OL 
ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 29 
Wilson '? has extended these facts to a general theory of colloids 
and adsorption, showing that all surfaces must possess a potential due 
to unbalanced chemical forces on the surface, and therefore in a liquid 
containing electrolytes must condense ions or particles of the one sign 
on its surface, and repel those of the opposite sign; and also showing 
that surfaces must therefore be surrounded with a film of liquid of different 
concentration to the bulk, to which the same considerations and equations 
are applicable as to the absorbed solution of colloid jellies. For fuller 
mathematical treatment the reader is referred to original papers. 
Some of the views just explained are so recent that their bearing on 
actual tanning processes has hardly had time to make itself felt in technical 
literature, and for its latest applications Papers by Procter and Wilson 
must be consulted,’ but a brief summary may here be given. 
As has been explained, the leather-hide, freed from epidermis, consists 
of a sort of felt of fibres of gelatigenous tissue, which are themselves 
bundles of finer fibrils cemented together by some substance nearly 
identical with, but somewhat more soluble than that of the fibrils them- 
selves. Treated with dilute alkalies or acids, this cementing substance 
is more or less completely dissolved, and the fibrils themselves are swollen. 
Accurate chemical investigation of skin is complicated by this fact of 
structure, for while the free acid or alkali absorbed in the jelly of the fibre 
is subject to the mathematical laws which have been explained, the 
interstices between the fibres are also filled with external solution by 
capillarity, and no accurate means has been found of measuring the 
proportion between the two. Hide swollen in acid or alkali is tense and 
firm, and containing its liquid in jelly-form in the fibres only parts with it 
under heavy pressure ; but when the fibres are dehydrated by neutralisa- 
tion, the skin becomes ‘ fallen’ or flaccid, and apparently much wetter, 
since the imbibed water is easily squeezed out. If in this condition the 
loose water is removed by soaking in alcohol or other dehydrating agents, 
the fibrils no longer adhere to each other, and a soft leather is produced, 
which, however, on again soaking in water, rapidly returns to its raw 
or ‘pelt ’ condition.’ If, however, a little stearic acid is dissolved 
in the alcohol so as to coat and partially waterproof the dehydrated 
fibrils, the leather at once becomes tolerably permanent. This led Knapp 
to the view that the process of tanning was merely an isolating and coating 
of the fibrils, and, though the explanation is incomplete, it unquestionably 
is part of the true one. 
In order to make a soft leather, it is therefore necessary to have the 
skin in a flaccid or unswollen condition, and, assuming that it has been 
swollen by lime, this is brought about essentially by neutralisation. 
The older processes depend on fermentations of bran, pigeon and dog 
dung, and the like, and just as liming serves the several purposes of 
swelling the hide, loosening the hair, and partially saponifying the 
fat, so these fermentation processes not merely neutralise the lime 
by weak acid, or salts of weak bases, but remove cementing substance 
2 Jour, Am. Chem. Soc., 1916, 38, 1982. o 
13 Procter, Koll. Beihefte, 1911, 2, 270; 7'.C.S., 1914, 105, 313; and W ilson, 
7'.C.8., 1916, 109, 307; ‘Swelling of Colloid Jellies,’ J.ALCA., 1916, 11, 399; 
and Burton, D., ‘The Swelling of Gelatinous Tissues,’ JSC ., 1916, 35, 404, 
14 Knapp, Natur und Wesen der Gerberei, Braunschweig, 1888 ; Meunier and 
Seyewetz, Coll., 1912, 11, 54. 
