The best method is to mix 4 parts of barytes with 1 part of bituminous coal and so much coal-tar that a plastic mass is formed, which is well kneaded and made into small cylinders 3 centimetres in diameter and 10 centimetres long. These cylinders are placed in layers in a cylindrical furnace with a good draught, which contains at the bottom a layer of coal 15 to 20 centimetres thick, then a layer of the cylinders, then again coal, and so on until the furnace is full. The lowest layer of coal is lighted, and the whole burnt at a bright red heat, when the barium sulphate is changed into sulphide. Hydrochloric acid is poured over the residue and the insoluble part, consisting chiefly of unaltered barytes, is used for the next operation. Witherite (barium carbonate) can be converted into barium chloride in a very simple manner. Hydrochloric acid is added, in which it dissolves with the evolution of carbonic acid. The solution is allowed to stand twenty-four hours with excess of witherite; the whole of the dissolved iron is thus precipitated. The solution is then filtered, evaporated down and left, when pure barium chloride crystallises out. Barium chloride and all soluble barium salts should only be dissolved in pure water (rain or distilled). Water which contains carbonates or sulphates always gives a turbid solution by precipitating barium carbonate or sulphate. ALUMINIUM COMPOUNDS. The compounds of the earth metal aluminium play a very important part in colour making, since they form beautifully coloured compounds with many organic colouring matters. Formerly alum was the only material used in colour factories for the preparation of the alumina compounds; at present aluminium sulphate is used, and when it is sufficiently pure it is the most valuable material, because it contains the greatest quantity of alumina. Under the designation of alum only one compound, the so-called potash alum, was at one time found in commerce, but now there are other alums, which contain soda or ammonia in place of potash. These salts are of equal use in colour making to potash alum. The preference is to be given to the compound which contains the largest proportion of alumina. The chief point to be observed in connection with alumina compounds for use in colour making is that they shall be free from iron, because iron oxide, which would be precipitated out of the solution along with the colours, in consequence of its red colour would spoil the shade of the pigment.