Gas-lime is principally active on account of the calcium sulphide which it contains, but is very variable in its strength, as both sulphydrate and sulphide are decomposed by the carbon dioxide always present in the gas, forming carbonates. Lime has nearly gone out of use for purifying gas, its place being now taken by iron oxide, but formerly gas-lime was a good deal used for unwooling the small lambskins used for the commoner sort of glove-kid, usually by painting a cream of it on the flesh side, but sometimes by immersing in a strong solution, which of course destroyed the wool. Its place is now taken by a solution of sodium sulphide of 15°-18° Tw. (approximately 30-35 per cent. crystals), thickened with lime to a soupy consistence, the use of which is much to be recommended for unwooling sheep-skins. The tank-waste from the Leblanc process, consisting principally of calcium sulphide, is, when fresh, quite insoluble, and has no depilatory powers; but when exposed to air and moisture, decompositions take place, resulting in the formation of sulphydrates and polysulphides, which form a solution which has been the subject of several patents for unhairing.[81] Polysulphides alone have probably no unhairing effect, but in conjunction with lime, sulphydrates are formed which rapidly loosen the hair. This fact was the basis of an ingenious and effective unhairing process used very many years ago by Mr. John Muir, of Beith, who, after liming for 24 hours in the usual way, submitted the hides to a pretty strong solution of weathered tank waste for 24 hours, and finally to water for 24 hours, to remove the surplus lime and sulphides. The sulphydrates formed in the hide attacked the hair-roots with little injury to the hair itself, and the hides contained so little lime that they could be tanned for dressing without bating, and made about 10 per cent. more weight than those treated in the ordinary way. Some trouble was occasioned by stains caused by impurities in the tank-waste. [81] Squire, E. P., 756, 1855; Claus, E. P., 1906, 1855. A somewhat similar unhairing mixture to that obtained from tank-waste, which is now seldom to be got, was patented by Prof. Lufkin,[82] who mixed equal parts of sulphur and soda-ash with a little water till combined, and then added 8 to 10 parts of lime, slaked and still hot. Schultz[83] states that such a mixture containing 10 lb. of sulphur, will unhair fifty hides in the same way, and in about the same time as an ordinary lime, the pelt being little plumped and easily reduced without bating by a few minutes’ wheeling in warm water. By boiling lime and sulphur with water a yellow solution is obtained which can be used in the same way as that from the tank-waste. A further quantity of water can be boiled on the same materials, more lime and sulphur being added as required. Polysulphides appear to have a marked effect in preventing plumping. [82] Eng. Pat. 2053, 1860. [83] ‘Leather Manufacture,’ p. 35.