vapor of the metal, for, as the salt is melted, the heat is sufficient to vaporize the metal liberated. The gas escapes through one tube and the metallic

Fig. 5.

LEVYTYPE.PHIES

vapor by the other. The vapor is led into a con

denser and solidified.

PART VII.

MANUFACTURE OF ALUMINA.

I Do not propose to give here all the methods which have been employed to get good clean alumina (Al2O3), but only those which may be recommended as being practical and economical on a large scale, not repeating the methods used at Salindres or by Mr. Webster, which will be found in connection with the full description of the processes used at Salindres and Birmingham. Most of the following is from Mierzinski, and may be taken as representing the present state of the industry.

By igniting an alum salt, as ammonia alum, there remains either a white powder or shining, sticky pieces which are very hard and dissolve with difficulty in weak acid or in concentrated solutions of alkali. Large quantities of this alumina may be obtained by calcining the salt in an oven similar in its principal details to a soda furnace.

Mierzinski then gives Mr. Webster's process of mixing the powdered alum with coal-tar, etc., which is given in full in Part IX.

Tilghman decomposes commercial sulphate of alumina, Al2(SO4)3.18H2O, by filling a red-hot fire

clay cylinder with it. This cylinder is lined inside with a magnesia fettling, is kept at a red heat, the sulphate put in in large lumps, and steam is passed through the retort, carrying with it vapor of NaCl. This last arrangement is effected by passing steam into a cast-iron retort in which NaCl is kept melted, and as the steam leaves this retort it carries vapor of the salt with it. It is preferable, however, to make a paste of the sulphate of alumina and the sodium chloride, forming it into small hollow cylinders, which are well dried, and then the fire-clay cylinder filled with these. Then, the cylinder being heated to whiteness, highly superheated steam is passed over it. The HCl which is formed is caught in a condensing apparatus, and there remains a mass of aluminate of soda, which is moistened with water and treated with a current of carbon dioxide and steam. By washing the mass, the soda goes into solution and hydrated alumina remains, which is washed well and is ready for use.

Most of the alumina is now made from the natural aluminous earths, beauxite and cryolite, the occurrence and properties of which have been already described. The manufacture from beauxite is fully described in the account of the process used at Salindres, on p. 158. We will give here the modern methods of making it from cryolite.

MANUFACTURE FROM CRYOLITE.

Dry Way.--The cryolite is pulverized, an easy operation, and to every 100 parts, 130 to 150 parts of chalk are added, and a suitable quantity of fluorspar is also used, which remains in the residue on washing after ignition. More chalk is used than is theoretically necessary, in order to make the mass less fusible and keep it porous. But, to avoid using too much chalk merely for this purpose, a certain quantity of coke may be put into the mixture. It is of the first importance that the mixture be very intimate and finely pulverized. It is of greater importance that the mixture be subjected to just the proper well-regulated temperature while being calcined. The cryolite will melt very easily, but this is to be avoided. On this account, the calcination cannot take place in an ordinary smelting furnace, because, in spite of stirring, the mass will melt at one place or another, while at another part of the hearth it is not even decomposed, because the heat at the fire-bridge is so much higher than at the farther end of the hearth. Thomson constructed a furnace for this special purpose (see Figs. 6 and 7), in which the flame from the fire first went under the bed of the furnace, then over the charge spread out on the bed, and finally in a flue over the roof of the hearth. The hearth has an area of nearly 9 square metres, being 4 metres long and 2.5 metres wide. It is charged

twelve times each day, each time with 500 kilos of mixture, thus roasting 6000 kilos daily, with a

for drying the soda solution to its crystallizing point, and the gases finally pass under an iron plate on which the chalk is dried. In this fur