2 7 S POPULAR SCIENCE MONTHLY. 



loidal gold and of arsenious sulphide, no coagulation occurs, but when 

 the suspensions of ferric hydroxide and arsenious sulphide, which we 

 have seen from their behavior on migration have opposite electric 

 charges, are poured together, there is immediate coagulation, and in a 

 few minutes the precipitate settles, leaving the liquid clear above. 



Crystalloid substances are also to be divided into two classes with 

 respect to their effect in coagulating colloids. Non-electrolytes, 

 whether organic or inorganic, have no tendency to produce coagula- 

 tion ; indeed, we have seen that organic substances, like ether, glycerine 

 or sugar, often increase the stability of the suspension. On the other 

 hand, strong electrolytes, that is, substances which are themselves 

 largely dissociated into electrically charged particles or ions, cause 

 coagulation, when their concentration in the solution becomes suffi- 

 ciently great. Although complete coagulation does not occur suddenly 

 as the quantity of electrolyte is increased, yet the interval between 

 the concentration at which, in a given time, the turbidity becomes 

 visibly greater and that at which the particles have become large 

 enough to settle out or to be retained by a filter is usually so small that 

 a fairly definite concentration can be specified at which each electrolyte 

 causes a certain, experimentally determinable, degree of coagulation in 

 a definite time. Now recent investigations have demonstrated the 

 remarkable fact that this coagulation-concentration is nearly the same 

 for different ions having the same electric charge (or valence), but 

 that it diminishes enormously with increase of the electric charge of 

 the ion of unlike sign to that of the colloid, while it is not affected 

 by a change in the electric charge on the ion of like sign. 



These principles are well illustrated by the results given in the 

 table, which were obtained by Freundlich, on the one hand with the 

 negative colloid, arsenious sulphide, and on the other with the positive 

 colloid, ferric hydroxide, by determining the concentration of various 

 salts which in two hours caused such coagulation as would prevent 

 any of the colloid from passing through a hardened filter. 



Coagulation-concentration in milli-equivalents per liter 



It will be seen by comparing the first two values in each column 

 that so long as the electric charge or valence of the ions of the salj; 

 remains the same, their chemical nature has no influence. It will be 

 further seen from the third value in each column that a variation of 

 the electric charge of the negative ion has no great influence upon the 



