3. Carbon blocks, closely united, form the anode, while the cathode consists of a metal tube inserted through the bottom of the melting vessel, and capable of being cooled by water or otherwise.

4. The melting vessel consists of a shallow iron cylinder open above and lined within with pure refractory compounds of aluminium.

5. The heat necessary for fusion is produced by the current that serves for electrolysis, a very high current density being employed, amounting to some 25,000 amperes per sq. metre [1.61 amperes per sq. in.] of cathode area.

6. The walls of the melting vessel must be kept so cool that the lining does not become dissolved in the bath.

7. The temperature of the electrolyte is kept as low as possible, because, apart from the waste of heat energy occasioned by the use of high temperatures, it is probable that the metal may, to some extent, redissolve in the bath in the condition of sub-oxide, this being fully oxidised again at the anode, and so occasioning a waste of metal. The possibility of the separation and volatilisation of alkali metals at high temperatures introduces another source of loss both of current and energy.

Relative Specific Gravities of Metal and Electrolyte.-It may appear astonishing that the aluminium separated by electrolysis from such salts as cryolite, or from solutions of alumina in fused cryolite, should collect on the bottom of the melting vessel, seeing that the usually accepted specific gravities of these substances would lead to the contrary expectation. The specific gravity, for example, of aluminium is taken as 2.7, and that of cryolite as 3. J. W. Richards has, however, experimentally determined the densities of the materials. employed both in the molten condition and after cooling. The results, which suffice to explain the apparent anomaly, are given in the following table :

Uses of Aluminium.-Although only a few years have elapsed since the price of aluminium has fallen to its present level from one that had always been prohibitive, the metal has already found a wide-spread application for household and kitchen utensils, articles of military equipment, art-work, especially in place of silversmith's work, scientific instruments, and (in the largest proportions of all) for the refining of metals, although the quantity added in each operation is very small. In the iron, steel, and copper foundry, aluminium is used to reduce the oxides present in the melted metals, so that a dense casting free from blow-holes may be obtained, while at the same time the working properties of the purified metal will have become improved. In casting iron it is to be noted that the addition of more aluminium than is necessary to reduce the oxide present in the charge tends to produce a separation of carbon in the form of graphite.

The metal has, however, found but limited application in engineering, on account of its low tenacity, which is not greatly increased by the addition of small quantities of other metals, such as copper. It would otherwise be especially adapted to the requirements of marine and aërial engineering. It is already sometimes used for the construction of the framework, slide-valve, valve and pump chambers of marine engines, and other metallic mountings used on board ship; and recently experiments have been made in the construction of launches and boats from aluminium plate [commonly alloyed with a small percentage of some other metal, such as nickel or copper. -TRANSLATOR].

CHAPTER II.

THE CERITE METALS.

CERIUM, LANTHANUM, DIDYMIUM.

Properties of the Cerite Metals.-The metals of this group have been, up to the present, little used in the arts, at least in the metallic form; but owing to the wide-spread employment of their oxides for lighting purposes in the incandescent gas-lamp, attention has naturally been drawn to the properties of the metals themselves. The discovery of new cerite deposits, or the publication of the fact that cerite is not so scarce in Sweden as is commonly supposed, might lead to some unexpected application of the metals contained in this mineral.

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Cerium (atomic weight 140; specific gravity 6.7) is a soft malleable metal, in colour resembling iron, fusing at about 800° C. Most noteworthy is its power of combining with the heavier metals, such as copper or iron, to produce dense alloys. In solid pieces it offers considerable resistance to atmospheric influences; but, on heating the fragments, they exhibit the various temper-colours of polished steel. Finely powdered cerium, on the contrary, oxidises very rapidly in the air, and, on filing the metal or shaving it with a knife, the detached filings or scrapings take fire and burn with a brilliant light. Fine wire made from the metal burns with a brilliancy exceeding even that of magnesium. Cerium in the form of powder causes only a slow decomposition of water when introduced into it, but the presence of salts, dissolved in the liquid, induces a very lively attack. This property should be noted in connection with the production of cerium, and with the possibility, which may easily arise, of obtaining the metal in pulverulent form, owing to the use of an electrolyte at too low a temperature. It dissolves very easily in diluted acids, but only to a slight extent in cold concentrated sulphuric or nitric acids. Cerium reduces the oxides of most metals and metalloids, which is a property worthy of remark in regard to its applications to metal refining and the making of alloys.

Lanthanum (atomic weight = 138.5; specific gravity = 6·1) possesses a colour resembling that of cerium, but it is less soft and malleable than the latter. It shows a tendency during its preparation to separate in the form of thin leaves. Its fusing point is higher than that of cerium. The chemical relations of

the two metals are very similar.

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Didymium (atomic weight 142; specific gravity = 6.5) is a clear grey metal, harder, less malleable, and less easily fusible than cerium, but resembling it in its chemical properties.

Preparation of the Cerite Metals.-The methods of producing these three metals show but few differences. The statement of the chemical text-books that the oxides of the cerite metals are not reducible by means of carbon is incorrect. It has been shown (pp. 88, 111, &c.) that all oxides are capable of being reduced by electrically-heated carbon; but since a large excess of carbon must be employed, combination of the reduced metal with that element is unavoidable. In consequence of this, the production of the cerite metals by such a process does not appear to be promising, for the product is exceedingly brittle, easily crumbled, and porous, and is therefore readily oxidised. The separation of the metal by Wöhler's method (treating the haloid salts with metallic sodium) would probably give better results, especially if a system analogous to that adopted by Grabau for aluminium extraction (p. 103) were employed.

Electrolytic Deposition of the Cerite Metals.- Electro

lysis, however, seems to offer the best solution of the problem, as in the case of aluminium. The cerite metals appear to form a group intermediate between those of magnesium and aluminium, in regard to the behaviour of those salts which would be likely to come into use for electrolysis. It is well understood that none of the chlorides of these metals can be obtained in the anhydrous condition by evaporating their solutions in water, since decomposition always occurs on drying. But, as in the case of magnesium-chloride, if a chemically equivalent quantity of the chloride of sodium or potassium, together with a little ammonium-chloride, be added, the solution of the cerium, lanthanum, or didymium oxide may be evaporated to dryness, and the dry residue may then be fused without decomposition. The melt then contains comparatively easily fused double chlorides of the cerite and alkali metals, and will be found to conduct the electric current well. But although it may have been easy to obtain the metals of the magnesium group, magnesium and lithium, almost absolutely pure, either by direct electrolysis or by electrolysis followed by fusion, it is not safe, with the methods of production hitherto described, to rely too much on the purity of the separated metal, if its reduction have been effected in quantities somewhat greater than would be possible in the small porcelain crucibles of the laboratory. It is, indeed, very improbable that Bunsen, Hillebrandt, and Norton, who were the first to reduce the cerite metals by electrolysis, obtained a product that was free from iron. They employed the following method, which was devised by Bunsen*:

The

The decomposing vessel in which the electrolysis of the molten chloride was to be accomplished, was arranged after the fashion of a Grove's element. The outer cell, which in the Grove's battery contains the zinc plate and sulphuric acid, is here an ordinary Hessian crucible of about 100 c.c. [34 fl. oz.] capacity, filled with a fused mixture of equivalent weights of sodium and potassium chlorides, in which a cylinder of thin sheet-iron serves as positive electrode in place of the zinc of Grove's cell. cylinder is 5 cm. [2 in. ] high, and 2-5 cm. [1 in.] across internally, and terminates in a strip which serves as a conductor, and must not be either soldered or riveted in place. Within the cylinder is a clay cell of the best quality, 9 cm. [3 in.] high and 2 to 2.6 cm. [ to 1 in.] wide, in which is placed the chloride to be decomposed. The negative electrode is immersed in this. to about two-thirds of the depth of the cell; the electrode consists of a thick iron wire, the end of which is filed down somewhat thinner, and round its end is twisted a piece of iron wire about as thick as a horse-hair, which projects some 15 mm. [ in.] beyond the stouter piece to which it is attached. A piece of a clay pipe-stem is now drawn so far over the thicker Pogg. Ann., 1850, vol. clv., p. 633.

wire, that only the fine wire* at the end projects out of the clay and comes in contact with the fused chloride that is to be reduced.

In the reduction of such chlorides as are easily converted into oxides by the action of water vapour, the fusion must never be effected by means of a gas flame. Even in the heating of chlorides that are less readily decomposed, it is better to avoid the use of gas flames, since the water vapour that they evolve is very liable to cause re-oxidation of the already reduced metal. The charcoal that is used to melt the electrolyte in these cells, therefore, must be thoroughly glowing, and should have given off all the hydrogen that it contained before starting the experiment. For the same reason, the chloride that is to form the electrolyte must be very thoroughly dried, and must then be heated in a platinum crucible with sal-ammoniac until the bulk of the latter salt has been expelled. It must be stored in closely stoppered bottles, and be guarded most carefully against the re-absorption of moisture. Finally, when the chloride is melted for the experiment, the contents of the clay cell are covered with a layer of powdered sal-ammoniac, which has previously been heated, and this salt is replaced as fast as it volatilises.

The yield of metal, and the size of the globules obtained, depend upon the temperature at which the fused chlorides are submitted to the action of the currents. If the clay cell be raised to a temperature exceeding the fusing points of the salt under electrolysis and the metal that is to be separated, the drops of metal, which form upon the surface of the negative electrode, fall to the bottom, and are there for the most part re-oxidised at the expense of the silica in the clay walls of the cell. The addition of fuel and the supply of sal-ammoniac are therefore so regulated that the upper part of the salt in the clay cell remains solid, while the lower part around the negative electrode is in a semi-solid or pasty condition. The metallic particles thus increase in size without sinking through the pasty mass, and may even grow into globules the size of a hazel nut if the experiment be carefully tended. The electrolytic decomposition should be started only when the melted salt is in the proper condition, because otherwise the reduced metal is liable to separate in a pulverulent form, and to mingle with the contents of the clay cell, so that the formation of larger metallic globules would be prevented.

The success of the reduction depends not only upon the temperature of the bath, but also upon the absolute intensity of the current employed. Four large carbon zinc elements suffice for the experiment. The clay cells of such elements should con

A piece of this wire, 1 cm. long, weighs about 4 mg. [1 in. weighs about grain.]