32 BULLETIN 1452, U. S. DEPARTMENT CF AGRICULTURE 
governed more by the kind - of replaceable cations present than by 
the total quantity. Several investigators, including Geclroiz and 
Kelley and Brown, have observed that when the exchangeable bases 
of the soil are replaced by sodium the reaction is alkaline and when 
hydrogen is substituted the reaction is acid. The correspondence 
between pH and total quantity of exchangeable bases that obtains 
in the case of the seven colloids studied is probably due to the fact 
that those colloids which under natural conditions, contain the 
smaller quantities of total exchangeable cations, usually contain ex- 
changeable hydrogen ions (or iron and aluminum), while the col- 
loids with the higher content of exchangeable cations are usually 
saturated with the monovalent and divalent bases. 
Although no determinations were made which served as a direct 
measure of the stability of the different sols or the dispersability 
of the dry colloids, it was evident in working with various soil 
colloids that the neutral or alkaline colloids were usually more 
easily dispersed than the distinctly acid ones and formed more 
stable sols. The dispersability and stability of the colloids, like 
the pH, is probably influenced by the character of the exchangeable 
ions. Gedroiz (19) and Kelley and Brown (29) have observed that 
the dispersability of colloids irom the wdiole soil is largely affected 
by the character of the exchangeable cations, a soil saturated with 
sodium being much more dispersable than one which has been satu- 
rated with hydrogen by an acid treatment. One might therefore 
regard the dispersability of the colloid and the pH as being both 
influenced by character of the exchangeable cations. However r 
high pH and high dispersability do not always go along together. 
A Ca-saturated colloid, for instance, may have a comparatively 
high pH but be of comparatively low dispersability. 
RELATIONS BETWEEN THE PROPERTIES OF SOIL COLLOIDS 
Most of the methods described in the previous pages for deter- 
mining the properties of the colloids are more or less arbitrary, and 
the values obtained can not be considered as constants in the same 
sense that the boiling point is a constant of a pure liquid. Also, the 
determinations given are in some cases very incomplete characteriza- 
tions of the properties. For instance, the adsorptive capacity of a 
colloid for water or ammonia vapor is not fully shown by the quan- 
tity adsorbed at a sing.e pressure of the vapor. The adsorptive 
property of each colloid woidd be better characterized by a curve 
showing the quantity adsorbed at different concentrations or press- 
ures. It is quite possible for two colloids to stand in one order on 
the basis of the quantity of water adsorbed at a given partial 
pressure and stand in a reverse order on the basis of the quantity ad- 
sorbed at another partial pressure. However, it was beyond the 
scope of this investigation to make a detailed study of any one 
property. 
