COLLOIDS AND ADSORPTION 



29 



bringing about coagulation by the same mechanism of 

 lowering the surface charge below the critical value. 

 Again the same valency rule applies, the higher the 

 valency of the ion the greater is its effect, except in the 

 cases of hydrogen and hydroxyl ions which for some 

 reason are much more effective than other ions. 



Similar coagulation can be brought about by adding 

 a positively charged colloid to a negatively charged one. 

 When the positively charged colloid is present in sufficient 

 quantity to lower the surface charge on the particles of 



CONCENTRATION OF ADDED ELECTROLYTE >- 



Fig. 4 



the other below the critical value coagulation occurs. 

 If addition of the positively charged colloid is continued 

 a time will come when the charge on the particles becomes 

 greater than the critical value on the positive side, and 

 the system will no longer coagulate but will remain 

 dispersed ; that is coagTilation will only occur within a 

 zone of concentration of the added colloid, the limits of 

 the zone being between the concentrations of added 

 colloid necessary to keep the surface charge between the 

 positive and negative critical values (see Fig. 4). The 



