28 REPORTS ON THE STATE OF SCIENCE.—1917. 
of diffusible particles present. In the external acid the numbers of 
hydrions and chloridions are obviously equal, while in the jelly the 
chloridion of the gelatine chloride is added to the equal hydrion and 
chloridion concentrations of the free acid present, thus making the final 
concentrations of these ions in the jelly unequal. Now, as the sum 
of two unequal factors is always greater than that of two equals giving 
the same produet, or, geometrically the perimeter of a square is always 
less than that of any other rectangle of equal area, and as the sides re- 
present the osmotic pressure, while the area represents the product, 
it is clear that the two equalities cannot at once be completely fulfilled, 
but in electro-chemical equilibrium the osmotic pressure must be in 
excess and the jelly must tend to swell unlimitedly and finally to dissolve. 
That it does not do so is a consequence of its colloid nature, which depends 
on cohesive attractions drawing the colloid particles together to poly- 
merised masses or to a continuous network, and which consequently 
opposes swelling and solution, while the diffusible ions are held to the 
colloid ions by electro-chemical attractions, and, as they cannot escape 
from the jelly, tend to drag it apart and dilute it by absorption of the 
external acid, from which they expel a part of its acid concentration. 
The equilibrium is therefore a very complex one, but finally depends 
on the excess of internal osmotic pressure being balanced against the 
internal attraction or cohesion of the colloid particles, both ions and 
molecules. For its mathematical discussion the reader must be referred 
to original papers by Procter and his pupils. It will, however, be obvious 
that as the external solution becomes more concentrated the proportion 
of absorbed acid (or salt) is increased, while that of gelatine chloride 
is limited to the quantity of gelatine present. The difference of con- 
centration of hydrion and chloridion in the jelly is therefore diminished, 
and it contracts under the influence of its own internal attractions. 
Precisely similar considerations apply to the action of alkalies on gelatine. 
Tonisable salts are formed by combination of the base with the carboxyl 
group of the proteid, and the osmotic equilibrium is with the cation 
and OH instead of with the anion and H. Neutral gelatine, as an ampho- 
teric body, of course ionises to a limited extent with water alone, and 
its dissociation constants are of the same order of quantity as those of 
the water with which it isin equilibrium. It is, however, slightly stronger 
as a base than as an acid, and consequently its neutral point of minimum 
swelling is slightly on the alkaline side. This has important bearings 
on manufacturing practice, the greatest flaccidity of the raw skin, which 
is required for the softest leather, being obtained in weakly alkaline liquids. 
It has been pointed out by Donnan '° that in consequence of the 
unequal distribution of positive and negative diffusible ions which has 
just been described, the surface of an acid or alkaline jelly in equili- 
brium has necessarily an electrical charge or potential, greatest at the 
maximum swelling, and such charges seem an essential condition of the 
colloid state. The surface is positive or negative according to whether 
the diffusible anion or cation is retained in the colloid. Thus gelatine 
and hide fibre are negative in alkaline and positive in acid solutions, and 
it will be shown later that this has an important bearing on the theory 
of leather manufacture."* 
10 Zeits. Hlektrochem., 1911, 17, 579. 
" See below and Papers under 13. 
