The following may be suggested as a working hypothesis. As both water and hydrochloric acid can pass freely in and out of the jelly, it must be in osmotic equilibrium with the outer solution in every respect, and neither the un-ionised hydrochloric acid of the solution, nor the small amount which may be formed by hydrolysis of the gelatine compound can have any effect on the swelling. So long as the outer solution is very dilute, by far the greater part of the acid present is absorbed and fixed by the gelatine, and almost the whole of the outer acid will be ionised, as well as a portion of that in combination with the gelatine. In the latter case, however, the ions will be unable to pass out of the jelly, and will therefore cause an internal osmotic pressure, and the gelatine will swell till the Cl-ions are in osmotic equilibrium with those of the outer solution. At the same time, this internal pressure of Cl-ions will oppose the entry of the Cl-ions (and therefore also of their associated H-ions) from the outer solution, and the acid solution absorbed mechanically will be somewhat less concentrated than that outside. As the concentration of the outer solution is increased, the pressure of the outer Cl-ions will repress the ionisation of the gelatine-chloride, and at the same time its tendency to hydrolyse. Thus the acid actually combined with the gelatine should somewhat increase, but the swelling should diminish, as is actually the case.[57] It is impossible to carry the concentration of the hydrochloric acid much above 5 grm. per liter without causing solution of the gelatine, but the addition of common salt to the outer solution should equally increase the pressure of its Cl-ions, and cause further diminution of swelling, the Na-ions in this case increasing the outside pressure in the same way as the hydrogen ions. In fact the addition of salt in sufficient quantity will reduce the swelling till the gelatine becomes quite solid, and retains only about its own weight of water, while at the same time the apparently combined acid largely increases. This cannot be attributed to any direct dehydrating action of the salt, since concentrated sodium chloride solutions have no dehydrating, but rather a swelling effect on gelatine in the absence of acid, and the concentration of the salt in the outer solution and in the jelly proves precisely the same within the limits of experimental error. Several other facts may be noted, tending to support the explanation which has been given. The tendency to swell gelatine is common to all acids of appreciable strength, and in all cases where the concentration of the acid could be increased to a moderate extent without causing solution of the jelly, the effect of a maximum swelling, diminishing as the concentration of the acid increased, has been observed. Other salts also produce similar effects to sodium chloride; thus the swelling caused by sulphuric acid is repressed by sodium sulphate.