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charged with various grinding and polishing powders, are employed for different stones, and in different stages of the operations. The wheel, or disc, or lap as it is called, is usually horizontal and is made to revolve with great rapidity. The grinding or polishing powder, mixed, according to its nature, with olive oil or water, becomes partially embedded in the lap. This powder, in the case of diamonds, must be of diamond itself, generally in the form of boart, a dark and rather porous variety of the mineral. The comparatively new and artificial compound of silicon and carbon known as carborundum is now largely used in the case of the harder stones, but emery, garnet-powder, tripolite, rotten stone, jeweller's rouge, pumice, putty-powder, and bole are in constant requisition for the grinding and polishing of stones less hard than the diamond. The whole subject of this mechanical treatment of stones, including splitting, dividing and shaping operations, is one which cannot be discussed here, involving as it does a large number of minute technical details of no interest from an artistic standpoint.