It must be clearly understood that all sweating depends on partial putrefaction. This is proved both by the plentiful production of ammonia in the pits, and by the fact that antiseptics, such as salt or carbolic acid, entirely prevent sweating till they are removed. Although the process undoubtedly has advantages, and especially so in the treatment of dried hides, it is an open question whether it gives the extreme gains over liming in weight and firmness, which are claimed by some of its advocates. An unhairing process, largely coming into use on the Continent, depends on the action of alkaline sulphides, and particularly sodium sulphide, upon the hair. While all the methods already spoken of involve the softening and destruction of the hair-sheaths, either by lime or by putrefaction, the sulphides are peculiar in attacking the hair itself; when strong, they disintegrate it rapidly and completely into a sort of paste. From very early times to the present day, arsenic sulphide ("rusma") mixed with lime has been used in unhairing skins for glove-leather and similar purposes. About 1840, Böttger concluded that the efficacy of arsenic sulphide was due simply to the sulphydrate of lime formed by combination of the sulphur with the lime, and proposed lime sulphydrate, formed by passing sulphuretted hydrogen into milk of lime, as a substitute for the poisonous and expensive arsenic compound. It proved a most effective depilatory, but has never obtained much hold in practice. This is probably due to the fact that it will not keep, oxidising rapidly on exposure to the air; hence it must be prepared as it is required, which is both troublesome and expensive. A minor objection is the unpleasant smell of sulphuretted hydrogen, which is inseparable from its use. It was proposed to replace it by sodium sulphide, which, though at first said to be only effective when mixed with lime, so as to produce calcic sulphide, has since proved a powerful depilatory alone. Its use has been greatly extended on the one hand by its production on a large scale, and in the crystallised form (at first by reduction of sulphate by heating with small coal), and on the other, by the great interest which Wilhelm Eitner, the able director of the Austrian Imperial Research Station for the Leather Trades, has taken in its introduction. The substance, as manufactured by De Haen, of List, Hanover, is in small crystals, coloured deep greenish-black, by iron sulphide, which must have been held in suspension at the time of crystallisation. If the salt be dissolved in water, and the solution be allowed to stand, this is gradually deposited as a black sediment, leaving the supernatant liquor perfectly clear and colourless. Sodium sulphide is now manufactured from tank waste in a much purer form by Schaffner and Helbig's process, of which Messrs. Gamble of St. Helens are sole licencees. The crystallised salt is SNa_{2}10Aq, and therefore contains 69·8 per cent. of water.