the resulting rod of metal. When lead and copper are present, the surface will exhibit a play of colors and will be crystalline, especially towards the centre. This assay yields about 10 per cent. less than the true percentage.

The Cornish assay is less accurate than the German, but an experienced assayer obtains results which compare well with those obtained on a large scale, and it allows him to form a judgment of the quality of tin which an ore may be expected to yield, and to fix the price to be paid for it accordingly.1

C. Levol's assay with potassium cyanide.2-A sufficient quantity of powdered potassium cyanide is rammed into a capacious porcelain or fire-clay crucible to form a layer of from 12 to 15 millimeters (0.47 to 0.59 inch) thick, 5 grammes (77.16 grains) of the powdered ore, intimately mixed with 5 times the quantity of potassium. cyanide, are added to that in the crucible, and the whole covered with a thin layer of the cyanide. The charge is then heated in a moderate fire until it fuses, and is kept in constant fusion for 10 minutes. The crucible is then taken out, and gently tapped to facilitate the formation of a single button, and allowed to cool. then freed from adhering slag by water. or lead is present, the ore must be freed from them before the reduction, by treating it with acid. This is the most accurate method of assaying tin (to within one-half per cent.), and can be executed in a very short time. In case the ore is siliceous, a mixture of 10 grammes (154.32 grains) of ore, 3 to 8 grammes (46.30 to 123.5 grains) of ferric oxide, and 40 grammes (617.53 grains) of potas sium cyanide is placed in a crucible (Fig. 49, p. 117) lined with charcoal. The mixture is first covered with potas

1 B. u. h. Ztg. 1862, p. 261.

The button is In case copper

2 Journ. für prakt. Chemie, xcv. 503.

sium cyanide and then with powdered charcoal. The cover is luted on and the charge heated at a high temperature for one-half to one hour.

53. WET ASSAYS.

A. Gravimetric assays.

1. 1 gramme (15.43 grains) of tinstone is digested with diluted aqua regia; the residue is washed by decantation and dried. The dry mass is then fused with 3 parts sulphur and 3 parts sodium carbonate. The soluble double sulpho-salt of sodium and tin is lixiviated with water, and the tin sulphide precipitated with hydrochloric acid. Sulphuretted hydrogen is then introduced into the liquid. the resulting precipitate is filtered, dried, roasted, and finally weighed as stannic acid, containing 78 per cent. tin. By another method the ore is digested in aqua regia for half an hour, then washed by decantation and dried. It is then introduced into a silver crucible, standing in a clay crucible, with 4 times the quantity of caustic potassa (this is dissolved in water, the very finely powdered tinstone is stirred into it, and brought to dryness). It is then fused at a low red heat for half an hour. The mass, when cold, is treated with diluted hydrochloric acid and evaporated to dryness. The dry mass is then taken up in some hydrochloric acid, gently heated, filtered, and precipitated with sulphuretted hydrogen. The tin sulphide is washed, dried, and roasted, some ammonium carbonate being added towards the end of the operation. The tin oxide is then weighed.

2. The ore is digested with aqua regia, the residue with some charcoal is placed in a porcelain crucible and heated to redness. The reduced tin is dissolved in hydrochloric acid, and precipitated from the solution with zinc,

which, in the form of a flat button fastened on the end of a copper wire, is suspended in the fluid. According to the proportion of free acid present, the tin will appear in brilliant needles, in scales, mossy or spongy; the latter condition indicates the termination of precipitation. The zinc button is now taken from the liquor, freed from the tin, and this is pressed together in an agate mortar. It is then dried and fused to a button with some stearine (Moissenet1). Or, the ore is digested with aqua regia. decomposed with potassium hydrate, and a solution of stannic chloride in hydrochloric acid is formed as above. The tin is precipitated by a rod of zinc. It is then washed and dried, treated with strong nitric acid, and evaporated to dryness. When cold, it is moistened with diluted nitric acid and filtered. The tin oxide is then. dried, ignited, and weighed. Lead and copper are removed by the aqua regia at the commencement of the operation, or when the precipitated tin is dissolved in nitric acid.

B. Volumetric assays.

1. Determination of tin by means of iodine.-A few drops of potassium iodide of any desired concentration are added to the acid solution of stannous chloride, and then a few drops of diluted starch paste. A solution of potassium bichromate of 0.02 or 0.01 gramme (0.3 or 0.15 grain) of the salt in 1 cubic centimeter (0.61 cubic inch) is now added drop by drop under constant stirring, until the separated iodine does not again disappear, and the starch assumes a blue color when all the stannous chloride has been converted into stannic chloride (3SnO+Cr2O = 3SnO2+ Cr2O3). The quantity of chromate decomposed by 100 parts of pure tin dissolved in

6

B. u. h. Ztg. 1861, p. 170.

hydrochloric acid is empirically determined (100 tin-83.2 chromate).

1 to 2 grammes (15.43 to 30.87 grains) of tinstone are placed with four times the quantity of potassium cyanide in a porcelain dish, and heated for fifteen to twenty minutes. The mass is then poured upon an iron plate and treated with water. The metallic residue (tin and iron)

is dissolved in hydrochloric acid, the tin precipitated with zinc, again dissolved in hydrochloric acid, and titrated with potassium bichromate in the presence of potassium iodide and starch. (Hart.)1

According to Lenssen, more accurate results may be obtained by dissolving stannous salts (stannous chloride) with an addition of tartaric acid, or potassiumsodium tartrate, in sodium bicarbonate. Some starch paste is added to the clear solution, and it is then titrated with solution of iodine until the blue color appears (SnO+21+ Na2O=SnO2+2NaI). The iodine solution is standarized by dissolving 12.7 grammes (196 grains) of pure iodine, and 20 to 30 grammes (308.64 to 462.97 grains) of potassium iodide in 1000 cubic centimeters (61.02 cubic inches) of distilled water. A quantity of pure tin, accurately weighed, is dissolved in hydrochloric acid, tartaric acid is added, and the solution supersaturated with sodium bicarbonate. Solution of starch is added, and the iodine solution is gradually added from the burette until the liquid becomes blue. Two atoms of iodine (254) correspond to one atom of tin (118). This assay may be especially recommended for the detection of small quantities of tin.

2. Determination of tin by means of potassium permanganate.-5 to 10 cubic centimeters (0.3 to 0.61 cubic inch) of solution of stannous chloride are treated with a

1 Dingler, ccx. 394.

2 Journ. f. prakt. Chem. lxxviii. 200. Mohr, Titrirmethode, 1874, p. 311.

boiling solution of ferric chloride containing free hydrochloric acid (SnCl2 + FeCl = SnCl4 + 2FeCl2). The ferrous chloride which is formed is titrated, after dilution with water, with standard solution of potassium permanganate, until the liquid becomes reddish (p. 127). The value of the standard solution is determined by placing 0.2 gramme (3.08 grains) of freshly precipitated tin in a platinum crucible, and dissolving it in hydrochloric acid, in a current of carbonic acid. Ferric chloride in excess is added, and solution of potassium permanganate is then added, until the last drop colors the fluid perceptibly. 2 equivalents of iron 1 equivalent of tin. A correction becomes necessary, as experience has shown that more potassium permanganate is consumed in titrating ferrous chloride than stannous chloride.

XI. BISMUTH.

54. ORES.

Native bismuth, bismuth glance, Bi2S, with 81.25 Bi; cupriferous bismuth, CuBiS2, with 62 Bi and 18.9 Cu; tetradymite, Bi2Te, with 51.94 Bi; bismuth ochre, Bi2O3, with 89.65 Bi, and others.

55. FIRE ASSAYS.

These are inaccurate, as bismuth votalizes, and, in case the ore is impure, foreign metals collect in the brittle button.

1. Ores and compounds free from sulphur (native bismuth, tetradymite, bismuthic cupel ash, etc.). 5 grammes (77.16 grains) of ore with two and a half to three times the quantity of black flux, or potash and flour, and 2.5 to 5