_Sodium and Potassium Hydrates._--From the earliest antiquity, wood-ashes, consisting mainly of potassium carbonate, have been used for unhairing, either alone or in conjunction with lime, and indeed the German name of the process (_Aeschern_) is derived from the fact. In more recent times, caustic soda, either ready formed, or causticised on the spot by the addition of lime, has often been recommended as a substitute for lime. Its action is very similar to lime, but, from its greater solubility, is far more powerful, and probably this has hitherto formed one of the greatest obstacles to its use, since a solution of the strength of lime-water is almost immediately exhausted, while a much stronger one is too violent in its action on the hides. Experiments made in the Author’s laboratory show that caustic soda, in solutions of the same strength as lime-water, dissolve considerably less hide substance than the latter, but it is more antiseptic than lime, and does not unhair readily without the aid of bacterial action (cp. p. 137). It also swells more violently, and it is difficult to keep the grain smooth and unwrinkled. Caustic soda has the great advantage that from its solubility, and that of its carbonates in water, it is much more easily and completely removed by washing than is the case with lime. It has been successfully applied in some instances to soften skins of which the texture is naturally too compact for moroccos and the softer leathers; and is usefully employed in softening dried goods (p. 115). Where caustic soda is required merely to “sharpen” limes, it is best added in the form of sodium carbonate (soda-ash or crystals), which are causticised by the lime in the pits. One-quarter or one-half per cent. on the weight of hides added in this way decidedly increases the plumping power of the lime. It may be noted that in the use of sodium sulphide in conjunction with lime, caustic soda is one of the products of its decomposition,[78] and is probably one great cause of the difference of effect of this material for sharpening limes as compared with red arsenic. [78] This has been denied, but is probably correct, though the actual reaction is not easy to prove analytically; but the effect on the hide is practically what is stated.