By calculating the equivalent of tin by these data, and by removing the iron from it, we obtain 59.105, which differs little from 59-00, the number generally adopted.

The result of these experiments is, that by passing an oxygen current through a concentrated solution of protochloride of tin, the oxygen is not fixed; in diluted solutions, on the contrary, the oxygen is rapidly absorbed by the stannous salt. Oxygen does not act on hydrated protoxide of tin. The variations noted in the strength found when protochloride of tin is oxidised by permanganate of potash are not observed in concentrated solutions; they take place only in presence of oxygen dissolved in water.

There are then three different processes for estimating tin by permanganate of potash :-1. To operate with water freed from air by boiling, protecting it from access of air while cooling. 2. To oxidise protoxide of tin in an alkaline medium. 3. To decompose protochloride of tin either by a salt of iron, as proposed by M. Stromeyer, or by a salt of

copper.

BLOWPIPE REACTIONS OF TIN.

Tin deposits an oxide upon charcoal which is feebly yellow, and moderately phosphorescent when hot; on cooling it is white, and almost touches the assay. It assumes, with cobalt solution, a bluish-green colour, which may be readily distinguished from that produced by zinc.

Tin Pyrites.-Tin is readily recognised in this mineral by exposing a small piece of it to the oxidising flame on charcoal. The assay at first exhales a sulphurous acid smell, afterwards becomes snow-white on the exterior, and a white coating is perceived on the support surrounding the specimen this sublimate is so abundant, that the charcoal is not seen in any part between it and the metallic bead. This deposit is not expelled in either flame; in other respects, its compartment is similar to the oxide of tin.

The best method for the detection of tin in tantalites and tin slags is by reduction with soda; but in such a case it is necessary to add a small portion of borax, to dissolve the

tantalic combinations, and prevent the reduction of iron. After the completion of the process, the tin is obtained by pulverisation and sifting. To be convinced that the metallic particles obtained are tin, dissolve protoxide of copper in microcosmic salt, add some of them to the flux, and then heat the whole upon charcoal in the reducing flame. If tin is present, the glass will be coloured reddish on cooling.

OXIDES OF TIN.-Alone, the protoxide, in the state of hydrate, lights and burns like tinder, becoming peroxidised. The peroxide does not fuse or undergo any change except in the reducing flame, which, if strong and long continued, entirely reduces it without the aid of any reagent. Nevertheless, this operation requires much practice and experience.

With borax it fuses with great difficulty and in small quantity, giving rise to a transparent and colourless glass, which remains so during cooling. The colour of the glass is not changed in the reducing flame.

With microcosmic salt it behaves as with borax.

Soda and oxide of tin combine with effervescence on the platinum wire. The result of this combination is a blebby infusible mass, which cannot be dissolved by a large quantity of borax. On charcoal it is easily reduced, and gives a grain of tin.

427

CHAPTER XII.

ASSAY OF ANTIMONY.

ANTIMONIAL substances susceptible of being assayed by the dry way are divisible into two classes.

CLASS I. In this class are comprised native antimony and all antimonial substances containing oxygen or chlorine, and but little or no sulphur.

These substances are the following:

Native antimony, Sb,

Oxide of antimony, Sb2O3,
Antimonious acid, Sb2O4,

Antimonic acid, Sb2O5.

CLASS II. includes the sulphide of antimony and all antimonial ores containing much sulphur.

Sulphide of antimony, Sb,S3,

Oxysulphide of antimony, Sb2O3+2Sb2S3,
Haidingerite, 2Sb,S,+3FeS.

ASSAY OF ORES OF THE FIRST CLASS.

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 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.

Oxidised matters which contain but a small quantity of sulphur can also be assayed in this manner; because the sulphide 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.

The ores of this class occur very seldom, and are only in rare cases subject to assaying.

ASSAY OF ORES OF THE SECOND CLASS.

As pure sulphide of antimony (antimonium crudum) as well as metallic antimony (regulus of antimony) are mercantile substances, the assays of the ores of this class have for their object the determination of both these bodies.

I. DETERMINATION OF THE PURE SULPHIDE OF ANTIMONY
(ANTIMONIUM Crudum).

Sulphide of antimony is almost the only mineral from which antimonium crudum is produced. This mineral generally occurs intermixed with very refractory gangue (gneiss, quartz, limestone, etc.). Sulphide of antimony fuses readily at a low red heat, and is not changed during fusion, if atmospheric air is precluded. At a white heat it volatilises without change of composition.

The assay of sulphide of antimony is now effected by a liquation process, i.e. by heating the mineral sufficiently in order to melt the sulphide of antimony, and, by this means, to separate it from the refractory gangue. The production

of sulphide of antimony on a large scale is done in the same way.

For the assaying purpose, two pots or crucibles are used, one standing in the other one, and leaving sufficient space between the two bottoms to receive the fused sulphide of antimony. The bottom of the inside crucible is furnished with holes. The mineral to be assayed is put into the inside crucible, the latter is then closed with a cover, and hermetically luted; the joints of the two crucibles are also luted. The under crucible is then put on the hearth of a furnace, enclosed with ashes or sand, in order to keep it cool, and the upper crucible, as far as it is outside of the under crucible, is covered with coal, and heated to a moderate red heat. The sulphide of antimony will then melt and collect in the under crucible, from which it may be taken out, after cooling, and weighed.

2. DETERMINATION OF REGULUS OF ANTIMONY.

This assay may be made in two ways: first, by roasting and fusing the oxidised 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 sulphide 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 sulphurous acid is given off, we may feel assured that it is perfectly roasted, because no sulphate is ever formed in this operation.

The roasted sulphide is then fused with three parts of black flux, or its equivalent.

Metallic iron very readily separates all the sulphur from sulphide of antimony; but as sulphide of iron has a specific gravity near that of antimony, the separation is very difficult to manage: a strong fire must be employed when the desulphurisation 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