ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 31 
stretching, forms a very perfect white leather, which, however, softens 
and swells at once in water through the removal of the restraining salt. 
It is not essential that the acid should be a ‘ strong’ one. Skins pickled 
with formic acid and salt by Mr. Seymour-Jones were sent on a voyage 
up the Amazon, and returned in perfect condition. It is obvious that 
if a skin swollen with some acid other than hydrochloric be subsequently 
treated with salt, a quadruple equilibrium results, most of the proteid salt 
being converted into chloride by the great excess of sodium chloride, with 
the formation of the sodium salt of its acid, each proteid salt being 
balanced against its own anion in the external solution. In a direct 
experiment with gelatine formate almost the whole of the formic acid 
was replaced by hydrochloric. It is probable that the so-called ‘ free’ 
hydrochloric acid in the gastric juice has been liberated in this way, and 
really exists as a salt of some weak colloid base. 
In the ordinary processes of production of ‘ alumed leathers’ it is 
impossible to work without considerable addition of salt, and the process 
is largely a pickling one, the hydrolysed acid of the aluminium salt com- 
bining with the skin and leaving a basic salt which is also absorbed, 
the quantitative relation between the two independent actions depending 
on the relative concentrations. If, instead of alum or normal aluminium 
sulphate, a basic alumina solution is used, salt can be reduced or dispensed 
with, and the tanning action depends less on pickling and more on the 
fixation of alumina. What has been said about alumina tannage applies 
with little variation to tannage with chrome and iron salts. 
As regards the fixation of alumina and chrome, there is little doubt 
that in the first instance it takes place in the form of basic insoluble 
salts and is largely physical. The more basic a solution of these metals 
and the more readily and completely it is precipitated by the withdrawal 
of a further portion of acid, the more heavily it tans. If we imagine 
a normal salt to diffuse into the skin, and its acid to combine with the 
amino group of the proteid, then the remaining insoluble basic salt must 
remain precipitated in and on the hide fibre. Whether this is the final 
stage may be doubtful—Wilson, in a recent Paper on ‘ Theories of Leather 
Chemistry,’'® suggests that ultimate combination takes place with the 
carboxyl group, and this view seems well in accordance with known facts. 
. L. Lumiére”? has shown that the maximum amounts of chrome 
and alumina which can be fixed by gelatine accord well with this view ; 
and Wilson points out that if, as he supposes, the uitimate gelatine mole- 
cule is monacid and monobasic, a divalent ion such as Ca**, joined to 
two gelatine molecules, should exert only the same osmotic pressure 
as the monovalent Na’, and hence its swelling effect should be much 
less as is known to be the case; while the trivalent Al‘ or Cr 
should swell still less and be yet more easily repressed ; and that therefore 
chrome or alumina gelatinates, if they exist, should be very stable and 
insoluble compounds. It is well known in practice that change in the 
direction of stability gradually takes place on storing or ‘ ageing ’ alumed 
leather, and probably the same is true of chrome, though not so easily 
demonstrated. 
Vegetable tannage appears to be of a more colloidal or physical character 
than that with alum,or chrome. Tannins, like the proteids, appear 
16 J.A.L.C.A., 1917, 12, 108. 
17 Brit. Jour. Phot., 1906, 58, 573; Abst., J.S.C.1., 1906, 25, 770. 
