Washing, as an assay, is practised on auriferous sands ; on all ores from the stamps, and even on aehliche already washed upon the great scale; to appreci ate more nicely the degree of purity they have acquired. The ores of tin in which the oxide is often disseminated in much earthy gangue, are well adapted to this species of assay, because the tin oxide is very dense. The mechanical assay may also be employed in reference to the ores whose metallic portion presents a uniform composition, provided- it also posiesses considerable specific gravity. Thus the ores of sulphuret of lead (galena) being susceptible of becoming almost pure sul phurets (within 1 or 2 per cent.) by mere washing skilfully conducted, the richness of that ore in pure galena, and conse quently in lead, at once conclud ed • since 120 of galeria contain 104 of lead, and 16 of sulphur. The sulphuret of antimony mingled with its gangue may be subjected to the same mode of assay, and the result will be still more direct, since the crude antimony is brought into the market after being freed from its gangue by a simple fusion. Of Assays by the dry way.—The assay by the dry way has for its object, to show the nature and proportion of the metals contained in a mineral substance. To make a good assay, however, it is indis pensably necessary to know what is the metal associated with it, and even within certain limits, the quantity of the foreign bodies. Only one metal is commonly looked after ; unless in the case of cer tain argentiferous ores. The mineralo gical examination of the substances under treatment, is most commonly sufficient to afford data in these respects ; but the assays may always be varied with different views, before stopping at a definite re sult; and in every instance, only such assays can be confided in, as have been verified by a double operation.
This mode of assaying requires only a little experience, with a simple appara tus ; and is of such a nature as to be prac tised currently in the smelting works. The air furnace and crucibles employed are described in all good elementary che mical books. These assays are usually performed with the addition of a flux to the ore, or some agent for separating the earthy from the metallic substances ; and they possess a peculiar advantage relative to the smelting operations, because they offer many analogies between results on the great scale and experiments on the small. This may even enable us often to deduce, from the manner in which the assay has succeeded with a certain flux, and at a certain degree of heat, valuable indications as to the treatment of the ore in the great way.
For assays in the dry way, both of stony and metallic minerals, the process of Dr. Abich deserves recommendation. It consists in mixing the pulverized mineral with 4 or 6 times its weight of carbonate of baryta in powder, fusing the mixture at a white heat, and then dissolving it, after it cools, in dilute muriatic acid. The most refractory minerals, even corundum, cyanate staurolite, zircon, and feldspar, yield readily to this treatment. This pro cess may be employed with advantage upon poor refractory ores. The platinum crucible, into which the mixed materials are put for fusion, should be placed in a Hessian crucible, and surrounded with good coke.
It never, however, furnishes exact re cults. To obtain them it is necessary to resort to assays by the moist tray, which are regular chemical operations, requiring much skill and experience in chemical analyses. The process generally consists of solution of the ore in acid, determining the insoluble earthy matters (gangue), then taking the clear solution, and sepa rating the several parts of the ore out of the solution by the proper re-agents. The limits of this work do not admit of enter ing into the processes necessary for indi vidual metals ; they arc found in the standard works on assaying.
Since the publication of the improved method of making cyanide of potassium, by Liebig, great advance has been made in assaying by the humid way, as this salt possesses the property of separating many metals in mixed solutions.
This salt is the best re-agent for de tecting nickel in cobalt. The solution of the two metals being acidulated, the cyanide is to be added until the precipi tate that first falls is redissolved. Dilute sulphuric acid is then added and the mixture being warmed and left in repose, a precipitate does not fail to appear sooner or later, which is a compound of nickel. Cyanide of potassium serves well to separate lead, bismuth, cadmium, and copper, four metals often associated in ores. On adding the cyanide in excess to the solution of these metals in nitric acid, lead and bismuth fall as carbonates, and may be parted from each other by sulphuric acid. Sulphuretted hydrogen ispassed in excess through the residuary solution, and the mixture being heated, a small quantity of cyanide is added ; yellow precipitate indicates cadmium; and a blackprecipitate falls on the addi tion of hydrochloric acid, if copper be present.
If into a crucible (containing the cyan ide fused by heat), a little of any metallic oxide be thrown at intervals, it will be al most immediately reduced to the regu line state. When the fluid mass is after ward decanted, the metal will be found mixed with the white saline matter, from which it may be separated by water. Even metallic sulphurets are reduced to the state of pure metals by being pro jected in a state of fine powder into the fused cyanide. When an iron ore is thus introduced, along with carbonate of pot ' ash or soda, and the mixture is heated to fusion, requires a strong red heat, the alumina and silica of the ore fuse into a slag iron from which, on cooling, the me tallic ron may be separated by the action of water, and then weighed. If manga nese exist in the ore, it remains in the state of protoxide ; to be determined by a separate process. When oxide of cop per is sprinkled on the surface of the fused cyanide, it is immediately reduced, with the disengagement of heat and light. The mixture being poured out of the cru cible and concreted, is to be ground and washed, when a pure regulus of copper will be obtained.
The process of reduction is peculiarly interesting with the oxide of antimony and tin ; being accomplished at a low red heat, hardly visible in daylight. Even the sulphurets of these metals are imme diately stripped of their sulphur, with the formation of sulpho-cyanide of potassium. Cyanide of potassium, mixed with car bonate of soda, is an excellent re-agent in blow-pipe operations for distinguish ing metals. The reductions take place with the utmost facility, and the fused mixture does not sink into the charcoal, as carbonate of soda alone is apt to do in such cases. Hence the grains or bends of metal are more visible, and can be bet ter examined.
When the cyanide is heated along with the nitrates and chlorates of potash), it causes a rapid decomposition, accompa nied with light and explosions.
Arsenic may be readily detected in the commercial sulphuret of antimony, by fusing it with three-fourths of its weight of the cyanide in a porcelain crucible over a spirit-lamp, when a regulns of an timony is obtained. The metal may then be easily tested for arsenic, since none of this volatile substance can have been lost, owing to the low temperature employed. When arsenious acid, or orpiment, or any of the arseniates, are mixed with six times their weight of the mixture of cyan ide and carbonate of soda in a tube with a bulb at one end, and heat applied with a spirit-lamp to the glass, very beautiful rings of metallic mirror are formed by the reduced arsenic. The arseniates of lead and peroxide of iron, however, do not answer the test.
When sulphates of lead and barytes, along with silicia, are mixed with four or five times their weight of the above mixed cyanide and carbonate, and 'fused, the sulphate of lead is reduced to the metal lic state, the sulphate of bar) tes becomes a carbonate, and the silica gets combined with the alkali into a soluble glass.