of Giles's Theory of Surf aee- Concentration* 501 
(2) The reaction goes to an end almost instantaneously. 
(3) Heat effects, even in concentrated solutions, are 
undetectable. Chemical action, on the other hand 
at relatively low temperatures is usually accompanied 
by evolution of heat. 
Characteristics icltich distinguish Adsorption 
from Absorption. 
The difference existing here (which has been attested by 
numerous determinations) may be stated thus : There is not 
a direct proportionality between the concentration of the 
solution or partial pressure of the solute and the amount 
adsorbed. To take an example from Walker and Appleyard's * 
paper on the adsorption of picric acid at the surface of silk. 
The quantities of picric acid (remaining after the reaction) 
in the aqueous solution and in the silk itself are estimated, 
the following being the results obtained : — 
Table I. 
milligrams Picric Acid milligrams Pi< 
in I c.c. solution. ' in 1 gram 
;ric Acid 
silk. 
Eatio T . 

(«) 
0064 13 
0005 
1-98 37 
0053 
70 75 
0-094 
Assuming the molecular weight of the picric acid to be the 
me in water and silk, the ratio 
had true absorption taken place. 
same in water and silk, the ratio j should have been constant 
II. Theoretical Discussion. 
Gibbs's Theory of Surf ace Concentration. 
A theoretical investigation of this subject from the stand- 
point of thermodynamics forms one of the chapters in Gibbs'st 
memoir on ' ; Equilibrium in Heterogeneous Systems/'' He 
* "Walker & Appleyard. he. cit. 
t ' Scientific Papers of J. Willard Gibbs." English edition, vol. i. 
p. 219. 
