1916] Sharp : Soluble Salts and Soil Colloids 319 



proposes to attribute the diffusion of tlie soil colloids to changes 

 in the composition of the medium, namely the soil solution. Such 

 a diffused condition of the soil colloids might be brought about 

 by an increase in the OH-ion content of the solution bathing the 

 soil particles. This concept is based on the fact that clay is a 

 negatively charged colloid, and according to the views now held 

 the further addition of negative ions to such a colloidal suspension 

 causes these particles to assume greater charges of like sign, so 

 that they repel each other and thus remain distributed through- 

 out the medium in a stabilized condition. On the other hand, it 

 is held that the introduction of ions bearing an opposite charge 

 to that of the colloidal particles neutralizes the charge associated 

 with the particles, so that they no longer repel each other but 

 gather together in aggregates or floccules. For a more complete 

 discussion and bibliography dealing with these phenomena, the 

 reader is referred to the work of Whitney and Ober.-* In a more 

 recent review Tolman-^ has advanced a clear conception of colloids 

 and their behavior, which affords us a satisfactory working basis 

 for studies on these substances. According to this author, the 

 surface tension existing between the particles and the surround- 

 ing liquid is the factor which determines the degree of dispersion 

 of the particles in the liquid. Thus systems of zero surface tension 

 are at equilibrium. Those possessing a negative surface tension 

 increase, automatically, their degree of dispersion until the zero 

 value is reached, while those of positive surface tension tend to 

 become less dispersed. Since the surface tension referred to is 

 the resultant of many forces, it may be readily affected in numer- 

 ous ways, as by the mechanical process of grinding, by heating, by 

 the addition of electrolytes, or by the passage of an electric cur- 

 rent. Through the application of these considerations we may be 

 able to decipher more clearly and definitely the problems involved 

 in the effects of salts on the physical condition of soils. 



It is po.ssible that deflocculants other than tlie OH-ion may 

 have been introduced into the soil solution by means of the salt- 

 and-water treatments, but the latter factor obviously appears as 

 the most significant defiocculating agent likely to be present 



24 Journ. Amer. Chem. Soc, vol. 23, p. 842, 1901. 

 Sojourn. Amer. Chem. Soc, vol. 35, no. 4, 1913. 



