heating, and power gas would be employed in making a gas of the above-named description. A single plant could thus supply a cheaper gas for heating and power, of which a portion could be mixed with acetylene for illuminating purposes."

Of the other chemical properties of acetylene, the following deserves the attention of the chemical industries. By the agency of platinum black acetylene unites directly with hydrogen to form ethane, CH. The direct conversion of the acetylene into ethylene, CH, by the addition of hydrogen is not so easily accomplished as would appear from recently published statements. But the possibility of producing alcohol from acetylene on a commercial scale by the formation of ethylene, the conversion of the latter into ethylsulphuric acid and the decomposition of this into alcohol, depends still upon many questions that have yet to be settled.

* [Note on the Use of Acetylene in the Liquid Form, and as an Enriching Agent for Illuminating Gas.—It must not be forgotten that acetylene is an endothermic compound, and is, in consequence, very unstable. As compared with ordinary heating or illuminating gas, a far smaller proportion, either of acetylene in air or of air in acetylene, suffices to produce a mixture that may be exploded by contact with a light. But Berthelot has shown that in proportion as acetylene is compressed it becomes more liable to true explosion of itself, without any admixture of oxygen; so that when the pressure exceeds 2 atmospheres, if any portion of the gas were locally heated to its temperature of decomposition, the action would spread through the whole of the gas. Such a result might even be produced by opening a cylinder of the compressed gas too suddenly into a very confined space. Liquid acetylene, when fired by an electrically heated wire, was dissociated so violently that in one case a pressure of over 35 tons per sq. in. was developed; and, by firing a charge of fulminate of mercury in the midst, the acetylene behaved like a high explosive (such as gun-cotton), shattering the steel flask in which it was contained. It is true that mere mechanical shock failed per se to explode even liquid acetylene; but in one instance, in which a cylinder was fractured, the mixture of gas and air became fired by the heat generated by friction between flying fragments of metal, and a violent explosion of the whole resulted. It is obvious that such accidents are likely to occur but rarely, yet the mere fact that there is so considerable an element of danger in connection with the use of acetylene, and that serious accidents have occurred, is likely to restrict the use of the material. Again, Lewes (Journal of Gas-Lighting, 1895, vol. lxv., p. 1067) has shown that although acetylene, when burned alone under favourable conditions, exhibits a luminosity of 240 candle-power (per 5 cub. ft.); yet when mixed with non-luminous (or inferior) gases it does not increase the luminosity in anything approaching to the ratio that it should, according to calculations based upon the proportions of the gases employed. This has been confirmed by Love (Journal of Gas-Lighting, 1895, vol. lxvi., p. 339), who found that in an extreme case the luminosity of the acetylene added in small quantity to a large volume of water-gas showed luminosity which, calculated per unit of acetylene employed, was only a little in excess of 5, instead of 230 candle-power. Acetylene is, therefore, hardly likely to come into extended use for the enrichment of illuminating gas in competition, for example, with such a material as oil gas, which may develop an enrichment value of 96 candle-power per unit employed, although, when burned alone, its luminosity is only 60 candle-power.-TRANSLATOR.]

The conversion of acetylene into benzene and higher hydrocarbons by polymerisation whilst passing through a heated tube, is quite out of the question for technical purposes, because the method of producing acetylene can never be sufficiently inexpensive to make it practicable. If all the coke manufacturers were to recover bye-products by using ovens of the Otto or other analogous type, the market would soon be flooded with benzene and other products of the distillation of coal.

Especially noteworthy is the fact that acetylene combines directly with nitrogen under the action of the electric spark to form hydrocyanic acid, C,H,+ N2 = 2HCN. It is even possible that under suitable conditions nitrogen might be similarly added to metallic carbides.

Acetylene may be converted into oxalic acid or acetic acid by treatment with oxidising agents in aqueous solution.

From all that has been written in this chapter, it is evident that both the carbides of the alkaline earths and acetylene, the product of their decomposition, deserve the most careful attention. But before far-reaching speculations can be made in this field, it is necessary to determine the cost of production, especially of the calcium carbide. It would be agreeable to find that the estimates made by the author and by Willson were too high. As soon as practical experience in this direction shall have been gained with large installations, the inferences to be drawn will become self-evident.

Finally, as to the locality in which the carbide industries will ultimately be developed, there is no doubt that in the first place the fortunate possessors of water-power will take up the manufacture. But there are many other sources of power unutilised, which at any moment might enter into competition, and among these are especially to be noted iron-works and coke-ovens. The waste gases of the blast-furnace and of the coke-oven at present supply, in most instances, more than sufficient power to drive the whole of the machinery of the works. And the majority of these works are intentionally somewhat extravagant in the installation of their boilers and machinery, in order to get rid of this troublesome profusion of gas. The surplus energy which passes away unused in many modern works of this description, exceeds not only hundreds, but even thousands, of horse-power units. There is here open, then, a field which, from the standpoint of national economy, can and will be at once successfully utilised by electro-chemistry. The electro-smelting process adapts itself peculiarly easily to the often varying conditions which obtain in some works, and it will be to the advantage of the iron trade to turn to its own uses the young industry of electro-metallurgy.

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PART II.THE EARTH METALS.

=

CHAPTER I.

ALUMINIUM.

Properties of the Metal.-Aluminium (Al; atomic weight = 27, specific gravity 2.6 to 2.74) is the most important of the earth metals. It possesses a white colour and a high lustre, and its fractured surface shows a crystalline structure. It fuses at about 700° C. Its low specific gravity renders it especially useful for many purposes. At ordinary temperatures it shows. a remarkable resistance to atmospheric action, and, even at higher temperatures, such as are required for melting and casting operations, the almost imperceptible film of oxide that is formed on the surface protects the metal from further oxidation. Water and dilute organic acids act scarcely at all upon aluminium, but the latter substances begin to attack it slowly on boiling. Nitric acid is almost without action upon the metal, sulphuric acid dissolves it slowly, but hydrochloric acid and caustic soda, on the contrary, attack it energetically. Aluminium precipitates most of the other metals from solutions of their salts, and reduces the majority of oxides when in the molten state, as well as those of carbon, silicon, and boron, the excess of aluminium alloying itself with the reduced substance.

39

Occurrence of Aluminium in Nature.-Aluminium is found in nature only in the combined state. All the compounds of aluminium may be derived from the oxide, Al,O,, and hydroxide, Al(OH); from the former are derived the sulphide, Al,S,, and the salts which contain alumina as the base; and from the latter the aluminates, which are salts containing the aluminium in the acid radical. Of the compounds that occur naturally, the oxide forms the basis of corundum, sapphire, and emery; the hydroxide is present in diaspore, bauxite, and hydrargillite; and the salts are thus represented :-The fluoride by cryolite; the sulphate by the

alums, alunite, and alum shale; the silicates by the felspars and their products of decomposition, the clays (kaolin, &c.) Among these minerals, corundum and sapphire are used as precious stones, emery for polishing and grinding, and felspar and clay for the manufacture of building-stones, firebricks, pottery, earthenware, stoneware, and porcelain. Chemical processes of a more or less simple kind are employed to utilise alunite and alum shale in the manufacture of alum; cryolite in that of soda and pure aluminium hydroxide and oxide; and the hydroxide in producing the pure oxide and hydroxide; whilst corundum was used in the Cowles process for the direct production of aluminium alloys. None of the minerals can be utilised for the extraction of pure aluminium.

The abundance of natural compounds of aluminium, the valuable properties of the metal, and, finally, the difficulties, which must not be under-estimated, connected with the successful decomposition of the raw material that is present in such abundance, sufficiently explain how it is that the number of researches, of inventions (either actual or merely patented), and of more or less timid suggestions, has become almost interminable. For the better estimation of the value of the processes which have been employed in the reduction of aluminium, it will be desirable to consider separately the different divisions or groups into which, from the metallurgical point of view, they may be classed, viz.:Precipitation, Reduction, and Electrolytic Processes.

Precipitation Processes of Extracting Aluminium.Having regard to the scope of this book, a short account of the more prominent processes of this class must suffice, for the precipitation of one metal by the addition of another is an operation which belongs to pure chemistry rather than to electro-metallurgy.

*

Oerstedt, in the year 1824, was the first to attempt the decomposition of aluminium chloride by means of potassium amalgam, but evidently without result; for other reliable experimenters, working according to his directions, failed to obtain any aluminium. Wöhler, however, in 1827, was successful in reducing the anhydrous chloride by means of potassium. Later, Deville‡ obviated the difficulties connected with the production and use of aluminium chloride by employing the double chloride of aluminium and sodium instead; and he further substituted the much cheaper metal sodium for the potassium recommended by Wöhler. The process was actually conducted on these lines for thirty years in France (first at Nanterre and later at Salindres), and for a time in * Oerstedt, Overs. o. d. Danske Vidensk. Selsk. Forhandl., &c., 1824-25. + Wöhler, Pogg. Ann., 1827, vol. xi., and Liebig's Ann., vol. liii. Ann. de Chimie et de Physique, 1854, vol. xlix. See also H. St. ClaireDeville, De l'Aluminium, Paris, 1859.

England also. In 1855 Rose* proposed the substitution of the mineral fluoride (cryolite) for the chloride; and in place of sodium, the use of which had been retained by Rose, Beketoff† employed magnesium. But the principal interest at present centres in Grabau's process, which is excellent in all its details. In this process, solutions of sulphate of alumina are first treated with cryolite, in order to obtain the aluminium entirely in the form of fluoride, according to the equation:

Al2(SO4)3 + Al2F6.6NaF = 2A12F6 + 3Na2SO4.

The aluminium fluoride, which is insoluble in water, is filtered off, washed, dried, and heated to an incipient red heat, and is then at once charged into a cold vessel lined with pure cryolite. The required quantity of dry sodium, in the form of a cube or cylinder, is now placed upon the hot powder, and the vessel is immediately covered up. The following reaction then occurs, accompanied by a great evolution of heat; but in other respects it proceeds quite quietly ::

The aluminium is afterwards found at the bottom of the vessel, melted to a regulus, and covered with a slag of cryolite, which has been completely fused owing to the heat of the reaction. The latter bye-product is available for the production of fresh quantities of aluminium fluoride. Of all the chemical processes, this is the only one which, given a cheap method of producing sodium, is ever likely to come into competition with the electro-chemical methods. The metal obtained by this process has the advantage of being unusually pure.

The Reduction Processes.-For a long time alumina was held to be non-reducible; and even to this day statements to that effect are to be found in the chemical text-books. This has probably resulted from an erroneous comprehension of the fact that pure metallic aluminium in a form that would be useful in the arts is never obtained by the direct reduction of the oxide. In addition to this, the high temperature necessary for reduction can only be (or, at least, is most advantageously) obtained by the conversion of electrical energy into heat. This circumstance has frequently given rise to the assumption that the reduction of aluminium is either entirely or in part due to electrolytic agency. But the production of the metal is really due, as will be shown later, entirely to a reduction of the oxide of aluminium by electrically heated carbon.

The idea of heating a material with a high electrical resistance by a powerful current, and of placing this resistance in the most Pogg. Ann., 1855, vol. xcvi. + Jahresbericht der Chemie, 1865.

*

German Patent 47,031. [English Patents 14,356, Oct. 21, 1887, and 15,593, Nov. 14, 1887.]