2 ART. 8. — S. KAWAMUßA : COAGULATION OF COLLOIDAL 



sensitive to electrolytes/ Most inorganic colloids belong to this 

 class, and the greater part of the experimental researches have 

 been carried ont with such. Of these the colloidal solution of 

 arsenious sulphide is the one most frequently studied on account 

 of the ease with which it can be obtained free from contamina- 

 tion, and on account of its stability. It is the positive and 

 negative ions which cause the coagulation. Where the suspended 

 particles are negatively charged, as is the case v^dth arsenious 

 sulphide, the coagulation is called forth by positive ions ; while 

 positively charged particles such as ferric hydroxide are pre- 

 cipitated by negative ions. The strength of the action evidently 

 depends on the valency of the ions, because the higher the 

 valency the stronger the action. And the difference between the 

 action of mono- and divalent ions, as w'ell as that between di- 

 and trivalent ions is enormous. 



Such are some of the salient features of the results hitherto 

 obtained. They are only quasiquantitative, because the time effects 

 are often very pronounced in the case of coagulating colloids 

 and preclude any exact determination. Moreover the methods 

 hitherto employed in determining the concentration of electrolytes 

 which call forth coagulation in a given colloidal solution leave 

 much to be desired. Some investigators have measured the 

 minimum concentration of the electrolytes at which distinct tur- 

 bidity is observed ; others have estimated the concentration at 

 which the coagulum separates out leaving clear supernatant liquid 

 free from suspended particles ; while still another has determined 

 the strength of the electrolyte solutions which so far coagulate 



1 Tliere are intermediate forms between susiiension coUoiels and Iiydropliile colloids. 

 These may be sensitive to electrolytes as is evidently the case with Icguminc, whose co- 

 agulum forms the principal constituent of our food-stufi' tôfu. 



