COLLOIDS 55 



ference in the properties of a colloidal solution from those of a suspen- 

 sion of the same substance. Thus, the difference between a colloidal 

 solution of platinum (prepared by allowing an electric arc to form be- 

 tween platinum electrodes in water) and pieces of platinum in water 

 depends on the fact that the surface of the platinum in the former case 

 has been increased many million times. When the subdivision becomes 

 still greater and the particles gain the size of molecules, the phenomena 

 due to surface development become suppressed and those due to con- 

 centration in unit volume become accentuated. The properties depend- 

 ent on osmotic pressure, diffusibility, etc., are exhibited by all dispersoids, 

 whether ions, molecules or particles, but some of these properties are 

 much more pronounced when the dispersoids are of large dimensions, 

 and others when they are small. In other words, the phenomena due to 

 surface, such as those of surface tension (see page 64), become apparent 

 only when the dispersoids have the properties of matter in mass; when 

 the dispersoids become molecular in size, they manifest the properties 

 characteristic of true solutions. 



4. Electric Properties of Colloids. Most colloids carry a charge, which 

 may be either positive or negative toward the dispersion medium. Both 

 crystalloids and colloids therefore carry electric charges; in the former 

 case, however, the charge does not reveal itself until the molecules in 

 solution have become dissociated, when each ion carries a charge of 

 opposite sign (see page 16), whereas in the case of colloids, each col- 

 loid particle usually carries a charge which is always of one sign, either 

 positive or negative. Colloids may therefore be grouped into positive 

 and negative, according to the charges which they carry, and there is 

 a third group in which the charge may be either positive or negative ac- 

 cording to the nature of the dispersion medium. 



A colloid not carrying a charge to begin with can be caused to assume 

 one by the action of electrolytes-, for the electrical properties of colloids, 

 as well as those of inert powders suspended in water, are readily in- 

 fluenced by the charges present in the ions of the dispersion medium. 

 The H- and OH' ions are especially liable to exert this influence. The 

 particles of inert powders in suspensions (kaolin, sulphur, etc.) carry 

 a positive charge when the water in which they are suspended is acidi- 

 fied, and a negative charge when it is made alkaline. In general, it may 

 be said that suspensions of most powders and of insoluble organic acids 

 in water (e. g., charcoal, cellulose, kaolin, caseinogen, mastic, free acid 

 of congo red, etc.) are electro-negative. Of true colloids ferric hydrox- 

 ide (ferrum dialysatum) and serum globulin are positive in acid solu- 

 tions: arsenious sulphide and serum globulin are negative in alkaline 

 solution, and serum globulin in neutral solutions has no charge. 



