The retort is filled with stove-dried balls, the lid is carefully luted, and the retort is heated gently till all the moisture is driven off. This complete desiccation is of great importance, and requires much time. Then chlorine, furnished by a battery of three generating vessels, is passed in. During the first hours, the gas is totally absorbed by the balls, and the double chloride distils regularly for about three hours, and runs into the earthen pots where it solidifies. Toward the end, the distillation is more difficult and less regular, and the chlorine is then only incompletely absorbed. After each operation there remains a little residue in the retort, which accumulates and is removed every two days, when two operations are made per day. One operation lasts at least twelve hours, and a retort lasts sometimes a month. The double chloride is kept in the pots in which it was condensed until the time it is to be used in the next operation; it is almost chemically pure, save traces of iron, and is easy to keep and handle.

IV. Reduction of the Double Chloride by Sodium.

The difficulty of this operation, at least from an industrial point of view, is to get a slag fusible enough and light enough to let the reduced metal easily sink through it and unite. This result has been reached by using cryolite, a white or grayish mineral originally from Greenland, very easy to

melt, formula A12F6.6NaF. This material forms with the NaCl resulting from the reaction a very fusible slag, in the midst of which the aluminium collects well, and falls to the bottom. In one operation the charge is

The double chloride and cryolite are pulverized, the sodium, cut into small pieces a little larger than the thumb, is divided into three equal parts, each part being put into a sheet-iron basket. The mixture of double chloride and cryolite, being pulverized, is divided into four equal parts, three of these are respectively put in each basket with the sodium, Fig. 11.

refractory brick, with an inclined hearth and a vaulted roof. This furnace is strongly braced by iron tie-rods, because of the concussions caused by the reaction. The flame may at any given moment be directed into a flue outside of the hearth. At the back part of the furnace, that is to say, on that side towards which the bed slopes, is a little brick wall which is built up for each reduction and is taken away in operating the running out of the metal and slag. A gutter of cast iron is placed immediately in front of the wall to facilitate running out the materials. All this side of the furnace ought to be opened or closed at pleasure by means of a damper. Lastly, there is an opening for charging in the roof, closed by a lid. At the time of an operation the furnace should be heated to low redness, then are introduced in rapid succession the contents of the three baskets containing sodium, etc., and lastly the fourth containing only double chloride and no sodium. Then all the openings of the furnace are closed, and a very vivid reaction accompanied by dull concussions immediately takes place. At the end of fifteen minutes, the reaction subsides, the dampers are opened, and the heat continued, meanwhile stirring the mass from time to time with an iron poker. At the end of three hours the reduction is ended, and the metal collects at the bottom of the liquid bath. Then the running out is proceeded with in three phases: First. Running off the upper part of the bath, which

consists of a fluid material completely free from reduced aluminium and constituting the white slag. To run this out a brick is taken away from the upper course of the little wall which terminates the hearth. These slags are received in an iron wagon. Second. Running out the aluminium. This is done by opening a small orifice left in the bottom of the brick wall, which was temporarily plugged up. The liquid metal is received in a castiron melting pot, the bottom of which has been previously heated to redness. This aluminium is immediately cast in a series of small rectangular cast-iron moulds. Third. Running out of the rest of the bath, which constitutes the gray slags. These were, like the white slags, formed by the NaCl and cryolite, but they contain in addition, isolated globules of aluminium. To run these out all the bricks of the little wall are taken away. This slag is received in the same melting-pot into which the aluminium was run, the latter having been already moulded Here it cools gradually, and after cooling there are always found at the bottom of the pot several grains of metal. In a good operation there are taken from one casting 10.5 kilos of aluminium, which is sold directly as commercial metal.

The foregoing description from Fremy sets forth. in its perfection the production of aluminium by means of sodium, and until very recently this was the only successful commercial process. A large

amount of aluminium is now produced by this process, and it, therefore, does not lack interest. The following data as to the expense of this process may be very appropriately inserted here, giving the cost at Salindres in 1872.

In 1872, 3600 kilos of Al were made at Salindres at the following average cost:—

α.

Soda

Manufacture of one kilo of Na.

9.35 kilos (a 32 fr. per 100 kilos

= 3 fr. 9 cent.

0.59 kilos @ 86 fr. per 100 kilos = 0 fr. 50.7 cent.

7. Manufacture of one kilo of Al.

Na . . 3.44 kilos @ 11.32 fr. per kilo

* A. Wurtz, Wagner's Jaresb., 1874, vol. xxi.