When sole-leather first goes into liquors, it is generally swollen with lime to some extent. If the liquors contain, as they usually do, sufficient free acid (acetic, lactic) in addition to the tannins, these combine with and neutralise the lime, and the pelt, without absolutely becoming flat and thin, loses its firmness, and becomes soft and spongy. This is a favourable condition for the absorption of tannin, but care should be taken not to allow the pelt to be squeezed or pressed, or water will be squeezed out, and the pelt will not easily resume its plumpness. As the tannage proceeds, both the tannin and the acid of the liquors penetrate deeper into the pelt, the former tending to contract and the latter to swell the fibres. Thus a given quantity of acid will cause the greater swelling, the less tannin is present; and therefore in strong tanning liquors more acid is required. The presence of certain products of bacterial putrefaction has a great but unexplained effect in preventing hide from swelling with acids; and in hot weather, much better swelling is obtained by sterilising and deliming the hides with one of the coal-tar products mentioned on pp. 30, 162. Boric acid may also be satisfactorily used for this purpose, but should not be allowed to get into sole-leather liquors, as it tends to produce a soft and loose tannage, and from its inorganic and indestructible character, is apt to accumulate in a yard in which it is used. The same reasons render unadvisable its introduction into any liquors which are to be returned to the leaches even in the tannage of dressing leather, though its presence in the colouring liquors is otherwise very useful in lessening the astringency of the tannins (“mellowing the liquors”), and making a fine grain. Its mode of action is by no means clearly explained, but is in some way connected with its tendency to produce “conjugated acids” (L.I.L.B., pp. 37, 46).