Lime by itself does not readily attack the hair bulbs, and the slowness of the process has led to the introduction of other chemicals, generally for use in conjunction with lime. The principal of these are sulphide of sodium and red arsenic. Mixtures of sulphide of sodium and lime, or red arsenic and lime are now largely used, the former for hides, calf, and sheep skins, and the latter for goat and kid skins. Both sulphide of sodium and arsenic dissolve keratinous matters (horns, hair, etc.) and workmen should, therefore, be provided with rubber gloves to prevent the loss of their finger nails. Sulphides naturally lower the commercial value of the hair removed and, if used alone, destroy it. In admixture, however, the hair has some value, although it is not so good as that removed by the use of pure lime. Against this, however, there is a great saving of time and less loss of hide substance and, therefore, increased weight of leather. Sulphide of sodium is prepared in crystallised or concentrated form; the former, about 30 per cent. strength, is preferred in Great Britain, while the latter--65 per cent. strength--is prepared for export, the main object being to save the cost of transit of 35 per cent. of water. About thirty different sulphide salts may be used, but the sodium and arsenic disulphides are the best, as it has been shown that the most rapid loosening of the hair occurs where the quantities of sulphur and alkali are nearly the same. There are two arsenic salts used in the trade, namely, realgar, or red sulphide of arsenic (As_{2}S_{2}) and orpiment, or the yellow sulphide (As_{2}S_{3}), but the former is often preferred as it is said to give better results than the latter. In practice, the proportion of arsenic used is 1 part in 20 parts of lime, although it naturally varies a little according to the class of skin under treatment. Sulphide of sodium is used in the proportion of 1 part to 10 of lime, or, if concentrated sulphide be used, 1 in 20. The quantities of lime, or lime and sulphide, used are estimated on the weight of the raw hides. For hides intended for sole leather, 5 per cent. of lime on the weight of hides is ample, while a little more may be used for hides intended for dressing leather (_i.e._, leather which has to be dressed or finished with a certain degree of flexibility for bags, boot uppers, etc.). When a mixture of lime and sulphide is used, 3 per cent. and 0.3 per cent. respectively is a satisfactory quantity. The action of this mixture on hides is complex and has not yet been definitely ascertained, but it is thought that the calcium sulphydrate formed by the chemical reaction between sulphide of sodium and calcium hydrate (slaked lime) is the active principle.