For the commoner classes of sole-leather, hair is frequently removed by painting on the hair side with a 15°-28° Tw. (30-40 per cent.) solution of (crystallised) sulphide of sodium thickened with lime, applied with a fibre-brush, and folding the hide in cushions in a damp place, or packing in a tub. The hair is reduced to paste in a few hours. The same effect is produced by drawing the hides through a similar solution without lime, of which sufficient is retained by the hair to destroy it. The workmen must be provided with indiarubber gloves to prevent the caustic effect of the solution on the skin and nails. Skins and lighter hides are conveniently unhaired by painting the mixture on the flesh side, when it will loosen the hair or wool in a few hours without destroying it. For dressing-leathers and the finer sorts of sole it is best employed as an addition to ordinary limes to the extent of ¹⁄₄-¹⁄₂ per cent. on the weight of the hides or skins, when the hair is loosened more rapidly than with lime alone, and with less loss of hide substance. Good samples of sulphide of sodium consist of pale-brown, almost colourless crystals, containing 28 to 32 per cent. of dry sodium sulphide, which readily deliquesce on exposure to air. Fused sodium sulphide can now be obtained, which contains nearly twice as much actual sulphide as the crystalline form. The dark green colour possessed by many samples of sodium sulphide is due to the presence of iron sulphide. If carefully used no serious harm can accrue from its presence. If allowed to stand a short time in solution the iron sulphide will settle out. _Calcium sulphydrate_, Ca(SH)₂, sometimes called Böttger’s _Grünkalk_, is a powerful depilatory, while it has probably less destructive action on the hide-fibre than even the sulphide of sodium, and would no doubt be largely used but for its unstable character. It is probably the principal active product produced by the use of sulphide of arsenic in conjunction with lime, though it is possible that a sulpharsenite may be formed. It may be produced by passing hydrogen sulphide (SH₂), into milk of lime. According to von Schroeder, it is _not_ formed by the reaction of sodium sulphide on lime solutions (see note, p. 136). It may be obtained crystallised, and is soluble in water, but is decomposed on boiling. The sulphide, CaS, is insoluble in water, but by the action of steam under pressure it is said to be converted into a mixture of equivalent parts of hydrate and sulphydrate. It may also be dissolved in a solution of hydrogen sulphide, forming a solution of sulphydrate. In this way it might be formed on a large scale from the “tank waste” of the Leblanc soda process.