COLLOIDS 551 



arate parts may, however, occur in a variety of other ways, in fact, 

 in any way whatever whereby such union is physically possible. 

 Thus, if the separate portions be of different size, the smaller one 

 will have a higher vapor pressure than the larger, and evaporation 

 from the smaller to the larger will take place until the whole of the 

 smaller portion has transferred itself to the larger one, and the re- 

 union is therefore complete. There is every reason to believe that if 

 the two portions of unequal size were made electrodes in a galvanic 

 cell, and this cell were then short circuited, that the smaller portion 

 would go into solution and again deposit upon the larger one. In 

 case the two portions were of the same size, these forms of recom- 

 bination, with the exception of that of direct coalescence, would not 

 occur if all other conditions were kept constant, but a slight differ- 

 ence of conditions in respect to the two portions would start the act 

 of recombination, which would then in general proceed to comple- 

 tion. The same tendency is noticed with all substances. Thus in a 

 liquid small crystals disappear while larger ones grow at their ex- 

 pense, and it may be stated that, other influences for the moment 

 ignored, the most stable configuration which can be assumed by a 

 given mass of any substance is that in which all of the substance is 

 in one portion, and that portion is spherical in form. This is equiv- 

 alent to saying that all bodies so arrange themselves as to expose the 

 least possible surface. The force which tends to bring about this 

 condition is called surface tension. In so far as surface tension 

 alone is concerned it follows that any colloidal solution must be un- 

 stable, and tend to condense itself until all of the dispersed matter 

 has aggregated itself together into a single mass of spherical form. 



But there are many other forces which may under certain con- 

 ditions act against surface tension. If the dispersed substance is 

 one that is crystalline, the directive forces of crystallization over- 

 come those of surface tension, and the form of stable configuration 

 will be that of the crystal instead of spherical, and equilibrium will 

 be established when all of the available substance has aggregated 

 itself together into one large crystal. We know, on the other hand, a 

 great many colloidal solutions which seem to be quite stable even in 

 very high degrees of dispersion. To explain such cases we must look 

 for other forces working against the force of surface tension. If the 

 dispersed particles in a colloidal solution are all charged with the 

 same kind of electricity, they will then repel one another with a 

 force which will vary inversely as the squares of their distances from 

 one another. This repulsion will then tend to work against any 

 coalescence or other sort of union between the disperse particles. We 

 have already seen that colloidal particles are in general charged 

 either positively or negatively, and this may be taken to some extent 

 as explaining the stability of such systems. Equilibrium results 

 when the surface tension is just counterbalanced by the electrical 



