It will be seen from the following table of the specific gravities of a few of the most important metals, minerals, rocks, &c., that there is, as a general rule, a fairly wide margin between the specific gravities of the minerals and their associated gangues:

Gold (variation due to the metals it may
be alloyed with),

Silver (native),.

Specific Gravity.

12 to 20

The wet processes of concentration may be conducted in two general ways:-In "Jigs" or "Hutches," or on "Tables." In the "Jigging" process the ore to be treated is fed into a frame with a gauze-sieve or perforated plate bottom. The

frame is placed inside a box or hutch, and the ore is subjected to a series of jerks or lifts in rapid succession from a column of water forced through the perforations. The light particles

are lifted up to the top and carried away by the water at the overflow. In working the "Jig," either the sieve is jerked up and down in a cistern of water, as in the "Hand Jig," or the water is forced up through the sieve by means of a piston, in the latter case the sieve is fixed.

In "Collom's" Jig, shown above in vertical section, the ore is fed on to screens b, where it is subjected to the action of uprising currents of water. The main water-supply is connected to rectangular chambers e (one only is shown in the fig.), which are provided with pistons d, and communicate with spaces a underneath the screens. Motion is given to the pistons by means of a rocking tappet-lever i, which is operated by means of a crank and connecting-rod from a shaft m.

The water is thus forced upwards through the screens in "jerks," and the lighter portion of the ore, that is, mainly the gangue, is washed away over the sides of the screens. The concentrates are removed from time to time off the screens, and any fine metalliferous portion of the ore that may pass through the meshes of the screens is received in the compartments a. This machine is not continuous in its action, as it must stop each time that the concentrates are removed. With regard to the "Table" concentrators, these may be divided into two groups-(1), with stationary or fixed tables; (2), with moving tables.

In the stationary table concentrators the crushed ore and water is allowed to flow over tables or shallow troughs of varying width, the narrowest being at the commencement of the flow, consequently the velocity of the current is greatest at this point, and the heaviest particles settle at this stage. The width of the tables is gradually increased, the velocity of flow being correspondingly diminished and the finest stuff is deposited at the tail end of the tables, the gangue or earthy matter being carried away by the water. These tables are often of great length and require a large quantity of water. When used in connection with gold-mining, cross-pieces of wood called "riffles" are inserted in the tables, mercury being placed at these points to catch up the gold and form an amalgam.

In the second group, the tables may receive a slight swinging motion in the direction of the flow of water, or they may be what are termed "Percussion Tables." In the latter the table is subjected to a sharp blow either at right angles to the direction of flow as in the "Rittinger" table, or in the direction of flow as in "Halley's" percussion table.

When re

In the "Rittinger" machine shown in plan and elevation in Plate III., the table is divided down the centre so as to form two separate tables A1, A2, the entire table is suspended by four rods or chains at the corners, and is moved sideways by means of a series of cams, K, fixed on a roller R. leased from a cam the table is returned by a spring, N, and strikes against a fixed block M. The ore and water is fed on at one corner from the dividing planes T1, the other planes T distributing cleaning water, the strongest flow being furthest away from the ore plane T. The particles of ore, &c., have two movements, one down the plane, due to the flow of water and inclination of table, and the other across the table, caused by the percussion, the result is a diagonal movement downwards. The heavier particles are thrown farther by the blow, and, owing to their greater weight, there is greater friction between them and the table than the lighter particles, consequently they are longer and more exposed to the forward movement than the latter in the same interval of time. They thus tend to travel outside the lighter particles, and gradually separate, according to their specific gravities, into bands, the separation being more perfect the nearer the edge of the table where the different grades of quality of ore are delivered into separate launders. The velocity of the stroke requires to be adjusted according to the size and character of the ore treated, varying from 1 foot per second for coarse stuff to 5 feet per second for fine. The flow of water is regulated so as to assist the separating action. The output of six double tables is from 10 to 12 tons per 24 hours. The inclination of the tables. may be adjusted to suit the character of the ore being

treated.

In the later types of wet concentrating machines, as in the "Frue Vanner" shown below, the ore in a fine state of division, together with a quantity of water, is fed on by a

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spreader, 1, near the top of an inclined travelling band e, which has an uphill movement, clean water being run on at the same time from a pipe, 2. The band is mounted in a frame which is subjected to a side shake; the amount of shake, travel, and inclination of band are so adjusted that most of the earthy matter is washed down the band, while the metalliferous portion is carried onwards and finally deposited in a tank of water, 4, through which the band is

made to pass, underneath the table. The depth of ore on the table is about half an inch, some of finer metallic particles are washed down with the waste, and are collected in settling tanks. These deposits are known as "slimes," and are sometimes very rich in mineral. So far it has been almost -impossible to extract the valuable portion of these slimes, owing to the difficulty of getting rid of the earthy matter without losing the mineral.

In other machines of the "Frue Vanner" type, the main difference seems to be in the character of the shake given to

the table. These machines are continuous in their action, and are considered to be the best wet concentrators, one machine will probably turn out 6 to 7 tons of concentrates per 24 hours.

Concentration by Dry Method.-It is a well-known fact that in some of the finest mining districts in the world, notably in Australia and Africa, there is a great scarcity of water during the whole or some portion of the year, consequently the concentrators previously described, which are dependent for their successful operation on a good and abundant water supply, are of little value. Assuming, however, that a good water supply exists, concentration by the wet method is not altogether satisfactory. The output, for instance, of a "Frue Vanner" type of concentrator is only about 6 tons per 24 hours, and even at that the degree of concentration is far from being perfect, at the same time the finer and often richer particles of the ore are apt to float on the water and be carried away. This is especially noticeable in gold mining, where there is great loss by reason of the difficulty in retaining the finest particles of the metal or "float-gold."

Many attempts have been made to produce an efficient dry concentrator, by causing a current of air to act upon the ore when falling in a thin stream across the direction of the current which carries away the lighter particles. In another machine the air current acts upon a thin layer of the ore when moving over a tray of shot which is agitated, the heavier particles sinking below the shot out of the action of the air current, the finer particles being aspirated off. In both of these methods, the separation has been very ineffectual, on account of the difficulty in obtaining a current of air of uniform velocity throughout. The unreliable results obtained from such machines have caused dry concentration to be somewhat discredited by mining experts.

The author and his co-inventor, Mr Thomas Clarkson, had their attention directed a few years ago to the importance of dry concentration, and from a careful examination of the dry concentrators invented up to that period, also from numerous experiments, were led to the conclusion that the main