If the hydrochloric acid were just sufficient to combine with the whole of the sodium, we should have an equilibrium containing much (ionised) sodium chloride and little sodium acetate, together with much free acetic acid, and little hydrochloric. Thus the “strong” acid would displace the weak one. Taking another example, we add sodium acetate to a solution of acetic acid. As the ionisation-pressure of the acetic acid is much less than that of sodium acetate, and both have a common acet-ion, the ionisation of the acetic acid will diminish, and more undissociated acetic acid will form, till by its concentration the two pressures are equalised. The total quantity of free acetic acid will be unchanged, but a less proportion of it will be ionised, and it will act like a weaker acid. This reduction of the activity of a weak acid by the addition of its neutral salt is often made use of by chemists. Instances in tanning practice are the use of excess of potassium dichromate with chromic acid in the chrome tanning process, the effect of neutral salts in “mellowing” the action of tanning liquors, and the use of salt in “pickling.” Let us now try to apply these facts to the physics of tanning, taking first the simplest cases, where electrolytic dissociation does not take place. We may consider the wet hide as made up of a mass of fibres of gelatine-jelly, with interspaces which are filled with water. In fact, for many purposes of experiment we may substitute for hide, mere sheets of swollen gelatine, so as to avoid the complications introduced by the water or solution mechanically retained between the fibres. If we place a sheet of dry gelatine in water, it swells, absorbing perhaps seven or eight times its weight of water, but does not appreciably dissolve. A condition of equilibrium is reached when the attraction of the water-molecules for the gelatine is equal to the sum of the cohesive attraction of the gelatine for itself and the internal attraction of the water outside. An increase of the cohesion of the gelatine would tend to make it contract and expel part of the water, and this contraction would tend further to increase both the cohesion of the gelatine, and its attraction on the diminished number of water molecules it contained, and clearly these causes would act in opposing directions. The equilibrium is therefore a very unstable one, and slight causes might be expected to produce great changes in the degree of swelling, which is indeed the case. If we increase the temperature we diminish the cohesion of the gelatine, till at a point it becomes less than its attraction for the water, and the jelly suddenly loses its solid condition and dissolves.