PHYSICAL CHEMISTRY 29 



"like glue"). Substances which usually are colloidal often can 

 be 'crystallized, whereas crystalloids also can be made to assume 

 colloidal form. It has become known that colloidal properties 

 are dependent on the fineness of subdivision of the dissolved 

 substance, that is, the size of the particles in the solution. If a 

 more or less coarse insoluble material is shaken up with water, 

 a portion remains suspended. This mixture is called a suspen- 

 sion. If the particles of this suspended material could now be 

 ground up so fine that they no longer could be seen, even with 

 the aid of a microscope, we would have a cloudy liquid, the 

 particles of which would not sink to the bottom on standing. 

 If the particles were sufficiently small, we now would have a 

 colloidal solution. If this process were continued further, we 

 would ultimately arrive at individual molecules. This would 

 then be called a true solution, since the dissolved particles now 

 would be so small that they could pass through the pores of a 

 parchment membrane. Fairly definite limits have been set for 

 the size of colloidal particles. 1 /x is 0.001 mm. and 1 w is 

 0.001 fi or one-millionth of a millimeter. The size of colloidal 

 particles has been set as ranging from 1 to 100 /x/x. 



In thus subdividing material into such extremely small par- 

 ticles the surface is enormously increased. If a cube of material 

 1 cm. on edge were divided into cubes 10 ^ on edge the surface 

 would be increased from 6 sq. cm. to 600 sq. meters and there 

 would be 10 18 particles. The peculiar properties of colloidal 

 solutions are thus largely dependent on the enormous increase 

 of surface, and the consequent importance of surface forces, 

 such as surface tension. 



Classification and Properties of Colloids. The colloids are 

 by no means a unified chemical group, for substances of the 

 most widely diverse chemical nature, such as metals, salts, acids, 

 bases, proteins, carbohydrates, etc., may form colloidal solu- 

 tions. The term "colloidal" refers in fact to a state of mat- 

 ter, and not to a class of compounds. Many substances of the 

 greatest biological importance form colloidal solutions, in fact 

 the constituents of the living cell are believed to be in a col- 



