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sometimes, marble and iron

POLISHING, the art of gloss or lustre to a b thing ; particularly a precious stone, marble, glass, a mirror, or the like.

For grinding and polishing steel, the grindstones used are made to rev 1 ve, either vertically or horizontally, with a velo city so great as to describe sometimes as much as sixty feet in a second. The steel is also, in some cases, drawn back wards and forwards horizontally on a circular surface ; and in order that the action may be equally distributed through out the surface, it is allowed to revolve on an axis by means of the friction ; its motion being confined to one direction by the action of a catch. Various substances, chiefly of mineral origin, are also used, on account of their hardness, as inter mediate materials, for grinding and polishing others. These arc diamond-dust, corundum, emery, tripoli, putty, glass, sand, flint, red ovyde of iron, or crocus martin, and prepared chalk. These are sometimes applied in loose powder, and sometimes fixed on wood, leather, or paper. Cuttlefish bone

and seal-skin arc furnished by the animal kingdom ; and Dutch rushes by the vegetable ; these are employed chiefly in polishing wood or ivory. Marble is made smooth by rub bing one piece on another, with the interposition of sand ; the polishing-blocks are sometimes caused to revolve by machinery in a trough, in which the marble is placed under water, and are draw n at the same time gradually to and from the centre; or the slab itself, with the frame on which in rests, is drawn slowly backwards and forwards, while the blocks are working in it. Granite is polished with iron rubbers, by means of sand, emery, and putty: but it is necessary to take care, during the operation, that the water, which trickles down from the rubbers, and carries with it some of the iron, may not collect below the columns, and stain them ; an inconvenience which may be wholly avoided by employing rubbers of glass.