=Aluminium Sulphate= (Sulphate of Alumina), Al₂(SO₄)₃.18H₂O = 664.—Any manufacturer who can obtain cheap china clay (kaolin) and sulphuric acid can himself prepare this compound with advantage. The apparatus used for this purpose is an iron pan containing sand, in which is placed a large earthenware dish. In this dish are put very finely ground kaolin and strong sulphuric acid, and the mixture is heated so strongly that the acid boils, evolving heavy, white vapours. It is absolutely necessary to heat in this manner in order to avoid dangerous accidents. Sulphuric acid bumps so violently on boiling that it may even break a thick earthenware dish. The use of a sand bath makes the bumping harmless. China clay, which consists of silicate of alumina, is decomposed by heating with sulphuric acid into silicic acid and sulphate of alumina. The original milky liquid becomes more transparent during boiling, and has at last the appearance of starch paste. Kaolin contains varying quantities of silica. The quantity of sulphuric acid necessary for its decomposition can only be found by trial. The quantities are chosen so that a small amount of kaolin remains undecomposed in order that the aluminium sulphate shall contain no free sulphuric acid. When the decomposition is finished the pan is allowed to cool and the solid mass is brought into a vat filled with water, in which it is stirred until dissolved; then the liquid is left until the jelly-like mass of silicic acid has sunk to the bottom, when the clear solution of aluminium sulphate is drawn off and can at once be used. If solid aluminium sulphate is required—and this is to be recommended when large quantities are to be prepared—the solution is evaporated in earthenware dishes until a portion solidifies when dropped on a cold plate. The molten aluminium sulphate is then cast in prismatic blocks, which are preserved in boxes. These blocks are of a pure white colour and very crystalline; they dissolve with difficulty in cold, but readily in hot water without residue. The solution has an acid taste, even when it contains no excess of sulphuric acid. When the blocks have a yellowish tinge, this denotes the presence of iron, and the solution must be freed from this impurity. Nowadays sulphate of alumina can be obtained so cheaply that it is hardly of advantage to make it. THE ALUMS. These are double salts of aluminium sulphate and potassium, sodium or ammonium sulphate. There are also other double sulphates known as alums which, in place of aluminium, contain chromium, iron, etc., but they are not of interest here. It may still be observed that all alums, whatever their composition, possess the property of crystallising together from mixed solutions, so that crystals can be obtained in which every existing alum is contained. The potassium, sodium and ammonium aluminium alums are used in colour making.