The following is a good rough method of assaying galena. Two hundred grains of the ore are finely powdered and mixed with 200 grains of carbonate of soda and 50 grains of argol in an ordinary crucible, which has beeu well black-leaded in the interior. Add a few bits of hoop•iron or a few nails, and then cover the mixture with 200 grains of common salt and 200 grains of anhydrous borax. Now place the crucible in the furnace and heat to bright redness. At this point, the oover of the furunce should be partially removed for a few minutes. Cover up again and heat once more to bright redness. When completely melted, the metal may be poured out in the ordinary way, after washing the pieces of iron. In employing this method, care must be taken to keep the top of the crucible as hot as its contents, by covering it completely with bits of coke.
Assay of Manganese.—Manganese ores are valuable in proportion as they possess the power of evolving chlorine, and as it is not the metal itself which possesses this power, but the peroxide, all assays of manganeae ores have for their object the determination of the available quantity of this substance present in them. Many methods are employed for this purpose, of which the following are the beat.
1. Fifty grains of the ore (which must be powdered very finely in an agate mortar) are placed in a small flask filled with a drying-tube containing chloride of calcium. Then add lT oz. of water and 11 oz. of strong sulphuric acid, and weigh the whole accurately. This done, remove the stopper and drying-tube and introduce 100 grains of pure oxalic acid, replacing the stopper immediately. Carbonic acid gas is now evolved with considerable effervescence, and is thoroughly dried as it passes through the drying-tube. When the action begins to slacken, the flask may be gently heated until no more gas comes off. When this is the case, and the flask has become perfectly cool, it is again weighed. The loss in weight represents the amount of carbonic acid gas disengaged, and therefore the amount of peroxide contained in 50 grains of tho ore.
2. Another method devised by Will and Fresenius is very commonly employed and gives capital results. Fig. 3 represents the apparatus used, consisting merely of two Bohemian glass flasks and three pieces of tubing. Fifty grains of the finely powdered ore are introduced into the flask A with 150 grains of neutral oxalate of potash. Into flask B is poured concentrated sulphuric acid until it is about one-third full. The flasks are then corked up and connected in the manner shown in the figure, and then accurately weighed. The tube b is then elosed with a pellet of wax, and a little of the acid in the flask B is eaused to flow over into A by sucking at the end of the tube d.
Carbonic acid gas is then formed by the action of the oxygen contained in the peroxide of man ganese, upon the oxalic acid, and it passes over into B, being deprived of any moisture which it may carry with it by the sulphuric acid in the flask ; through d it escapes into the air. Thisds repeated until no more gas is evolved. The plug is then removed from b and air is drawn through the apparatus by sueking at d. When quite cold, the whole is again weighed, the loss in weight, as in the former case, representing the quantity of peroxide of manganese in the ore taken.
3. A more accurate method than either of the preceding, though a longer one, consists in com bining the ore with iron and then estimating, by means of a standard solution of bichromate of potash, as in the case of iron ores, the excess of iron. The process is as follows :—Dissolve in pure dilute sulphuric acid (1 part of acid to 3 parts of water) about 30 grains (carefully weighed) of pure, clean iron wire, in a large flask, haviog a tightly-fitting stopper in which is fixed a tube, bent so that the end of it dips into a small beaker containing water. Heat the flask until all the wire is dissolved. Now remove the stopper and tube, and add to the contents of the flask a quantity of the powdered Ore equal to the iron previously taken ; then replace the stopper and let the flask simmer gently on a sand-bath until the manganese is all dissolved, or the only residue is a small quantity of white sand. The apparatus is then removed from the bath, and, as it cools, the water contained in the small beaker is allowed to flow into the flask, in order to thoroughly rinse out the tube. The contents of the flask are next diluted and allowed to become quite cool. Now, estimate the excess of iron unoxidized by the manganese, by means of the bichromate solution, of which 1000 grains = 10 grains of iron ; this is done exactly in the same way as described under Iron (p. 354). The difference between the weight of wire miginally taken and the weight indicated by the bichromate solution, represents the amount of iron which has been converted into peroxide by the action of the manganese ore, and from this we can calculate the amount of peroxide in the 50 grains of ore taken for the assay. Supposing that we find 7 grains of iron remain unoxidized, then 50 — 7 = 43 grains of iron oxidized by,the manganese. Now, 43 grains of iron correspond to 33.07 grains of peroxide of manganese, therefore, as 50 grains of the ore were originally taken, the percentage of peroxide in the ore is 76.9.