PROPERTIES OF THE COLLOIDAL SOIL MATERIAL 27 
is from four to -five times that of the exchangeable. If the H or 
Fe and Al replaced by the salt treatment had been included in the 
exchangeable bases of these acid colloids, then the nonexchangeable 
Ca-f-Mg+K+Xa would probably have approximated 2.5 times 
the total exchangeable bases of the acid colloids also. For instance, 
data given in a previous paper (39, p. 463) show that the total 
exchange capacities of other samples of the Sassafras and Norfolk 
colloids were between one-third and one-half as great as the nonex- 
changeable bases given in this paper. If a balance obtains between 
exchangeable and nonexchangeable bases, there might be almost as 
good a correlation between nonexchangeable bases and a property 
as between exchangeable bases and the given property, although the 
property might be directly dependent on the exchangeable bases. 
It would seem from the work of Gedroiz (15) that certain proper- 
ties, such as swelling, may be more directly dependent on the ex- 
changeable bases. 
ELECTRICAL CHARGE 
The electrical charge of a colloid is an especially important prop- 
erty, since it largely determines the behavior of the material in the 
sol condition. The sign and magnitude of charge, however, is 
greatly affected by the medium in which the colloid is dispersed and 
by the presence of electrolytes. Variations in the charge of particles 
can readily be followed by measuring the speed with which the par- 
ticles m" grate under the influence of an electric current. 
In pure water the colloidal materials from all soils are electronega- 
tive. In electrophoresis determinations the particles move toward 
the anode with a speed of the same order of magnitude as that of the 
slower ions, the velocity seldom being greater than 3 microns per 
second under a potential gradient of 1 volt/centimeter. 
The charge on the part'cles of soil colloids is markedly affected by 
the addition of electroh r tes to the water suspensions. Electrolytes 
might be classed in three groups on the basis of their effects on soil 
colloids: (1) Electrolytes which in low concentrations increase the 
negative charge, such as the alkali hydroxides, phosphates, and car- 
bonates; (2) those which even in low concentrations reduce the 
charge without changing the sign, such as most compounds of the al- 
kaline earths; (3) those which are capable of changing the sign of 
the charge, such. as iron, aluminum, ancl thorium compounds, the basic 
dyes and acid gelatin. Although a given electrolyte apparently has 
qualitatively the same effect on all so'l colloids, there are quantita- 
tive differences in the action of a given electrolyte on different col- 
loidal soil materials. 
The various colloidal soil materials might be characterized with re- 
pect to their electrical behavior by differences in the intensity of the 
negative charge they assume in distilled water and by the quantity 
of a given electrolyte required to reduce the charge to a given value. 
Determinations of this kind are readily made by cataphoresis. Inas- 
much as the isoelectric point is a critical point in the behavior of 
colloids, the different soil colloids were compared on the basis of the 
quantities of an electrolyte required to bring them to electrical neu- 
trality. Obviously only an electrolyte of Group 3 could be used for 
this purpose, and preliminary tests showed methylene blue was satis- 
factory. 
