MECHANICAL concentration may be carried out by hydraulic or pneumatic means. Water is usually preferred where circumstances will permit, although it is a denser medium than air. With the former one has to contend with slimes, with the latter, dust; both of which are objectionable substances. To avoid forming these, ore should not be reduced finer than necessary to free it from the gangue, thus limiting the quantity of finely comminuted ore to be treated, and the number of machines for dealing with it; for a certain bulk of stone has its surface largely increased in proportion to the fineness of its particles, and therefore requires a larger concentrating area. The specific gravity of different minerals is the quality most largely utilized in concentration; but the shape of the particles is also an important factor which must be taken into consideration in the treatment of ores. Machines used for concentration may be stationary or movable. The latter may have a regular forward or rotatory movement, or may be given a pulsating, oscillating or shaking motion. There is little difficulty in concentrating the larger grains of different classes of mineral, for the larger the grain the better will the specific gravity of the mineral act.