195. This paste is made of calcined alumina and an aluminate of lime, the latter obtained by heating together equal parts of chalk and alumina to a high heat. By taking about four parts calcined alumina and one of aluminate of lime well pulverized and sieved, moistening with a little water, there is obtained a paste with which the inside of an earthen crucible is quickly and easily coated. The paste is spread evenly with a porcelain spatula, and compressed strongly until its surface has become well polished. It is allowed to dry, and then heated to bright redness to season the coating, which does not melt, and protects the crucible completely against the action of the aluminium and fluorspar, A crucible will serve several times in succession provided that the new material is put in as soon as the previous charge is cast. The advantages of doing this are that the mixture and the sodium are put into a crucible already heated up, and so lose less by volatilization because the heating is done more quickly, and the crucible is drier than if a new one had been used or than if it had been let cool. A new crucible should be heated to at least 300° or 400° before being used. The saline slag contains a large quantity of calcium chloride, which can be washed away by water, and an insoluble material from which aluminium fluoride can be volatilized.

"Yet the operation just described, which was a great improvement on previous ones, requires many precautions and a certain skill of manipulation to

succeed every time.

But, nothing is more easy or simple than to substitute cryolite for the fluorspar. Then the operation is much easier. The amount of metal produced is not much larger, although the button often weighs 22 grammes, yet if cryolite can only be obtained in abundance in a continuous supply, the process which I will describe will become most economical. The charge is made up as before, except introducing cryolite for CaF2. In one of our operations we obtained, with 76 grms. of sodium, a button weighing 22 grms. and 4 grms. in globules, giving a yield of one Al to two and eighttenths parts sodium, which is very near to that indicated by theory. The metal obtained was of excellent quality. However, it contained a little iron coming from the Al2C16, which had not been purified perfectly. But iron does not injure the properties of the metal as copper does; and, save a little bluish coloration, it does not alter its appearance or its resistance to physical and chemical agencies.

"Process with cryolite alone: The process adopted in the works at Amfreville, near Rouen, directed by Tissier Bros., is the same as that described by Percy and Rose. The details which I give are taken from MM. Tissier's own account of their process." (Deville then gives the details of the process outlined by Rose (see p. 103), of reducing in iron crucibles; which it is not necessary to repeat.)

"I obtained a good specimen of commercial aluminium thus extracted from cryolite; and M. Demon

dèur has been so kind as to make an analysis of it, with the following results: Si 4.4; Fe 0.8; Al 94.8.

"M. Rose has recommended iron vessels for this operation, because of the rapidity with which alkaline fluorides attack earthen crucibles and so introduce considerable silicon into the metal. Unfortunately, these iron crucibles introduce iron into the metal. This is an evil inherent to this method, at least in the present state of the industry. The inconveniences of this method result in part from the high temperature required to complete the operation, and from the crucible being in direct contact with the fire, by which its sides are heated hotter than the metal in the crucible. The metal itself, placed in the lower part of the fire, is hotter than the slag. This, according to my observations, is an essentially injurious condition. The slag ought to be cool, the metal still less heated, and the sides of the vessel where the fusion occurs ought to be as cold as possible. The yield from cryolite, according to Rose's and my own observations, is also very small. M. Rose obtained from 10 of cryolite and 4 of Na about 0.5 of Al. This is due to the affinity of aluminium for fluorine, which must be very strong not only with relation to its affinity for sodium but even for calcium, and this affinity appears to increase with the temperature, as was found in my laboratory. Cryolite is convenient to employ as a flux to add to the mixture which is fused, especially when operating on a small scale;

but it is fortunate that it is not indispensable, for no one would wish to establish an industry on the employment of a material which is of uncertain supply."

We here close what Deville has written on the use of cryolite. The process was that used by Tissier Bros. at Rouen, but was finally abandoned there and the works closed. We find a little improvement on Deville's process suggested by Wöhler, in which he shows how to perform the reduction in an earthen crucible. The finely pulverized cryolite is mixed with an equal weight of a flux containing 7 NaCl to 9 KCl. This mixture is then placed in alternate layers with sodium in the crucible, 50 parts of the mixture to 10 of sodium, and heated gradually just to its fusing point. The metal thus obtained is free from silicon, but only one-third of the aluminium in the cryolite is obtained. In spite of the small yield, this method was used for some time by Tissier Bros. Cryolite has also been treated at Nanterre, by a different process, but the aluminium produced contained phosphorus. So, while the exclusive use of cryolite in the preparation of aluminium is now renounced, it has retained the office of a flux.

Watts gives the following paragraph in connection with the reduction of cryolite: "A peculiar apparatus for effecting the reduction of aluminium,

* Ann. der Chem. und Pharm. 99, 255.

either from Al2C16.2NaCl or from cryolite, the object of which is to prevent loss of sodium by ignition, has been invented and patented by W. F. Gerhard.* It consists of a reverberatory furnace having two hearths, or of two crucibles, or of two reverberatory furnaces, placed one above the other and communicating by an iron pipe. In the lower is placed a mixture of sodium with the aluminium compound, and in the upper a stratum of NaCl, or of a mixture of NaCl and cryolite, or of the slag obtained in a previous operation. This charge, when melted, is made to run into the lower furnace in quantity sufficient to completely cover the mixture contained therein, and so to protect it from the air. The mixture thus covered is reduced as by the usual operation."

Watts thus summarizes the use of cryolite: "The chief inducement for using it as a source of aluminium is that it is a natural product obtained with tolerable facility, and enables the manufacturer to dispense with the troublesome and costly preparation of Al2C16.2NaCl. But the metal thus obtained is less pure than that obtained by other processes. If earthenware crucibles are used, the metal is contaminated with silicon, because the sodium fluoride produced acts strongly on the siliceous matter of the crucible, while if an iron crucible be used, the metal takes up some iron.

* Eng. Pat. 1858, No. 2247.