EE eS 
ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 25 
of gelatine) and physically is a network of fibres of colloidal jelly. In 
its natural state it has an outer coat of epidermis, with its hair and seba- 
ceous and sudoriferous glands, but it is not necessary in this Report to 
consider in detail the chemical, mechanical, and bacteriological processes 
which are used to free it from these appendages or to separate its fibre- 
bundles into their smaller constituent fibres. These gelatinous fibres 
in the dried raw hide adhere together to form an almost homogeneous 
horny mass, and the problem of the tanner is so to treat them, either by 
chemical change or by surface-coating, as to prevent their adhesion, so 
that on drying they remain isolated and free to move, and the skin conse- 
quently flexible and porous, and at the same time without tendency to 
putrefactive change. While the durability of wet raw hide is measured 
in days or weeks, Roman boots have been dug up which are hardly in 
worse condition than those which the tramp leaves, worn out, at the road- 
side. The methods employed by the tanner, though very similar in their 
effects, are so various that no single explanation, physical or chemical, 
will cover all of them ; and often various actions are combined to produce 
the desired result. 
The first general problem, then, regards the nature of the jelly state, 
which has many peculiarities. Wan Bemmelen? and Biitschli? believed 
it to be a network or cellular structure of microscopic dimensions, 
and this view long held the field, but is now abandoned for that which 
regards it as a solid or semi-solid solution of which the colloid and water 
(or some other solvent) are the constituents. The question is still an open 
one whether the colloid is in the form of ‘ micelle’ (submicroscopic 
particles) or of large conjugated or polymerised molecules, but this is 
mainly a matter of terms, and at least it is clear that the mixture is so 
intimate that both constituents are within the range of molecular and 
electro-chemical forces. 
The colloid most fully investigated in this relation is gelatine, which 
in its chemical constitution is almost identical with hide fibre, while its 
homogeneous character renders exact quantitative study much more 
possible. Soaked in water at laboratory temperature, it does not dissolve, 
but swells to a definite volume dependent to some extent on the particular 
sample and the temperature. When the temperature is raised above 25°, 
the jelly melts and becomes miscible in water in all proportions, though 
even when diluted considerably beyond its original equilibrium volume, 
it still ‘sets’ on cooling to a coherent elastic mass. Gelatine, both as 
jelly and solution, always shows a slight Tyndall effect, reflecting a beam 
of light sideways; but the ultramicroscope shows no defined particles. 
Arisz* has shown that the Tyndall effect increases with concentration 
and with lowered temperature, but without any break or sudden change 
at the setting point. The viscosity shows a similar increase, with no 
actual break, but a rapid rise at the temperature of gelatinisation, below 
which it speedily becomes too great to measure by ordinary methods. 
Both these effects are reversed on gradual heating, but there is a ‘lag’ 
in both directions, a cooling solution only acquiring its full viscosity and 
1 Z. Anorg. Chem., 1896, 18, 304; 18, 15. 
2 Untersuchungen iiber mikroskopische Schaume und das Protoplasma, Leipzig, 
1892. Verh. des naturh.~med. Vereins zw Heidelberg, 1892, N.F. 5, 28-41; ibid., 
42-43; ibid., 1893, 89-102; ibid., 1896, 457-472; ibid., 1894, 230-292. 
§ Kolloidchem. Bethefte, 1915, 7, 22. 
