in the slag, in part, and the remainder alloys with the separated tin, and forms a button which must be analysed by the humid method, which will be presently described.

Analysis by the humid method.

The following method of analysing tin stones, was contrived by Klaproth. A certain amount of the ore in the finest possible state of division, is mixed with six times its weight of caustic potash, and heated to redness in a silver crucible. The fused mass is acted on by means of water and hydrochloric acid; and if any remain undissolved it must be re-fused with more caustic potash, when it will be readily soluble in more hydrochloric acid.

The solution must be evaporated to a pasty consistence, re-acted on by water, and filtered. Silica remains on the filter. A bar of zinc must then be placed in the solution, which will precipitate tin copper and lead. These may be separated by the action of hot nitric acid, which will convert the copper and lead into soluble nitrates, but the tin, into an insoluble oxide, which must be washed, dried, ignited, and weighed; it contains 78.61 per cent. of metallic tin.

The button of iron and tin, obtained by the treatment of tin slag with iron filings, may be analysed by ebullition with concentrated hydrochloric acid. The solution must be filtered from the insoluble residue, if there be any, and treated with sulphuretted hydrogen gas, which will throw down all the tin as sulphuret, without touching the iron. The precipitated sulphuret must be treated with concentrated boiling

nitric acid, which will transform it into peroxide of tin, which must be washed, dried, and ignited. From its weight the per centage of tin is calculated.

Or a bar of zinc may be placed in the solution in hydrochloric acid, which will throw down its contained tin; this process, however, is not so accurate as the former.

CHAPTER X.

ASSAY OF ANTIMONIAL ORES.

ANTIMONIAL substances susceptible of being assayed by the dry way, are divisible into two classes: the first comprises all those bodies containing oxygen or chlorine, and but little or no sulphur; the second, the sulphuret, and all antimonial ores containing much sulphur.

Class 1.-All the oxides of antimony are very readily reduced by charcoal, so that their assay presents no difficulty. The assay is conducted in precisely the same manner as that of oxide of lead, only as antimony is much more volatile than lead, the heat must be managed with much care, and the assay taken from the fire as soon as finished. When all suitable precautions are taken, the loss of antimony is not very considerable; but Berthier says it is never less than from 5 to 6 per cent. This, I think, is too high. Thus the pure protoxide gives 77 per cent. of metal, and antimonious acid 75. The reduction is readily made, without addition, in a charcoal crucible; but when the substance to be assayed is mingled with impurities, some flux must be added. It succeeds equally well with 3 parts of black flux, with 1 part of tartar, with 1 part of carbonate of soda, and 15

per cent. of charcoal, or any other equivalent reducing flux.

When the substance under assay contains oxide of iron, the latter oxide is more or less reduced, and the metallic iron alloys with the antimony.

Oxidized matters which contain but a small quantity of sulphur, can also be assayed in this manner; because the sulphuret gives up to black flux, the small quantity of antimony which it contains, so that but little remains in the slag. The common glass of antimony produces by this method of assay, 70 per cent. of antimony, and occasionally even more than that.

Class 2.-The assay of the members of this class may be made in two ways: first, by roasting and fusing the oxidated matter with black flux; secondly, by fusing the crude ore with iron, or iron scales, with or without the addition of black flux.

The roasting of sulphuret of antimony requires much care, for it is very fusible and volatile, as is also the oxide its decomposition gives rise to. The heat ought to be very low during the operation, and the substance continually stirred. When no more sulphureous acid is given off, we may feel assured that it is perfectly roasted, because no sulphate is ever formed in this operation.

The roasted sulphuret is then fused with 3 parts of black flux, or its equivalent.

Metallic iron separates very readily all the sulphur from sulphuret of antimony; but as sulphuret of iron has a specific gravity very like that of antimony, the separation is very difficult to manage; a strong fire

must be employed when the desulphuration is complete, to keep the whole body in full fusion, for a considerable time. With these precautions two buttons are obtained, which separate very well. The one white, and in large plates, which is antimony; and the other is a bronze yellow, a little brighter than the ordinary sulphuret of iron, because it is mixed with a little metallic antimony. During the operation, a very considerable proportion of antimony is always volatilized, which, by this process, is an inconvenience impossible to avoid.

It is, nevertheless, practised in the large way in some factories, but a good result is not generally obtained. It however appears, that when all the necessary precautions are taken, that it can be employed with profit.

The first precaution which is indispensable is, mixing with the sulphuret only the precise proportion of iron necessary to effect its decomposition, which quantity amounts to about 42 per cent. of its weight. If more be placed, the antimony having a great tendency to play the part of an electro-negative element will combine with the surplus, and an antimoniuret of iron result, part of which will remain in the antimony and part in the slag.

Further, the iron ought not to be rusty, and to be in the finest possible state of division. If the masses be large, a portion of sulphuret of antimony is volatilized before they can be fully attacked. In general 63 per cent. of antimony can be extracted from sulphuret by the aid of iron in the small way, but on the large scale it seems that 55 per cent. is the maximum.