Other metals are deposited electrolytically in exactly the same way. The metal to be deposited is joined to the positive pole and the article to be plated to the negative pole of the battery. Both are suspended in a solution of salt, generally the sulphate, of the metal which is to be deposited. Thus, for nickel plating, a piece of sheet nickel would be used in conjunction with a solution of sulphate of nickel or, better, a solution of nickel ammonium sulphate, made by crystallizing ammonium and nickel sulphates together. The current required is small; indeed, if it is too strong, the deposit adheres loosely to the article, and the result is, therefore, not satisfactory. Electrotype blocks are also made by a similar process. An impression of the article to be reproduced is made in wax, or some suitable plastic material, and polished with very fine graphite or black lead, in order to give a conducting surface. It is then suspended in a solution of copper sulphate and joined to the negative pole of the battery; a plate of copper connected with the positive pole is suspended in the same solution. When a weak current is passed, copper is deposited on the black-leaded surface and grows gradually in thickness, until at length it can be stripped off, giving a positive replica of the object. INDEX A Acetic acid (glacial), 73 Acids, early notions of, 1 ——, fatty, 78 ——, mineral, 68 ——, vegetable, 68 Agate, 61 Air-saltpetre, 42 Alkali Acts, 44 ——, caustic, 96 ——, metals, 95 ——, mild, 80 —— waste, 87 Alkalis, properties, 3 Aluminium acetate, 73 Alums, the, 26 Amethyst, 61 Ammonal, 36 Ammonia, 97 ——, synthetic, 99 Ammonite, 99 Ammonium carbonate, 99 —— chloride, 98 —— nitrate, 99 —— sulphate, 99 Anhydride, an, 21 Anode, 103 Argol, 76 Asbestos, 63 ——, platinized, 19 Ash, black, 84 ——, pearl, 93 ——, soda, 10, 92 Atolls, 51 Atomized water, 18 B Bacon, Roger, 32 Basic slag, 58 Basil Valentine, 12 Beryl, 63 Black liquor, 74 Blasting gelatine, 35 Bleaching powder, 46 Blue-john, 47 Boiler scale, 54 Bonbonnes, 31 Bone, 56 —— ash, 57 —— black, 56 —— meal, 56 Borax, 59 Bordeaux mixture, 7 Boric acid, 58 Boyle, Robert, 2 Burgundy mixture, 6