THE COLLOIDAL CONDITION 207 



out the liquid for an indefinite period of time. The reason for 

 this phenomenon undoubtedly lies in the fact that these minute 

 particles carry an electric charge, which is of the same sign for all 

 of the particles and results in a repellent action which keeps the 

 particles in constant motion. This constant motion may easily 

 be conceived to keep the particles uniformly distributed through- 

 out the liquid, just as constant shaking would keep those of a 

 mechanical suspension uniformly distributed through the mixture. 



The sign of the electric charge on the particles of a sol may be 

 either negative or positive, depending upon the chemical nature 

 and dialectric constants of the two phases of the system. The 

 proportion of the total electric charge of the system which is of 

 the opposite sign to that borne by the dispersed particles is, of 

 course, borne by the liquid which constitutes the other phase. 

 The origin of this electric charge on the colloidal particles is, as yet, 

 not known with certainty; but it seems probable that it is due to a 

 partial ionization of these small particles, similar to, but not so 

 complete as, that which takes place when compounds which are 

 soluble go into true solution in water, or other solvents which 

 bring about the dissociation of dissolved substances. 



The conditions necessary to bring a solid substance into a 

 colloidal mixture with some liquid, or, in other words, to produce a 

 suspensoid sol, require that the proportion of liquid to solid shall 

 be large and some means of disintegrating the material which is to 

 be dispersed into very fine particles. Many common chemical 

 reactions, if carried out in very dilute solutions, result in the pro- 

 duction of sols, especially if a small amount of some emulsoid is 

 present in the reacting mixture; sols produced in this way are very 

 stable, and the emulsoid which is used in stabilizing the sol is 

 known as a " protective colloid." Direct methods of disintegra- 

 tion; such as reduction by chemical agents, discharge of a strong 

 electrical current through the substance which is to be dispersed 

 while it is submerged in the liquid, alternate treatment of finely 

 ground material with alkali and acid so as to frequently change 

 the electric charge, etc., are utilized for bringing inorganic com- 

 pounds into the colloidal state. 



Suspensoids usually contain less than 1 per cent of the solid 

 dispersed through the liquid. In fact, extreme dilution is one 

 of the necessary conditions for suspensoid-formation. ' 



Emulsoids are much more easily produced than are suspensoids. 



