ARSENIC. (FR., Arsenic ; GER., Arsenilc.) Symbol, As. Combining weight, 75.—Arsenic is an iron-grey, brittle subStance, possessing metallic lustre. It occurs native, in veins, in crystalline rocks, and the older schists. It is found in this country as the oxide and sulphides, in association with ores of tin and copper, and on the continent, with those of cobalt and nickel. Arsenic itself is a substance of no commercial importance, but some of its compounds, as the oxide, commonly called " white arsenic," or "arsenious acid," and the sulphides, orpiment and realgar, are largely used for various industrial purposes.
White Arsenic.—The nature of white arsenic, or arsenic, has been fully treated of under the head of Arsenious Acid (see Acids); but it remains to describe here the processes by which it is obtained in this country and abroad. A,s already stated in the article referred to, the sources employed iu England are chiefly the arsenical pyrites, or mispichel, which is smelted solely for the arsenic which it contains, and the ores of tin and copper, from which arsenic is obtained as a bye product from the various smelting processes. The manufacture in this country is confined to Cornwall, Devon, and South Wales. In the former county, the Botallack and South Wheal Crony tin mines, and the East Pool and West Wheal Scion copper mines produce large quantities of arsenic annually. In Devonshire, the Devon Great Consols, Wheal Friendship, and Maria and Fortescue copper mines, yield still larger quantities ; the produce of the former mine is well known as the very hest refined arsenic (" D.G.C." brand).
Arsenic sublimes at 218° (424° F.) ; hut in order to effect the thorough roasting of the ore, the temperature must he raised to low redness, but not beyond, since any increase of temperature above that which is absolutely necessary for sublimation, must be compensated for by a greater length of flue, in order that the vapours may be sufficiently cooled in their progress to be entirely deposited. This, of course, applies only to the ores (as mispickel) which are roasted solely for the sake of the arsenic which they contain. When tin and copper ores are employed, and arsenic is yielded
merely as a bye-product, a much greater beat is required, and consequently the series of flues and condensing chambers must be longer in proportion, in order that the requisite space may be afforded for the cooling of the superheated vapours.
The furnace in which the arsenical ores are most largely roasted is known as Ox]and and Hocking's calciner, and is shown in Figs. 273, 274, and 275. It consists of a long wrought-iron cylinder, lined with firebrick, 3 ft. inside diameter and 32 ft. long, placed at an inclination of 1 in 16 to 1 in 24, according to the nature of the stuff to be treated, and supported upon rollers, upon which it is made to revolve at a very slow speed of six or eight revolutions per hour. The ore is supplied into the higher end of the cylinder, through a hopper fitted with a feeding-screw, and gradually traverses the length of the cylinder to the lower end, where it falls into a chamber, from which it is removed for further treatment. The heating furnace opens into the lower end of the cylinder and the volatilized arsenic and sulphur, &c., are carried off by a flue from the upper end this flue is extended to a considerable distance, and divided by baffle walls into a succession of chambers, in which the arsenic is deposited and periodically collected. The time taken for the ore to pass through the calciner is from three to six hours. The firebrick lining of the calciner is constructed with four longitudinal ribs, projecting internally, as shown in the transverse section, Fig. 275, and extending two-thirds of the length from the lower end, as shown in Fig. 273 ; in the revolutions these have the effect of continuously stirring the stuff and exposing the whole of it to the heat. In this calciner, the stuff being supplied at the upper end, farthest from the heating furnace, is exposed first to the lowest heat, and afterwards to a gradually increasing heat, as it works its way along to the hotter end of the calciner ; by this means the most advantageous effect is obtained from the fuel consumed in the furnace.