THE COLLOIDAL STATE. 49 



is precipitated. The meeting of two electrolytes with a 

 common ion favors precipitation, while certain non- 

 electrolytes, such as urea and sugar, inhibit through 

 their presence the precipitating power of electrolytes, and 

 even cause already existing precipitates to go back into 

 solution. These differences between the two changes 

 in state may be made still clearer by citing a few 

 examples. Thus gelation is greatly inhibited through 

 the presence of bromides, while precipitation through 

 electrolytes is markedly increased as soon as the added 

 bromide contains a common ion. The combination 

 sodium acetate sodium bromide with the common Na 

 ion is in this way a more powerful precipitating agent 

 than the acetate itself. Again, dextrose favors gelation 

 in that it elevates the gelation-point and increases the 

 gelation velocity, and yet it inhibits the formation of a 

 precipitate through a precipitating electrolyte as soon as 

 the sugar is present in sufficient amount. 



The independence of the two changes in state evidences 

 itself in this also, that through suitable selection of experi- 

 mental conditions it is possible to produce precipitates 

 in a solid and clear gelatine just as in a liquid one, and 

 this without a change in the state of aggregation of the 

 solid gelatine. 



It seems to me that what has been said proves without 

 question that between the precipitation and the gelation 

 of a colloid, at the basis of which fundamentally similar 

 changes have been supposed to lie, there exist in reality 

 profound differences through which the assumption of a 

 related origin of the two phenomena has been rendered 

 most improbable. In the same direction will be found 

 to point an analysis of those facts which have always 



