Sodium carbonate occurs in commerce in three forms: “soda ash,” a more or less pure dry sodium carbonate; “soda crystals,” or washing soda, Na₂CO₃·10Aq, containing 62·95 per cent. of water of crystallisation, and efflorescing in the air; and Gaskell and Deacon’s “crystal soda,” Na₂CO₃·1Aq, containing only 14·5 per cent. of water of crystallisation. It must be remembered that where carbonate is used in conjunction with lime it becomes causticised and converted into NaOH. _Sulphides._--The practice of using realgar, or red sulphide of arsenic (Ger. _Rusma_) as an addition to limes for fine leathers is one of considerable antiquity. It has the property of loosening the hair and epidermis structures with less solution of cement-substance than lime alone, and hence produces a leather of fuller and closer texture. It will, however, be convenient to defer the consideration of this agent till after that of some of the more modern and simpler substitutes, such as the sulphides of sodium and calcium. Sulphides of the alkalies and alkaline earths, if used in strong solution, say 5 per cent. or upwards, have the effect of very rapidly reducing the harder keratin-structures, such as hair and wool, to a pulp, attacking first the interior cells, so that the hair crumples up like a string of sausages, and in a few hours, or even, with very strong solution, in a few minutes, the whole mass becomes so completely disintegrated that it can be swept off the hide with a broom, or washed off in a tumbler. At the same time, the action on the substance of the hide, and especially on the cementing substance, is very slight, though the grain is swollen and temporarily rendered somewhat tender. On the other hand, when used in weak solutions, say ¹⁄₄ per cent. and under, in conjunction with lime, the hair is but little injured, while the hair-roots and dirt are rapidly loosened, and results are obtained very similar to those with arsenic. _Sodium Sulphide_ (Na₂S·9OH₂).[80]--For the methods of valuation and determination of sodium sulphide, see L.I.L.B., p. 28. [80] In the Laboratory Book the water of crystallisation is given as 10 Aq. Later researches show that pure crystals of the commercial sulphide only contain 9 Aq., or 67·5 per cent. of water. Hides suspended in solutions of sulphide of sodium of 2 to 3 per cent. strength unhair rapidly.