PROPERTIES OF THE COLLOIDAL SOIL MATERIAL 31 
Ca, respectively. The Na-saturatecl colloid had a charge of 50.7 
millivolts in distilled water, while the charge of the Ca-saturated 
colloid was only 15.5 millivolts. This point is discussed in more 
detail in a recent publication by Mattson (37). 
Although the character of the exchangeable bases largely affects 
the electrical behavior of the colloid, the charge of the particles 
probably does not vary exactly with the proportions of these bases, as 
determined by the usual base-exchange methods. According to cur- 
rent theory, "the charge on a particle is a direct function of the 
character of the double layer. Anions and the nature of the adsorb- 
ing surface are obviously Important in their influence on the double 
layer as well as the cations. Also, while the cations in the double 
layer are evidently part of the exchangeable bases, it is possible 
that only parts of the exchangeable bases, as now determined, are 
present in the double layer at any one time. If this is so, the pro- 
portions of the various cations in the double layer are not neces- 
sarily the same as the proportions of the exchangeable ba<:es. For 
.instance, the form of the curves obtained in the experiment with 
methylene blue (fig. 1) shows that the charge of the particles was 
not greatly affected by the substitution of methylene blue cation 
until most of the exchangeable bases were replaced. In tests with 
Na and Ca saturated colloids, the kind of cation in the double 
layer probably followed the kind of " exchangeable " base, since there 
was only one exchangeable base present. 
HYDROGEN-ION CONCENTRATION 
Numerous investigations have shown that the properties of col- 
loids, particularly those of the lyophile type, are markedly affected 
by variations in the hydrogen-ion concentration of the dispersion 
medium. It was beyond the scope of this investigation to study the 
effect of varying hydrogen-ion concentrations upon the various 
properties of the soil colloids. However, the pH values which the 
colloids impart to distilled water is apparently more or less char- 
acteristic of the different colloids. Determinations were therefore 
made of the pH values of the different sols. The colorimetric method 
was used. These are given in Table 17. 
Table 17. — Hydrogen-ion concentration of soil-colloid sols 
Kind of colloid 
pH 
Kind of colloid 
PH 
Fallon . 
8.2 
7.1 
7.0 
6.9 
5.7 
5.6 
Norfolk 
5.3 
Sharkey 
The range in hydrogen-ion concentration of the different colloids 
covers the values shown by most agricultural soils, except the 
strongly acid ones. There is some degree of correlation between the 
pH values of the colloids and other properties, particularly the total 
content of exchangeable monovalent and divalent bases. 
The correspondence between total quantity of exchangeable bases 
and pH is evidently not a causal relation. The pH is obviously 
