The best use of cryolite is as a flux in the preparation of aluminium from AlC16.2NaCl, in which case the slag is not sodium fluoride but aluminium fluoride, which acts but slightly on the containing vessel."

GENERAL REMARKS.

I have now given the metallurgy of aluminium through what may be called its experimental stage up to its practical industrial manufacture. Up to this period, which I will place at about 1859, the object has been to produce the metal at any cost, only produce it. "To learn how" engrossed the attention of the investigators, who troubled themselves very little about the ultimate cost. They must learn first how to do the thing and afterwards devote their energies to cheapening the process discovered. But, in 1859, the works at Amfreville, near Rouen, under the direction of the Tissiers, is producing aluminium from cryolite; Morin & Co., at Nanterre, are making it, though not in such large quantities as Tissier, but they soon after move to Salindres, and set up so large a plant that a year or so afterwards the Tissiers were driven from the business. Such is then the state of the industry. We find that in the next fifteen or twenty years very little advance is chronicled. At Salindres, the processes given by Deville were used somewhat improved and perfected, but yet the

same processes. It is only within the last ten years that any improvements of a radical nature, such as Webster's, Frishmuth's, and Cowles, have been brought into the industry.

So, from now on we will treat the subject in the order usually adopted in presenting it; i. e., first give a short sketch of the metallurgy of sodium up to the present time, then a review of the manufacture of alumina and its conversion into Al2C16.2NaCl, ending with a full description of the process as now carried on at Salindres, and a few attempts which have been made to improve it. Afterwards, leaving the old Deville process and its improvements, I will give as full an account as I have been able to gather of the various methods proposed to produce aluminium without the use of sodium.

PART VI.

THE MANUFACTURE OF SODIUM.

As already observed, we will not go extensively into the metallurgy of this metal. Some years ago, in order to treat fully of the metallurgy of aluminium, it would have been as necessary to accompany it with all the details of the manufacture of sodium as to give the details of the reduction of the aluminium, because the manufacture of the former was carried on solely in connection with that of the latter. But now sodium has come out of the list of chemical curiosities and has become an article of commerce, used for many other purposes than the reduction of aluminium, though that is still its chief use. So we regard the manufacture of sodium as a separate metallurgical subject, still intimately connected with that of aluminium, but yet so far distinct from it as to deserve a metallurgical treatise of its own. Moreover, the metallurgy of sodium is very much as Deville left it, it has been very little improved since then, and so almost all the details of its manufacture are to be found in English in any good book on chemistry. To such works I refer the reader for fuller accounts than are

given here. The following summary is taken principally from Mierzinski.

Sodium was first isolated by Davy by the use of electricity in the year 1808.* Later, Gay Lussac and Thenard made it by decomposing at a very high temperature a mixture of Na2CO3 and iron filings. On April 30, 1808, Curaudau announced that he had succeeded in producing potassium or sodium without using iron, simply by decomposing K2CO3 or Na2CO3 by means of animal charcoal. Brünner continuing this investigation used instead .of animal charcoal the so-called black flux, the product obtained by calcining crude tartar from wine barrels. He was the first to use the wrought-iron mercury bottles. The mixture was heated white hot in a furnace, the sodium volatilized and was condensed in an iron tube screwed into the top of the flask, which projected from the furnace and was cooled with water. In Brünner's experiments he only obtained three per cent. of the weight of the mixture as metallic sodium, the rest of the metal being lost as vapor.

Donny and Mareska gave the condenser the form which with a few modifications it retains to-day. It was of iron, 4 millimetres thick, and was made in the shape of a book, having a length of about 100 centimetres, breadth 50, and depth 6 (see Fig. 2). This form is now so well known that a further

* Phil. Trans., 1808.

Recherches Physico-chemiques, 1810.

description is unnecessary. With this condenser the greatest difficulty of the process was removed, and the operation could be carried on in safety.

Fig. 2.

This apparatus was devised and used by Donny and Mareska in 1854, with the supervision of Deville, and the whole process as used by them is the same that the Tissier Bros. took with them and operated at their works at Rouen, and their description accords with that given by Deville, which is as follows:

The Na2CO3 is first well dried at a high temperature, then mixed with well dried pulverized charcoal and chalk, ground to the finest powder, the success of the operation depending on the fineness of this mixture. The proportions of these to use is various. One simple mixture is of