in the chemical reactions 

 of the colloids which differs from that of 

 simple inorganic relationships is the time taken 

 for the changes. A reaction in which there 

 is a great change of energy comparatively, 

 for a small amount of matter altered, runs 

 very swiftly. Hydrogen and oxygen unite 

 to form water with a practically instantaneous 

 explosion, and then all is over and the system 

 is inert. A moment of fierce activity followed 

 by absolute rest. But in colloidal reactions, 

 there is slow movement of the reaction for 

 days. In many cases it is never complete, 

 and slight changes acting on the system can 

 send it slowly pulsating up and down about 

 an equilibrium point. 



Such changes are seen even in inorganic 

 colloids, such, for example, as colloidal silicic 

 acid, as studied by Graham. Silicic acid as 

 ordinarily obtained, readily gives up water 

 and forms silica, of which flint and sand are 

 composed. This substance, needless to say, 

 is insoluble in water, but if treated with strong 

 alkali it forms alkaline silicates. An example 

 of such is sodium silicate, better known in 

 solution in water as water glass, now used 



