very poor, is re-worked in round buddles; a third layer is redollied, whilst the bottom is found sufficiently clean to go to the furnace. The middles, or central zone of stuff from the round buddle, is again re-worked and divided, until it is rendered free from impurities, whilst the tails are thrown away as worthless.

The water employed in these various operations holds for some time in suspension, a large number of fine particles, which are deposited in slime pits. The coarser particles are ultimately taken together with the raggings from some of the jigging hutches, and fragments of slate, quartz, or other vein stuff, containing spots of ore, to the stamping mill, where they are pounded by heads weighing 4 cwts. each, making 54 blows per minute, and falling 8 inches. From the stamps the stuff flows into pits, and is worked through a strake, and after being tyed, trunked, submitted to the round bundle, and dollied as may be found expedient, the dressing of the whole of the vein stuff is effected. About 25 parts of waste to 1 of ore is removed by these various operations at an average cost of 45s. per ton of crop ore, and £7. 7s. 54d. per ton of slime and halvan ore, whilst the average value of the whole for 1856 was £18. per ton.

The following statistics relative to cost of dressing lead ores in Cardiganshire, in 1855, are compiled from Messrs. Taylors' valuable and elaborate statistics on this subject:

:

Many ingenious machines have been constructed, with a view of facilitating the process of dressing slimes, and rendering tin stuff of a low percentage remunerative. Among the recently invented apparatus found practically successful, may be noticed that of Captain John Webb, at the Hewas Mines, and the patent cleansing frame of Captain Richard Hancock, lately in use at Polgooth.

For pulverizing, whether employed in reducing the variable tin stuffs of Cornwall, or the hard auriferous quartz of Australia and California, no contrivance has been found more simple or effective than the ordinary stamping mill. A striking proof of the economic value of this machine is afforded by statistics relating to the Polberro Mines :

The average number of heads employed during 5 years was 70, which reduced no less than 106,249 tons of stuff for washing purposes. This quantity averaged 20th Ibs., or 0.89 per cent., of merchantable black tin per ton. On this produce the profit per head amounted to £210. for the total period, or £42. per head per annum. The proportion of profit to the cost was as 1 to 5.3; hence, had the mine operated with 57 instead of 70 heads, the difference being 13, it would only have met its cost at the average sale price for the ore of £63. 2s. 4d. per ton.

MOISTURE IN ORES.

The amount of moisture existing in ores, on effecting a sale within the ordinary period after completing the dressing operations, varies with the constituents, and size of the particles of which it is composed.

The following results are derived from a series of averages, and will perhaps be valuable as affording a general idea upon this subject:

Cwm Erfin, per 21 cwt. =

76 lbs. of water = 3.2 per cent.

= 2.7

64

54

Average per 21 cwt. =

72 lbs. of water = 3.0 per cent.

ASSAYING.

ASSAY OF IRON ORES.

FOR the assay of iron ores a common wind furnace is required. This, in order that it may have a good draught, should have a chimney at least 30 feet in height. Furnaces for this purpose are often built of such a size that four assays may be made at the

The

same time; by which arrangement not only a better heat is obtained, but the loss of fuel attendant on the alternate heating and cooling of a large mass of masonry is to a great extent avoided. A convenient size for such a furnace is 14 inches square, in the hearth, A, and 2 feet in depth to the surface of the fire-bars. chimney, B, may be of almost any convenient size, provided it is sufficiently large; but when its area is less than one-third that of the fire-place, the draught cannot generally be depended on. The quantity of air passing through the apparatus is regulated by a damper, c, fixed in the chimney, and placed at a convenient height from the ground. Coke is commonly the fuel employed in such furnaces, and a proper distance for the bars to be placed from each other is in that case ths of an inch; if, however, a mixture of coke and charcoal be used, the grate should be made a little closer.

Wind Furnace.

The above furnace must be accompanied by an assortment of tongs to be employed for various purposes. For the removal of the assays at the close of the operation, very strong tongs are required. These being long in the handles, and provided with stout bent jaws, may be used as soon as the crucibles are sufficiently hardened to bear removal; but as at this stage of the operation the furnace is exceedingly hot, it is sometimes difficult to look into the fire, in order to see where to apply the jaws of the tongs. This inconvenience is lessened by using a wooden shield, near the centre of which is inserted a piece of common window glass;

when the shield is employed, it should be held before the person using the tongs by an assistant. By this contrivance the heat is effectually prevented from reaching the face, whilst the small glazed aperture allows of the interior of the furnace being distinctly seen. If the opening be closed by a sheet of mica or a fragment of coloured glass, the glowing crucible may be looked at without danger to the sight.

Preliminary Operations.-The assay of an iron ore should yield on a small scale similar results to those obtained on the large, and, consequently, has the advantage over the humid process, of affording results of a more practical character. Before operating in the dry way, it is, however, necessary to obtain some definite idea of the composition of the ore to be treated, so as to determine what fluxes should be added for the purpose of yielding, not only a fusible slag, but also the largest possible proportion of metallic iron.

The ores of iron may be divided into three classes :— 1st.-Those which contain that metal in the state of anhydrous oxide, such as oligistic and magnetic iron ores.

2nd.-Those which chiefly consist of hydrated sesquioxide.

3rd.-Spathose and clay ironstones, in which carbonate of the protoxide is the chief ingredient.

When the ore to be examined belongs to the first class, the amount of siliceous gangue is not readily determined by ebullition with hydrochloric acid, since the anhydrous peroxide and magnetic iron ores are but sparingly soluble in that reagent. In this case, the earthy carbonates will be dissolved, and may be estimated by the loss sustained on boiling a known quantity of the ore in weak nitric acid, and deducting the water expelled by ignition in a platinum crucible.

The preliminary examination of minerals of the second class may be conducted as follows:-Attack, in the cold, 50 grains of the ore, finely pulverized, either by acetic or very dilute nitric acid. This will dissolve only the carbonates of lime and magnesia, without interfering with either the oxide of iron or siliceous matters. Should it not contain any earthy carbonate, no effervescence will ensue on the addition of an acid; and in that case we may pass on to the next stage of the operation; but if effervescence takes place, weak acid is to be gradually added until it entirely ceases. When this point has been reached, the residue is thrown on a filter, washed with a little water, dried, and calcined. On weighing this product and deducting its weight from the original 50 grains operated on, we determine the collective amount of water, and the carbonates of lime, and magnesia present in the ore. Of the finely pulverized ore 50 other grains are now placed in a platinum