namely, I, the south west of Cornwall, beyond Truro; 2, the neighbourhood of St. Austell; and 3, the neighbourhood of Dartmoor, in Devonshire.

The first group is by far the richest and the best explored. The formation most abundant in tin mines is principally granite, though there are numerous exceptions to this. The most ancient metalliferous veins are the great tin veins in Cornwall; yet these are not of one formation, but belong to two or more different systems. Their direction is, however, nearly the same, but some of them dip towards the north, and others towards the south. The Cornish miners were formerly of opinion that tin occurred in the upper portions of the mineral lodes only, and mines were abandoned when in sinking, the miners came to the yellow copper pyrites, which were said to have cut out the tin. Within the last few years, however, tin has been found at very great depths below the surface and beneath the copper. Dolcoath mine is a remarkable example of this. This mine was worked first as a tin mine for a very long period; then as a copper mine for half a century; and finally, on exploring still deeper, the lode was found to become richer in tin, and is now worked to great advantage.

Alluvial Tin Ore, Stream Tin.-Peroxide of tin occurs disseminated both in the alluvium which covers the gentle slopes of the hills adjoining the rich tin mines, and in the alluvium which fills the valleys that wind round their base; and in these deposits the tin stone has been so abundant, that for centuries the whole of the Cornish tin was derived from them, and still is to some extent. The most important explorations of alluvial tin ore are grouped in the environs of St. Just and St. Austell, where they are called stream works, because water is the chief agent employed to separate the metallic oxide from the sand and gravel.

Synopsis of Processes for the Extraction of Tin from its Ores.

The object of these processes is to reduce, at a suitable temperature, the oxide of tin contained in the tin ore, and to transform the earthy and metallic foreign associates into a slag as

thin as possible, so as to allow a thorough separation of the reduced particles of tin; though simple in themselves, these operations are rendered more or less imperfect by the following circumstances:

1. Certain Properties of the Oxide of Tin.-This requires for its reduction a very strong red heat, approaching to whiteness, at which most of the foreign metallic oxides, which are frequently associated with tin ore, are also reduced, many of them, indeed, at a much lower temperature. These reduced metals combine with the tin and impair its quality, or they form deposits on the hearth of the furnace, and injure the process. Again, as tin oxidises so easily, oxide of tin is formed at the high temperature required for reduction, and this oxide becomes partly scorified, and is partially carried away mechanically owing to its fine division. The cupola furnaces must therefore be so constructed as to allow a quick reduction of the oxide of tin, and to remove the reduced tin as quickly as possible from the space where the blast reacts. Furthermore, as oxide of tin combines as a base with silica, and also, though less markedly, as an acid with oxide of iron, manganese, lime, &c., its tendency to scorification is very great, forming combinations from which the tin may be partly extracted only at a high temperature, when the foreign metallic oxides present (chiefly oxide of iron), are also reduced in larger quantities.

2. The Quality and Quantity of the Earthy Associates. -As tin ore occurs in older formations which are rich in silica, and therefore difficult to fuse, the formation of slag is rendered difficult. If ferruginous fluxes are added to facilitate the fusibility, a great deal of iron will become reduced, and some oxide of tin scorified by the protoxide of iron. An addition of lime acts similarly with regard to the scorification of tin; the least of two evils is therefore chosen, and the formation of a slag rich in silica (bisilicate) preferred. This slag being pasty and difficult to fuse, retains many grains of tin, the extraction of which necessitates a re-smelting or a dressing. When dressing, the scorified oxide of the tin is considered as lost.

3. The Quality of the Metallic Associates.-These are present either in an oxidised state, as magnetic iron ore, tungsten; in a sulphuretted state, as iron and copper pyrites, galena, copper glance; in a metallic state, as bismuth, copper; or in combination with arsenic, as arsenical pyrites. They are partly scorified by the smelting process, and partly decomposed, alloying the tin, such as iron, copper, bismuth, lead, arsenic, tungsten, &c.; or are volatilised, and form in the furnace deposits which injure the process. As these metallic associates have nearly the same specific gravity as tin ore, and as the tin ore usually occurs finely disseminated in gangue, it cannot be sufficiently separated by a mechanical dressing; they are, therefore, partially separated by combined roasting and washing processes previous to the smelting. Tungsten has to be separated by a special smelting process; if it remains in the ore, part of the tungstic acid combines with protoxide of iron and manganese, and enters the slags, but another part becomes reduced and alloys with the tin.

The processes for the extraction of tin may therefore be classified into certain preliminary operations, into the reduction smelting in cupola or reverberatory furnaces, and into the further treatment of the smelted products.

I. PREPARATION OF TIN ORES.

A mere mechanical preparation is only sufficient for stream tin and a few kinds of mine tin, such as the ore from Carclase, Wheel Bal, and Irtendron, in Cornwall.

In most cases a farther chemical treatment is required to extract the foreign admixtures. The following operations may be adopted :-a. A soft burning of the fragments of ore. b. A mechanical dressing of the raw or burned ore. c. A roasting of the schlich resulting from the latter operation. d. Subsequent washing. e. A treatment of the roasted mass with acids or with sulphate of sodium for extracting tungsten.

a. Soft Burning of the Ore previous to its Dressing.— The tin ore of Altenberg, in Saxony, containing or per cent of tin and the before-mentioned associated minerals, and of great compactness, is burned in open heaps. Upon a layer of ore 0'3 metre thick and 12 metres square, a layer

of wood shavings o'r metre thick is placed, and upon these a layer o'7 metre thick of brush-wood; next comes a layer 15 metre thick, of ore in grains the size of two cubic decimetres, and the side walls are also furnished with layers of this ore one metre thick; the whole is then covered with a layer, o'3 metre thick, of the ore of the size of the fist; the heap burns two days.

If mounds are used, they are furnished with a foundation of wood 14 or 16 inches thick, upon which a layer of tin ore 2 to 3 feet thick is placed, and covered with small coal 4 or 5 inches thick.

In England roasting furnaces are employed for this burning, the ores being burned for about six or eight hours.

The object of this partial burning is to save time and expense, nearly three-fourths of the ore being thrown away after dressing from the first burning.

b. Mechanical Dressing of Tin Ores.*-The dressing operations vary in different localities, and depend on the quality of the ore, whether it is more or less rich, finely or coarsely disseminated, and on the quantity and quality of the associated minerals. As tin ore is seldom massive, a gigging apparatus cannot be employed, and the tin ore has to be at once stamped more or less fine; but first it is submitted to a hand sorting, which is adopted in Bohemia for separating tungsten, and in Cornwall for separating the pure from the impure ore containing copper and arsenical pyrites and the gangue. The wet dressing of the ore, being rather complicated, causes a considerable loss of metal, amounting in Saxony to about 25 or 30 per cent, and in Cornwall from 17 to 20 per cent.

A comparison of the mining and dressing of the tin ores in Cornwall, Bohemia, Saxony, and the Bretagne, has been made by J. Huguenin, (Coup d'œil sur la Géologie du Morbihan: Paris, 1862, page 63).

c. Roasting the Concentrated Tin Schlich.-Schlich obtained from pure ores is smelted direct, but that containing

B. u. h. Ztg., 1855, pp. 93, 162; 1859, p. 181; 1862, p. 145; 1856, p. 62. Freib. Jahrb., 1830, pp. 217, 235. Berggeist, 1861, No. 68. Preuss. Zeitschr., Bd. 9, p. 251.

pyrites is previously roasted, as it still retains all the tungsten, the greater part of the arsenical pyrites, and much iron and copper pyrites, bismuth glance, together with some native copper and bismuth. The roasting in reverberatory furnaces is resorted to for the volatilisation of sulphur and arsenic, and the formation of oxides which may be washed off, being of less specific gravity than tin ore. The tin ore is but slightly modified by the roasting process; only at the commencement some little sulphide of tin is formed, which afterwards becomes converted into sulphite or sulphate of tin. This may be extracted by muriatic acid, but is lost in the subsequent dressing. In Cornwall the concentrated schlich, which is there called tin-witts, suffers a loss of about 1.5 per cent owing to this circumstance.

At the beginning, the roasting process is carried on at a low temperature, while the mass is continually stirred, in order to avoid caking. The temperature is then increased, and, if required, some coal is admixed in order to decompose the sulphates and arseniates that have been formed in the first period. The coal must not be added if it is intended to collect white arsenic in larger quantity, as a by-product in the condensation flues with which the roasting furnace is connected, or when treating ores containing copper pyrites with the intention of lixiviating sulphate of copper from the roasting mass for the production of cement copper. Ores containing iron pyrites only are more easily roasted than ores associated with copper and arsenical pyrites.