Oct. 16, 1916 
Acidity and Adsorption in Soils 
137 
that the OH ions of a solution of calcium hydroxid have been removed in 
large measure from the solution by the soil, while those in Table VIII 
demonstrate a simultaneous loss of calcium from the solution. Evidently 
both the Ca and OH ions have been removed to a notable extent from 
the solution by the soil. Two explanations of these phenomena suggest 
themselves. One explanation would assume a condensation of the cal¬ 
cium hydroxid as a whole on the surfaces of the soil particles. The 
other, in accordance with the ideas of Sullivan (32), Van Bemmelen (34), 
and Lemberg (32, p. 20-23), considers that the calcium hydroxid has 
been chemically united with some of the soil constituents, forming direct 
addition compounds. In view of the evidence embodied in the literature, 
the second theory seems the more tenable. In many ways the chemical 
explanation appears to be more logical in accounting for the fixation of 
bases by soils. 
Moreover, the theories which regard adsorption in soils as a chemical 
phenomenon have received an element of support in the proposed chemi¬ 
cal structure of the silicates. Clarke and Steiger (7) have shown that the 
composition of the silicates is such that it frequently admits of an ex¬ 
change of bases. This has also been recognized by soil chemists. Like¬ 
wise, from the structural formulae for silicates proposed by the above 
investigators, it is evident that acid salts of the various silicic acids might 
contain replaceable hydrogen. Loew (20) attributes the acidity of certain 
Porto Rican soils to the presence of acid silicates. As previously men¬ 
tioned, Sullivan (32) has also pointed out that a basic hydrate may form 
v an addition compound with silicates. 
The theory of selective adsorption of a single ion which has been 
advanced by various investigators is in its final analysis not entirely com¬ 
prehensible. One objection to this theory lies in the disregard of the 
ionic equilibrium. For example, it is frequently assumed that from a 
solution of a neutral salt one ion may be withdrawn by a colloid inde¬ 
pendently of its equivalent, oppositely charged ion. Thus if K + be 
selectively removed from a solution of potassium chlorid (KC 1 ), then the 
above assumption may be diagramatically represented as follows: 
(Colloid—K)+C 1 +H 2 0 
u _ . 
H+OH 
Obviously, such a system is electrically unbalanced. In order to 
meet this difficulty, the electrical double-layer theory of Helmholtz has 
been proposed. A modification of this theory has been especially urged 
by Billitzer (2). In its simplest form the electrical double-layer theory 
assumes that one ion may become more closely associated with a col¬ 
loidal particle than the oppositely charged ion. The force which binds 
the ion to the colloid is not well understood, but there is some justifi¬ 
cation for believing that the most closely associated ion imparts its 
