The production of caustic soda by an electrical method still remains to be fully developed. A process which gives only a 20 per cent. solution cannot be looked upon as final. In the meantime, other methods have been tried, in some of which fused salt is used in place of brine in order to give caustic soda in a more concentrated form. For a description of these methods, the reader must consult some of the larger works mentioned in the preface. Here we can only say that very great difficulties have been encountered, particularly in the construction of a satisfactory porous diaphragm or, alternately, in devising methods in which this can be dispensed with.
Another interesting application of electrolysis is furnished by the use of copper sulphate in industry. When this salt is dissolved in water, it breaks up into copper ions (positive) and an equal number of negative ions, composed of 1 atom of sulphur and 4 atoms of oxygen (SO″4). Under the influence of the current copper ions travel to the cathode, and there by the gain of two electrons become copper atoms. Now, since copper is not soluble in copper sulphate solution, and is not volatile except at very high temperatures, it is deposited on the cathode in a perfectly even and continuous film when the strength of the current is suitably adjusted. This film continues to grow in thickness as long as the conditions for its deposition are maintained. If the current employed is not suitable, the metallic film is not coherent, and the copper may appear as a red powder at the bottom of the cell. Any other metal or impurity which might be present in the unrefined copper falls to the bottom of the tank.
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]
C Calcium acetate, [5] —— bicarbonate, [54] —— carbonate, [50] —— fluoride, [47] —— nitrate, [29] —— phosphate, [56] —— sulphate, [27] Calc spar, [50] Caliche, [29] Calico printing, [26] Carbon, [49] Carbonic acid, [49] —— —— gas, [49] Castner process, [105] Catalytic action, [20] Cathode, [103] Cat’s-eye, [61] Cavendish, H., [40] Cellulose, [46] Chalcedony, [61] Chalk, [50] Chert, [66] Chili-saltpetre, [29], [39] China clay, [62] Citric acid, [77] Chlorides, [47] Chlorine, [46] Chrome yellow, [28] —— red, [28] Compound, [7] Compounds, binary, [8] Contact action, [20] —— process, [18] Copper refining, [107] —— sulphate, [5], [27] Coral reefs, [51] Cordite, [34] Cream of tartar, [76] Crops, rotation of, [37] Crystallization, water of, [9] Crystals, [9]
D Davy, Sir Humphry, [95] Derbyshire spar, [47] Devitrification, [65] Dynamite, [35]
E Efflorescence, [82] Electrode, [103] Electrolysis, [102] Electrons, [103] Electrotype blocks, [107] Element, definition of, [7] Elements, list of, [8] Explosives, [32]