Concentration was formerly effected chiefly by a crude process of washing. In this process the sand is thrown on to a sort of sieve, fixed over the upper end of a long wooden trough, by one workman; a jet of water is directed on to the sieve, washing the sand through it. The heavier particles fall to the bottom of the trough, whilst the lighter are washed right through. A second workman continually turns over the sand left in the box and on the sieve; at the end of a day’s work the ‘concentrate’ is collected. This averages from 15 to 70 per cent. of monazite, according to the nature and amount of the heavy minerals accompanying it in the sand. The concentrate is dried either on rubber or oiled cloths in the sun, or on an iron plate covering a trough in which a fire is lighted. The iron minerals are then picked out by means of a magnet, and the sand filled into sacks for transport. Before treatment for thorium nitrate, the sand is at the present day further concentrated by powerful magnetic separators. In a few cases the older method of concentration by hand-washing has been abandoned for machine concentration, the Wilfley table being sometimes employed. The principle here is exactly the same, the sand being fed into a hopper by means of a moving belt and thence on to a machine-shaken table from which running water constantly removes the particles, sorting them according to their specific gravity. Further separation of the dried concentrate has been effected by three kinds of separators.[121] The first was of the Edison, or fall-and-deflection type; in this the sand is allowed to flow in a thin vertical stream past a horizontal magnet, which deflects the minerals containing iron; these fall on one side of a partition, the part richer in monazite on the other. The second was an electrostatic machine; the heated sand is borne on a moving belt underneath a rotating vulcanite cylinder, excited by felt-covered rubbers; the lighter particles are attracted to the cylinder, and dropped on one side, the heavier passing on. Neither of these machines is of much value in effecting concentration, and neither is in general use. [121] See Pratt and Sterrett, _Trans. Amer. Inst. Min. Eng._ 1909, ~40~, 313.