OPTICAL AND ELECTRICAL PROPERTIES OF COLLOIDS 87 



and salting out. Cholesterin and lecithin may be regarded as such 

 transitional substances, whose flocculation has been thoroughly 

 studied by O. FORGES and E. NEUBAUER.* Cholesterin closely ap- 

 proaches the hydrophobe colloids, lecithin the hydrophile, and on 

 this account the former is irreversibly precipitated by salts and the 

 latter reversibly. Yet the concentration of alkaline earths required 

 to precipitate lecithin is considerably less than for the more hydro- 

 phile albumin, and conversely much greater concentration of salt is 

 required for the flocculation of cholesterin than is the case with true 

 hydrophobe sols. In the case of lecithin, the "irregular series" 

 occurs even with neutral salts (magnesium and ammonium sulphate) . 

 As will be seen in Chapter XIII, such transitions from hydrophobe 

 suspensions to hydrophile colloids may be artificially produced with 

 emulsions of bacteria. 



Much has been written on the theory of salting out and flocculation. 

 No one theory accounts for all the individual facts, yet the following 

 explanation is generally useful. Flocculation is brought about by 

 the coming together of small particles to form larger complexes. 

 These agglomerations always occur under the influence of electric 

 forces and in fact the optimum for reversible salting out is in the iso- 

 electric zone (see p. 158). The process, must, therefore, be brought 

 about by a discharge of the particles. RISDALE ELLIS has shown by 

 researches on oil emulsions, that the charge at the interface between 

 water and the dispersed phase is probably reduced to a minimum by 

 the addition of precipitating electrolytes. The smaller this charge 

 is at the interfaces (electric double layer), the more readily the 

 double layer is broken down, resulting in a union of the suspended 

 fluid or solid particles. 



In the case of anodic colloids, cations of an electrolyte, and in the 

 case of cathodic colloids, anions or an oppositely charged colloid, 

 lessen or neutralize the electric charge, so that the particles may 

 unite. (See H. FRETJNDLICH'S " Kapillarchemie " and Wo. OSTWALD'S 

 "Grundriss der Kolloidehemie.") 



Irreversible precipitation of metal hydrosols frequently occurs out- 

 side the isoelectric zone and the electrolyte threshold is not as sharp 

 as with reversible hydrosols. 



Radioactive Substances as Colloids. 



It has been demonstrated by electric migration and dialysis that 

 radioactive substances occur in colloidal solution. In view of the 

 great biological significance of such substance we shall explain the 

 facts more fully. 



