The powdering of small is performed by large stone rollers, inclosed in cases of wood. N1Then reduced to powder, it is of a fine blue colour, and is sometimes culled azure blue. It is employed not only in the arts, to impart its colour to substances with which it is fused, but likewise in washing, to prevent linen from becom ing yellow.
Cobalt is obtained in the metallic state by deflagrat ing 4 parts or smalt, 2 of nitre, and -1 of charcoal. The residue of the deflagration must be again subjected to a similar process, and afterwards fused with 2 pins of black flux. The product of this operation, weighing about one•fifth of the smalt, is mixed with one-sixth of its weight of nitre, and the same quantity of black oxide of manganese, and exposed to a strong heat in a covered crucible for about an hour. By this process it is freed fi um iron; it still, however, retains a small quantity of arsenic ; or the product of the deflagration may be dissolved in nitric acid, and carbonate of po tassa added to the solution, as long as a brownish pre cipitate is thrown down ; when, however, the precipi tate falls of a violet colour, the addition or the salt must be discontinued. By this means we get quit of the iron, and leave the cobalt in union with nitric acid. The nitrate is then exposed to heat, the acid is expelled, and the oxide may be reduced by fusion with black flux. By these processes cobalt is obtained sufficiently pure for many purposes.
Assay —The assay of cobalt ore may be performed, though not with great accuracy, in the dry way. The ore is mixed with saw-dust, and roasted to expel the ar senic, and the residue afterwards exposed to an intense heat for about 15 minutes, with its own weight of a mixture of carbonate of potassa and tartar. The metal lic button collected below is cobalt. The scoria of this operation contains a considerable quantity of metal ; the whole of the cobalt is therefore not procured, and it is even doubtful if by this means the whole of the arsenic is expelled.
.alercury.
Different methods are practised for procuring mer cury from its ores; they are however all very simple.
In Spain the cinnabar is collected and divided into three portions.
1st, The richest part of the ore.
21, That which contains less metal.
3d, The powder of the two first.
These are exposed to heat in a furnace, by which the mercury is expelled. The furnace employed in this operation is of a particular construction. It con sists of a long horizontal building, divided into an un der and an upper compartment, by a grating of iron. On this are placed flat rough stones, over which ore No. 2. is first put, and then ore No. 1. A layer of ore No. 2. is placed above this; and on the top of the whole is laid ore No. 3. made into a sort of bricks, with clay kneaded and dried. Wood is then kindled in the lower compartment of the furnace, by which the moisture is driven off. The fire is then continued till the sulphur begins to burn; after which the heat ex cited by the combustion of the sulphur is sufficient to volatilize the mercury, which is condensed in the re ceiver attached to the furnace. Along with the mer cury there is collected a quantity of sooty matter, which is removed by placing the metal on an inclined table.
In this process there is a considerable loss of mer cury, as the sooty matter which is thrown away re tains a great deal of the metal.
In Germany, the finer part of the ore is separated from the coarser part, and is reduced to powder. It is then mixed with about one•fifth of slaked lime, and put into iron retorts, each of which holds about one half cwt. From 40 to 50 of these are built into a fur race, and have receivers adapted to them. Heat is applied to the retorts, by which watery vapour is at first expelled. The receivers are then luted by means of clay, and the. mercury which comes over is condensed in them. By this process 100 lbs. of ore yield from 6 to 10 oz. of mercury.
Assay.—The assay of the ore of mercury is very simple. Cinnabar, the ore usually employed for yielding the metal, is reduced to powder, and digested in nitro muriatic acid, composed of one of nitric and of muriatic acid, diluted with water, till the whole of the soluble matter is extracted. Cal bonatc of potassa is then added to the solution, and the precipitate, after being dried, is mixed with lamp-black, and exposed to heat in a retort, to which a receiver is adapted. By this means the mercury is expelled in the metallic state.