COLLOIDS 555 



trolytes on this charge may also be observed. It is very commonly 

 true that the addition of a precipitating electrolyte to a sol reduces 

 materially the charge of the particles. This is not, however, always 

 true, and the relationships are more complicated than has generally 

 been assumed. Xor is it always true that the complete neutraliza- 

 tion of the electrical charge on dispersed particles results in floccula- 

 tion. For example, acetic acid may be added to arsenic sulphid sols 

 in such quantities as to completely neutralize the negative charges of 

 the particles and, further, so much acid may be added that the par- 

 ticles acquire a very considerable positive charge, all this without 

 the least signs of flocculation. Ultimately so much acid may be 

 added as to cause flocculation. 



6. In the flocculation of sols by electrolytes there is frequently 

 observed a curious effect known as the "zone-phenomenon/' It is 

 observed when increasing amounts of certain electrolytes are 

 added that at a certain concentration flocculation will be brought 

 about, while if the concentration be greater flocculation will not 

 occur, although still further increase of concentration will result in 

 another flocculation zone. The phenomenon is most common when 

 the electrolyte used is a salt that shows marked hydrolysis, such, for 

 example, as ferric chlorid. If a negative sol be treated with a solu- 

 tion of this salt it is obvious that there will be three precipitating 

 influences present, namely, the hydrogen ions and the colloidal ferric 

 hydroxid, both of which are formed by the hydrolysis of the ferric 

 chlorid and the unhydrolyzed ferric chlorid. Since the amounts of 

 these precipitating substances in a given solution vary with the con- 

 centration, and since each has its own concentration function in 

 precipitation, it will be seen that the zone phenomenon may be ac- 

 counted for in such cases. Since, however, the zone phenomenon 

 occurs in many cases where strongly hydrolyzed electrolytes are 

 not used, as, for example, in the agglutination of bacteria by 

 citric and some other acids, the explanation is not wholly sufficient. 

 There are also many curious phenomena concerned with the 

 action of electrolytes on sols which have as yet been very little 

 investigated, and which will probably throw considerable light on 

 the subject. 



The Mutual Reactions of Colloids The conduct of mixtures of 

 two different sols is of very great interest and variety, both in 

 the absence and in the presence of electrolytes. A number of 

 particular cases may be distinguished, and these will be taken up 

 seriatim. 



1. When two positive or two negative sols are mixed together 

 nothing very much seems to happen. It is generally considered that 

 the addition of one sol to another of like electrical properties results 

 in no action. Whether this is wholly true or not is doubtful, but it 

 is at least true that in all cases investigated up to the present time 



