CONCENTRATION. 131 



In the ordinary " jigging " apparatus a sieve containing 

 broken or crushed ore is worked up and down in water, or 

 has water pushed through from underneath, and the heavier 

 parts which do not fall through the meshes, follow the same 

 law as the above. Thus with a series of " jiggers," each 

 at a lower level than the next, and kept in motion by means 

 of an eccentric or revolving crank, the "stuff" can be not 

 only " sized" but also sorted as to weight of particles. 



Another plan of concentration is that of allowing running 

 water to wash the ore along an inclined plane, as in the 

 ordinary amalgamation tables, or in the "Broad Tom" 

 or " Long Tom " sluice (natural or artificial) ; and the 

 same principle that the lighter portions are washed 

 farther down than the heavier ones which remain near the 

 top applies to the " buddle " for treating slimes or finely 

 divided ore. In the simplest form of "buddle" the ore 

 is passed through a vertical passage on to the apex of a cone 

 with slightly inclined sides, the heavier matter settling near 

 the apex. 



It is unnecessary to describe the many various kinds of 

 concentrators which have been or are being used. Mention, 

 however, may be made of the well-known Hendy's concen- 

 trator, in which the oscillating shallow pan is constructed 

 in a specially designed curve towards the centre. In this 

 apparatus the heaviest portions sink to the bottom and 

 the lighter flow through an outlet in the centre. 



In the True Vanner, a shaking motion is imparted, the 

 finely divided matter being placed on the upper end of a 

 revolving band. In "percussion" tables the action of 

 water and percussion causes the heavier matter to be retained 

 at the head of the table and the lighter farther removed. 

 To classify mineral matter according to size, the ordinary 

 sieve, trommels (revolving sieves, cylindrical, conical, single, 

 or continuous), are much used for coarsely broken ore. 



There are other classifiers in which the varying velocity 

 of the grains in water, &c., are made use of, and in some 

 concentrators atmospheric assistance and gravitation, and 

 also centrifugal force, play a part. 



