The effect of acids and alkalies has been studied by Procter and others on actual pelt as well as on gelatine, and has been found to be qualitatively, if not quantitatively quite similar to that on gelatine, though from the acid retained mechanically in the interfibrous spaces, exact quantitative determination is more difficult. The amount of swelling produced is not proportional to the strength of the acid, some weak and little ionised acids such as lactic producing larger swelling than stronger acids such as hydrochloric and sulphuric, of which the ionic pressure in the external solution is greater. Dilute solutions generally produce greater swelling than more concentrated, so that where swelling is required without destructive effect on the fibre, dilute solutions of such weak acids are to be preferred, and the presence of neutral salts is to be avoided. On the other hand, where it is desired to remove lime, or to bring the pelt into an acid condition without swelling, the addition of neutral salts, and especially of chlorides is advantageous. A very important application of this principle is the “pickling” of sheep-skins, and especially of sheep-grains, in order to preserve them for export. The principle of this operation is that the skins are first swollen slightly with sulphuric acid, and the swelling is then reduced by salt, either added, or used in a subsequent bath. In practice, salt is now generally also added to the first bath to moderate the swelling. A suitable strength for the “rising solution” is about 80 grm. common salt, and 7·5 grm. sulphuric acid per litre. 100 c.c. of this solution will therefore require about 15 c.c. of N/1 alkali to neutralise it, and it should be tested after each lot of skins, and maintained at the same strength by suitable additions of acid. The acid absorbed by the skins is mainly hydrochloric, sodium sulphate accumulating in the bath. The salt is not absorbed by the skins in the same way as the acid, but will be continually diluted by the water they bring in, and occasional additions of salt must therefore be made, the density being maintained at about 65° Bkr. (1·065 sp. gr.) After paddling or being stirred in this bath for about ¹⁄₂ or ³⁄₄ hour the skins are transferred to saturated brine, and stirred in it till fully fallen in thickness, the density of the liquid being maintained by excess of salt. They may be allowed to remain some hours in the saturated brine with advantage.