The answer is the amount of nickel in weight of speiss which has been found in a given weight of ore.
Co, 193 : 10 grains : : 118 x.
The answer is the amount of metallic cobalt iu weight of speiss (Co,As) which has been found in a given weight of ore. As, however, cobalt is usually returned as protoxide, then we have— Co Co CoO.
found • . 37:5 : x.
The answer is the weight of protoxide of cobalt.
Assay of Copper.—The assay of copper ores is conducted in many different ways. Those methods which are performed in the dry way are uncertain and ivaeeurate, and will therefore not be described. The best method, and the one which is exclusively employed in the laboratories at the mines in Chili, consists in dissolving a weighed quantity of the powdered ore in hydrochloric acid, and precipitating the copper in the metallic state means of iron wire. It is performed in the following way :— The sample, carefully taken, is powdered roughly in a clean iron mortar, and afterwards very finely in an agate or steel mortar. 100 grm. are then weighed out (if the ore be very rich, 50 grm. will be sufficient), and washed with as little water as possible into a flask of Bohemian glass, of about 8 oz. capacity. A small quantity, say two or three teaspoonfuls, of pure nitric acid is next poured in in order to set free the sulphur which the ore contains, and which must be entirely got rid of. When the action of the acid has almost ceased, the flask must be placed upon a warm sand bath, and heated gently. The sulphur, if any be present, will now be observed floating upon the surface ; if it be not perfectly yellow in colour, a few drops more nitric acid must be added, and the liquid heated again until the globule of sulphur, previously quite black, has become bright yellow. The liquid in the flask must then be allowed to stand on the bath till perfectly dry.
This dono, a little pure hydrochloric acid, slightly warm and about equal in quantity to the nitric acid previously added, is run in, and the contents are once more dried off by the beat of the bath, the object of this being to expel the traces of nitric acid remaining. After driving out the acid fumes from the flask by blowing in gently through a glees tube, n little boiling water containing hydrochloric acid is ponred in, and the contents are boiled gently until the yellow Paste adhering to the eider of the flask is completely detached. This done, the liquid in the flask must be filtered into a large beaker in the following way :—Pour into the filter (shown in Fig. 282) about half the contents, and when it has run through, wash it down with a little boiling water ; now pour in the rest, taking care not to lose a single drop, and leaving as much as possible of the undissolved residue behind in tho flask. Wash this residue with boiling water several
times, pouring the washings into the filter, and finally wash in the residue ; then work the filter down from the top with a jet of boiling water from the wash-bottle. In order to be sure that no copper has been left in the flask, add a few drops of dilute nitric acid, and then a little solution of ammonia ; the presence of copper is shown by a deep-blue coloration. When the Slier bas been thoroughly washed, the beaker containing the filtrate is heated gently in the bath. A piece of pure iron wire, perfectly clean, and about 4 io. in length, must now be heated on the sand bath, and carefully introduced into the beaker, the contents of which must be nearly boiling. After standing some little time, the copper is precipitated, and forms a thick coating on the wire; this coating is easily detached by gently shaking the flask, so as to expose a fresh surface. When tiro copper appears to be all deposited, a few drops of hydrochloric acid may be added, and the solution boiled again for a minute in order to extract the last particles. This done, the clear liquid is care fully decanted into a clean evaporating dish, and, if no copper has been poured out with it, rejected. Boiling water is added to the flask, and again poured carefully away. The iron and copper are now to be washed into a porcelain capsule fitted with a lid and handle, and of about one pint capacity. In this the wire is rubbed gently with the fingers, so as to detach the whole of the copper, which should come off readily in thin cakes ; the wire is then thrown away. The copper must he washed in the capsule by adding and decanting boiling water, taking care not to pour away the least particle. After two or three washings, a little hydrochloric acid must be added to dissolve any iron which may be present ; and, as it is important that none should he left, the addition of hydro chloric acid, with subsequent washing, must be continued until no blue colour is produced by tiro addition of a drop or two of a weak solution of ferrocyanide of potassium to the acidified liquid. Finally, the copper is washed again with hot water, and drained off as dry as possible. Nothing now remains but to dry and weigh it. Five minutes' gentle heating on the sand-bath in the covered capsule suffices to dry the copper, and it is then brushed carefully into the pan of the balance with a camel's hair brush, and accurately weighed ; the weight gives, of course, the exact percentage if 100 grm. were originally taken.