500 Mr. W. C. M. Lewis : Experimental Examination 
possible to measure interfacial- or surface-tension — a funda- 
mental factor in the thermodynamic consideration of the 
question. 
Jlie Characteristics of Adsorption Phenomena. 
The earlier investigations on adsorption dealt in general 
with the adsorption of gases on the surfaces of various solid 
materials. The case of vapours is of much more recent date, 
the most important instances being those of Travers * and 
Trouton f- Modern investigators have, however, dealt 
chiefly with the question of the adsorption of dissolved sub- 
stances at the surface of some solid body in contact with the 
solution. Among the numerous recent memoirs on the 
subject may be mentioned those of Krister J, Schmidt §, 
Walker & Appleyard ||, Biltz % Lagergren **, and in par- 
ticular Freundlich ft. Freundlich/s investigations contain 
the most systematic and accurate determinations yet recorded 
of the adsorption at a charcoal surface of a series of organic 
substances in aqueous solution. The general method was to 
prepare a solution of a certain substance (say a dye-stuffy and 
to a known volume of this solution to add a certain quantity 
of very finely powdered pure blood-charcoal. A certain 
amount of the dye is removed by the charcoal, the change in 
concentration being determined colorimetrically. 
Among Freundliclr's most important results may be men- 
tioned that the equilibrium state is reached exceedingly 
rapidly — in about five seconds ; that, using the same solid 
body the quantity adsorbed varies with the chemical con- 
stitution of the solute, e.g. aromatic acids are adsorbed in 
greater quantity than aliphatic ; and finally, that the effects 
differed with the solvent used. 
We may sum up the characteristics of adsorption which 
differentiate it from chemical combination thus : — 
(1) The order in which a series of dissolved substances 
are adsorbed does not differ even when one sub- 
stitutes as the adsorbing material, bodies as unlike 
as charcoal, silk, clay, and cotton. This is unlikely 
on the assumption of chemical combination. 
* Travers, Proc. Roy. Soc. series A, vol. Ixxviii. p. 9 (1907). 
+ Trouton, Proc. Roy Soc. series A, vol. lxxvii. p. 292 (1906). 
X Kiister, Zeitschrift Phys. Chemie, vol. xiii. p. 445 (1894). 
§ Schmidt, Zeit. Phys. Chem. vol. xv. p. 60 (1894). 
II Walker & Appleyard, Journ. Chem. Soc. vol. lxix. p. 1334 (1896). 
If Biltz, Per. cl. deut. chem. Gesell. vol. xxxvii. p. 1706 (1904) ; 
vol. xxxviii. p. 2963 (1905). 
** Lagergren, Zeit. Phys. Chem. vol. xxxii. p. 174 (1900). 
tt Freundlich, Zeit. Phys. Chem. vol. lvii. p. 385 (1906); vol. lix. 
p. 284 (1907). 
