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28 GENERAL AND PHYSICO-CHEMICAL.
always > 1. (If n = \ then the formula would be —=k and we would be dealing
with a so-called solid solution.)
Appleyard and Walker l
have studied the adsorption of organic
acids from aqueous and alcoholic solutions by means of silk; the divi-
sion was found to correspond to the above formula for adsorption.
Freundlich 2
has also carefully tested the adsorption of crystalloids
by carbon. From these experiments it was shown that the equilibrium
could be quickly attained from both sides, i.e., that the process was readily
reversible. The above-given formula was found sufficiently accurate
for the case wr
here only the total quantity of the dissolved (to adsorb)
substance varied. The series in which the organic acids were adsorbed
by silk, as found by Appleyard and Walker, were pratically the same
as with carbon. The influence of temperature was slight.
According to Kuster,3
the combination between starch and iodine
is to be considered as an adsorption compound, and Biltz 4
finds for the
division of AS2O3 between iron hydroxide (1) and water (2) the for-
mula —= 0.631.
The theoretical foundations for the adsorption phenomenon are
not especially clear. Generally the adsorption is considered as con-
nected with segregation and surface tension phenomenon. At the con-
tact surface between a solid body and solution a surface tension exists
which is considered as positive, i.e., this attempts to diminish the
contact surface. The surface energy used thereby tends to be a min-
imum potential energy. As the product from size of surface and surface
tension are the same, and as the first cannot change, the surface energy
can only be diminished by a reduction of the tension. If, therefore,
the tension is diminished by increasing the concentration of a sub-
stance dissolved in a fluid, then this substance tries to collect itself
at the surface in greater concentration than in other parts of the fluid
(Ostwald,5 Freundlich 6
). In regard to the surface tension of solid-
fluid we only know that it is positive, but can otherwise show great
differences (Ostwald,7 Hulett 8
). According to this theory the facts
are that certain solid substances possess the ability of adsorbing dis-
1
Journ. Chem. Soc, 69, 1334 (1896).
2
Debar die Adsorption in Losungen, Leipzig (1906).
» Ann. d. Chem. u. Pharm., 283, 360 (1894).
* Ber. d. d. chem. Gesellsch., 37, 3138 (1904).
•Lehrh. d. allg. Chem., 2. Aufl., 2. Bd., 3. Teil, 237 (1906).
8
Ueber Adsorption in Losungen, 50-51.
7
Zeitschr. f. physik. Chem., 34, 495, 1900.
•Ibid., 37, 385 (1901).
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