‘ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 27 
constants) ; but which is really due to a complicated osmotic equilibrium 
which must be further explained.® 
Gelatine, being amphoteric, acts as a very weak alkali in presence of 
hydrochloric acid, and forms a gelatine chloride, which like most salts 
is highly ionised (in fact to practically the same extent as hydrochloric 
acid itself). The base, however, is very weak, its ionisation being of the 
same order as that of water, and consequently the salt is largely hydrolysed, 
and can only exist in the presence of free acid. Thus gelatine base, 
gelatine chloride, and free hydrochloric acid are necessarily prok in 
such a jelly in proportions determined by the concentration of the acid, 
and instead of a definite point of neutrality such as is given by a strong 
base, we have only a curve approaching, but never reaching, complete 
neutralisation. This is the explanation of the apparent indefiniteness 
of proteid compounds, which has led many chemists to deny the existence 
of definite proteid salts. By determining the hydrolysis curve and 
calculating the upper limit to which it tends, it is possible to calculate 
quite definite combining equivalents. Procter’ in this way, on the 
assumption that gelatine had a second valency negligible at low acid 
concentrations, found a combining weight of 839, while Wilson,’ from 
the same experiments, but regarding the apparent second valency as due 
to incipient decomposition or experimental error, found 768. It is not 
to be supposed, however, that these comparatively low weights represent 
the real complexity of the probably polymerised molecule in aqueous 
solution, but merely the smallest molecular division chemically possible. 
It has been stated that the swelling of gelatine in acids is due to a 
complicated osmotic equilibrium, and that it reaches a maximum at 
a very small acid concentration, and is repressed if the concentration 
is increased. A similar repression is caused by the addition of any salt 
with the same anion to the outer solution, and as neutral salts have 
no decomposing effect on gelatine, the repression can be carried much 
further than with acid; thus a chloride jelly treated with sodium or 
potassium chloride is reduced to a horny mass. Seeing that the jelly 
is almost as permeable as water both to ionised and unionised salts and 
acids, it is hard to see how this repressive osmotic pressure is exerted. 
The following is the explanation :— 
In equilibrium between a jelly and its external solution not only 
must all osmotic pressures be equally balanced, but, as has been shown 
by Donnan,’ the electro-chemical condition must be fulfilled that 
the products of the concentrations of any pair of diffusible anions and 
cations common to both phases must be equal. Thus with gelatine 
chloride and free acid the chloridions multiplied by the hydrions must 
be equal in the jelly and the external acid. On the other hand, the 
osmotic pressures depend not on the products but simply on the swm 
° It may be well to point out here that the ‘ adsorption formula’ just quoted is 
absolutely void of theoretical basis, as regards adsorption, but is a mathematical 
expression which will closely represent any chemical or physical phenomenon which 
proceeds at a diminishing ratio, It is, for instance, the exact law of distribution of 
a solute between two immiscible solvents, in one of which its molecular complexity 
is p times that in the other, 
77.C.8., 1914, 105, 320. 
8 J.A.L.C.A., 1917, 12, 108, 
9 Zeits. Elektrochem., 1911, 17, 572; Donnan and Harris, 7'.0.S., 1911, 99, 1575. 
