50 Proceedings of the Royal Society of Edinburgh. [Sess. 
some cases to exceed unity may be ascribed to the difficulties of measure- 
ment in the region of low pressures. 
In the case of adsorption from solutions we are dealing with a double 
adsorption, namely, adsorption of solute and adsorption of solvent. As 
with gases, what is ordinarily measured is the excess of the surface concen- 
tration over the concentration outside the adsorbent ; but in the case of 
solutions, the difference between the excess and the actual concentration 
in the adsorption layer may be considerable. Denoting by u and w the 
amount in grams of solute and solvent adsorbed per gram adsorbent when 
in equilibrium with a solution containing c grams solute per gram solution 
we have the excess of the solute per gram adsorbent given by 
u n = u - to . 
1 - c 
u Q is directly measurable from the change in concentration of the solution 
on immersion of the adsorbent, and is in 
Q Q 
fact m. ® , where m is the 
1 — c 
Ur 
mass of solution per gram adsorbent, and c 0 the initial concentration, 
or an approximation to it, is usually called the amount adsorbed, and will 
be denoted by a. In practice it has been frequently found that in dilute 
solutions a is positive and increases with c. Careful examination of the 
(log c, log a) curve once more indicates that the curve is not a straight line, 
but is decidedly concave to the log c axis. This may be exemplified by 
Table III. 
c 
u 0 
1 
n 
. 
0 0000122 
0-0042 
347 
73 
0-52 
704 
90 
•31 
•000128 
•0108 
•28 
305 
130 
•23 
490 
142 
T8 
916 
155 
T4 
•00124 
165 
•21 
159 
171 
T4 
the following observations on the adsorption at 25° C. of hydrochloric acid 
from aqueous solution by blood charcoal. Hence if we use a = a 0 Cn to 
express the adsorption curve, — is again only a mean value of the logar- 
71 / 
ithmic curve gradient, which steadily increases as c diminishes. 
An examination of the values of the exponent ~ tabulated for different 
