Besides salting them, there are other ways of curing hides and skins, and a large proportion is simply dried or dry-salted, the salt in the latter method being applied while the hides are moist. Dry hides, whether "flint" (i.e., simply dried) or salted, require special treatment to make them soft and pliable, without which it would be impossible to convert them into leather. Soaking in plain water is insufficient, as it would need too much time, during which there would be a great loss of gelatine. The use of chemicals and mechanical motion are required; sometimes chemicals alone will thoroughly soften the hides, but this is not often the case. Both alkalies and acids may be used for softening dried hides, and it is difficult to say which gives the better result, although the former are frequently used, mainly because treatment with acids is a comparatively recent innovation. Both kinds are effective, but the use of acids retards the loosening of the hair, although it has been proved experimentally that acid-softened hides give a slightly improved yield of leather compared with the effect of the alkaline process--a noteworthy advantage where the finished leather is to be sold by weight. The acid generally used--formic--has antiseptic properties, and can have no harmful effect on animal tissues, as is generally the case where mineral acids, especially sulphuric, are used. Sulphurous acid is also said to be a good softening agent for hides, but it is rarely used. The most satisfactory alkalies are sulphide of sodium (crystallised or concentrated) and caustic soda, and of these two, the sulphide is preferred in nine cases out of ten. The quantities used vary according to the condition of the hides, but the average is about 1 lb. of caustic soda, or 1 lb. of concentrated sulphide of sodium (65 per cent.), or 2 lb. of crystallised sulphide of sodium (30 per cent.) for every 100 gal. of water. Of formic acid, 1 lb. is sufficient for 100 gal. of water. It is possible to soften hides by these means without mechanical acid, but the time is considerably shortened by "drumming" the hides, _i.e._, placing them in a round or, preferably, a square tumbler fitted inside with shelves or staves (Fig. 13). The drum is rotated mechanically for a few hours, during which time the hides are thoroughly kneaded and softened by the alkali. Tanners often use a machine known as the "faller stocks" (Fig. 14), which kneads the hides very thoroughly, though somewhat drastically. The drum method is preferable, providing the hides can be suitably softened. An old method, and one that is used now by some of the small firms, is to "break over" (_i.e._, vigorously scrape) the hides with a curved blunt knife fitted into two wooden handles. For this manual operation, the hide is placed, flesh side up, over the tanner's beam (Fig. 15) and is then scraped with the knife. [Illustration: FIG. 15 TANNER'S BEAM]