138 
Journal of Agricultural Research 
Vol. VII, No. 3 
charge to the colloidal particle. The charge thus produced on the 
surface of the particle is balanced by oppositely charged ions in the 
immediate sphere of attraction. In this manner the conditions of 
electrical equilibrium are accounted for. 
Parker (24) has avoided some of the difficulties involved in explaining 
the electrical equilibria on the assumption that one ion alone is removed 
by a colloid. He claims that potassium chlorid in aqueous solution is 
hydrolyzed, yielding hydrochloric acid and potassium hydroxid. The 
latter is then considered to be withdrawn from the solution as a whole. 
Obviously, this does not explain the mechanism by which the potassium 
hydroxid is combined with the colloid, although it does take into account 
the electrical equilibria and the presence of acid in the solution. It 
does not, however, exclude the possibility of a direct chemical addition 
product of potassium hydroxid with some of the soil constituents. 
Either of the last two theories may account for the presence of acid 
in the filtrate or supernatant liquid derived from a colloid in contact 
with a salt solution. Whichever explanation may be preferable, for 
practical purposes the final result is the same—that is, the colloid has 
retained equivalent quantities of negative and positive ions, and chem¬ 
ically equivalent quantities remain in solution. 
Since these theories of adsorption necessitate the removal of both 
positive and negative ions, the usefulness of the idea of selective adsorp¬ 
tion as applied to many types of soil reactions may be questioned, 
especially in view of the possibility that chemical reactions, at least 
to a considerable extent, may account for the observed phenomena. 
This statement is not to be construed as denying the probability of 
a partial condensation of a chemical compound on solid surfaces. A 
clear exposition of this type of phenomena is given by Patten (25). 
The magnitude of such condensations is exceedingly variable, for it de¬ 
pends upon the physical conditions of the particular system, and, hence, 
it is difficult to estimate its significance in any specific instahce. In¬ 
deed, Robertson (27) is inclined to believe that surface condensation 
accounts for only a small portion of the total amount of substance 
combined with the adsorbing body. In his opinion most reactions 
designated by the terms “absorption” and “adsorption” obey the usual 
laws formulated for chemical reactions. The influence of surface in 
accelerating and possibly changing the nature of these reactions is not 
denied. This solution of the problem, as stated by Robertson, is not 
admitted by certain other investigators, notably Wolfgang Ostwald (23). 
Apparently, no final decision on this matter has been reached by physical 
chemists. 
It has been the aim of the present paper to set forth certain results and 
methods of investigation which may serve to throw additional light on 
some of the unsolved problems of soil fertility. Especially has the 
method of attack made possible an investigation of the important ques- 
