THE COLLOIDAL PROPERTIES OF SOIL 161 



As a rule the suspensoids are readily precipitated by 

 traces of electrolytes ; emulsoids, on the other hand, are not, 

 but usually require salting out. The phenomena have been 

 explained by W. B. Hardy (122) on an electrical hypothesis 

 and by Freundlich as the result of selective adsorption of the 

 coagulating ion. 1 



Hardy (122, p. 238) supposes that the particles of sus- 

 pensoids remain suspended so long as they carry an electric 

 charge, i.e. so long as there is a difference in electric potential 

 between them and the liquid in which they are immersed. 

 But they precipitate immediately this charge is neutralised by 

 an opposite charge, or to be more precise, as soon as they 

 attain the same potential as the liquid. This point where the 

 difference in electric potential has disappeared is called the 

 isoelectric point. 



The flocculation of clay has been much investigated. 

 Schloesing in 1870 (245^) discovered the main facts and de- 

 monstrated the reversibility of the process. References to sub- 

 sequent work will be found in Wolkoff's paper. 2 The 

 general result is that clay behaves like an electro- negative 

 colloid ; it is deposited at the anode on the passage of an 

 electric current through its suspension, 3 and it is flocculated 

 by positively charged ions, but beyond a certain point it is 

 deflocculated or rendered still more disperse by negatively 

 charged hydroxyl ions. The flocculating effect depends more 

 on the valency than the atomic weight of the cation ; univalent 

 cations a*e less effective than bivalent, which in turn are less 

 effective than trivalent ions ; 4 there is, however, no simple 

 proportionality between valency and flocculation (Hall and 

 Morison, 1 2od). In some cases, if not all, some absorption of the 

 flocculating ions occurs and the phenomena are therefore more 



1 For a discussion of these views see W. C. McC. Lewis, A System of 

 Physical Chemistry, Vol. I., p. 362 (Longmans). 

 2 So/7 Sci., 1916, I, 585-601. 



3 Advantage is taken of this property to purify clay from admixed coarser 

 grains of silica. The process is known as electro-osmosis. 



4 Among recent papers see Leoncini (172); Wo. Ostwald, Kolloid Zeitsch.,. 

 1920, 26, 69-81 ; N. Bach, J. Chim. Phys., 1920, 18, 46-64. 



II 



