ities, composed of an almost impalpable agglutinated powder, are used as oil-stones.
Such are the Water of Ayr, and Blue stones, and the Turkey oil-stone.
The old Royal exchange of London was pavel with this stone, and when it was burned, about 1834, the pavement yielded a handsome amount towards the re-erection.
The Nova Scotia and Berea, Ohio, grindstones are largely employed in this country; these may be obtained of very large size and uniform quality.
The Washita, Arkansas, stone is of the very finest quality, being sharp and clean, and is made into a great variety of forms; grindstones, whetstones, hones, and slips of various sizes and forms.
In dressing rough grindstones, the process of hacking is employed.
Hacking consists in notching the projecting parts with a short-handled hammer resembling an adze.
Laps used in polishing stones, etc., are jarred by holding an old knife against their edges, so as to vibrate or chatter, making a slight indenta