charged into the furnace, which has been previously heated, and is slowly melted till the sump is filled; the zinc is then rapidly ladled into iron moulds by means of ladles coated with a mixture of soap and chalk. Most of the so-formed plates keep sufficiently hot to be rolled, and plates which are too cold must be heated in a separate furnace up to the temperature of about 125° C., which is sufficient to keep the zinc ductile during rolling.

The resulting scum or zinc ash is worked up by the reducing process. The temperature at which the zinc is cast has an essential influence upon its nature, i.e., upon its power of resisting acids, water, atmospheric air,* &c., also upon its behaviour in rolling.

According to Bolley+ and Begemann, zinc melted at a very low temperature has a granular fracture, a higher specific gravity, a greater ductility, and is less soluble in dilute acids. Zinc poured out at high temperature, on the other hand, has a laminated crystalline fracture, a lower specific gravity, a greater brittleness, and is easily soluble in acids. At temperatures above 200° C., zinc is brittle; it becomes ductile and fit for rolling and other purposes, when kept at a temperature between 100° and 150° C. The fewer times zinc is heated before its treatment by rollers, &c., the more ductile it will be. The bending of a rolled zinc plate does not make any noise, but a crepitating noise will be observed after having heated the plate, probably due to a rearrangement of the crystals that have been formed by the influence of the temperature. Zinc which has been heated almost red hot shows a laminated texture if it be rapidly cooled, and zinc heated nearly to its melting point has a fine granulated texture if slowly cooled; the specific gravities are likewise modified.

The presence of certain impurities renders zinc harder. Karsten states that carbon has no influence on the nature of zinc. Taylor, Eliot, and Storer have discovered sulphur

+ Polyt. Centr., 1856, p. 1398. ERDM., J. f. pr. Chem., Bd. 66, p. 451.

Dingl., Bd. 142, p. 100.

in many kinds of zinc, chiefly in those produced from blende. A certain amount of sulphur seems to facilitate the combination of zinc with iron, in case the zinc is kept fluid for a longer time in iron pots, or is in contact with iron tools. Zinc-white produced from such zinc shows fine rose or cherry-red spots of oxidised iron. Schäuffele* discovered arsenic in zinc, a small amount of which renders zinc brittle. According to Cooke, the arsenic promotes its crystallisation; antimony has a similar behaviour.

Tin is seldom found in zinc; one per cent renders zinc unfit for rolling at the required temperature; per cent of copper renders zinc so hard that it is difficult to roll, and yields very brittle plates.

A large amount of cadmium renders zinc brittle and of a fine granular texture. Mentzel states that zinc with from 5 to 15 per cent of cadmium may be rolled into plates one line thick, without cracking. As both metals can only be separated by repeated distillations, in which cadmium first. volatilises, and as this causes a great loss, experiments have been tried at the zinc works at Birkengang, near Stolberg, to work zinc with from three to eight per cent of cadmium by the wet way, which is based upon the fact that when treating mixtures of zinc, cadmium, and lead with acids, the zinc first becomes dissolved, and the cadmium which may be in solution, as well as the lead, are again precipitated by zinc. The latter may then be precipitated by lime as fine white oxide of zinc, whilst the residue of cadmium and lead may be distilled, yielding cadmium.

Iron is frequently contained in zinc, and the presence of o'13 or 0.14 per cent does not, according to Karsten, greatly influence its quality, but an amount of above 0.2 per cent renders it harder, and necessitates a careful stronger heating. for the purpose of rolling. If lead be present at the same time, iron is more injurious. Its presence may be detected by grey spots on the fresh surface of zinc. Eliot and Storer found from 0.05 to 0'21 per cent of iron in zinc; Karsten

found o 24 per cent, and Percy 1.64 per cent. Iron may be separated from zinc by distilling in clay vessels.

Lead is found in zinc still more frequently than iron, and injures its firmness. According to Karsten, all zinc contains traces of lead, usually from 0°3 to 3 per cent. When containing 1 per cent it may still be rolled, but yields soft plates. A greater amount causes it to crack. If lead and zinc are melted together, they separate again according to their specific gravities, but the lead always retains some zinc, and the zinc some lead. Matthiessen and Rose state these amounts to be respectively 16 and 12 per cent. Bismuth behaves like lead; when bismuth and zinc are melted together and separated again, bismuth will retain from 9 to 14 per cent of zinc, and zinc 2'4 per cent of bismuth.

When choosing zinc for different uses, these foreign admixtures must be taken into consideration as well as the already mentioned influence of the temperature upon castings and rolled plates. The presence of foreign substances, chiefly arsenic, iron, and lead, and the temperature, are of most importance when rolling the zinc. In casting zinc, the chief consideration is the temperature, which must be so regulated as to avoid the formation of burned zinc. A greater amount of lead, cadmium, sulphur, and iron is deleterious to the manufacture of zinc-white, the lead giving a reddish, and the iron a yellowish hue. Rolled zinc used for coating vessels and buildings, &c., must be as free as possible from iron and lead, as the smallest amount of these substances facilitates the corroding of the metal by galvanic action.

b. Reduction of the Zinc Dust.

Zinc dust is a mixture of very finely divided metallic zinc, with eight or ten per cent of oxide of zinc, and contains the volatile metals, such as arsenic, cadmium, &c., which existed in the zinc ore, and sometimes also the chlorides, iodides, and bromides originating from the coal used for reducing the oxide of zinc. As the zinc dust collected in the receivers frequently amounts to 9 or 10 per cent of the whole production, the working it up is very important, especially in zinc works where blende and impure calamine are treated.

According to Thum,* zinc dust of Borbeck contains 97.82 per cent of zinc, o'16 per cent of iron, o'23 per cent of lead, and o'08 per cent of cadmium and arsenic; zinc dust of Birkengangt contains from 9 to 12 per cent of cadmium.

The most common way of working it up, is by adding it to the mixture in the distillation vessels. Although the impurities of the zinc dust are thus again added to the ores, the admixture is in such proportions that the quality of the resulting zinc is little affected. In the Belgian process the zinc dust mixed up with ore and coal is worked in the uppermost tier of retorts, which are only discharged every 24 hours; by this process the zinc dust yields about 80 per cent of zinc.

In some few cases the zinc dust is worked up in separate furnaces constructed for the purpose by Montefiore. This furnace consists of a number of vertical tubes into which clay cylinders fixed to iron rods are fitted. The zinc dust is put into the tubes, which are then heated to the melting point of zinc; the cylinder is now pressed upon the zinc dust, separating the melted zinc, which is then tapped off. In this way about 90 per cent of zinc is extracted from the zinc dust, but as this zinc contains all the foreign substances of the zinc ore, it is of very inferior quality, and unfit either for the purpose of rolling or the manufacture of zincwhite; wherefore this mode of working up the zinc dust is only used in zinc works where, as at Bleiberg, the zinc resulting from the ores is so hard as to require refining in reverberatory furnaces; the zinc produced from the zinc dust is then added to the refining process. From 14 to 18 cwts. of zinc dust may be worked up in 12 hours by 12 cylinders, consuming from 11 to 14 cubic feet of coal.

The deposits formed in the adapters are worked up in the distillation process. The residues of the reducing vessels contain from 2 to 9 per cent of zinc, or even more, and are mostly thrown aside, but are sometimes submitted to a hand

B. u. h. Ztg., 1859, p. 409.

+ Ibid., 1862, p. 307.

Ibid., 1859, p. 409; 1860, p. 29. Revue Univers., 2 Jahrb., 1 Lief., 1858.

sorting; if they are the result of working argentiferous and plumbiferous blende, it may be profitable to wash them. In 1859, the production of zinc in Europe amounted to the following:

In Dr. Ure's Dictionary, iii., 1127, the general consumption of zinc throughout the world is estimated to be 67,000 tons per annum, of which about 44,000 tons take the shape of rolled sheets; they are employed as follows, each quantity being rather below the fact :

Roofing and architectural pur

23,000 tons.
3,500
2,500 ""

Fifteen years ago the quantity used for roofing did not exceed 5,000 tons; none was employed for ship sheathing, or lining packing cases, and stamped ornaments in zinc only date from 1852.