33. - Of Composition Wheels the corundum wheels deserve the first notice, they consist of particles of corundum cemented into a mass by means of shell lac, and which composition variously prepared is nearly the universal grindstone and polisher of the East Indies. The reader is referred for the details of their preparation to the article Corundum in this Catalogue.

34. - Barclay's Artificial Emery Stones. - The manufacture of these very useful grinding and polishing wheels, is fully described under the head Emery article 11: in most respects they are superior to the corundum wheels of the Asiatics described under the head Corundum. They may each be made of various degrees of coarseness and rapidity of cut; when properly compounded their texture is very uniform and free from the hard veins and flaws that sometime occur in grindstones.

35. - Opticians sometimes employ fine crocus made into a solid body with wax, and moulded or turned into form (see the article on Lenses and Specula) in Chap. XXXIII. Section 4. The wax polisher is generally used with water, which greatly prevents the destruction of its surface and also assists in carrying off those particles of glass or metal which do not become embedded in the polisher. The introduction of this composition is ascribed to Mr. Varley.

36. - Crocus, mixed with powdered chalk and melted glue, constitute a composition employed by Mr. Bass in the formation of little wheels, employed by him in sharpening the long slender straight blades of his cork cutting machine. In cutting each cork the knife sweeps by against a square piece of cork, which, during the time, makes one revolution, and the four angles are removed in one piece. The knife in proceeding to and fro, is rubbed on its upper side by three of the crocus wheels which revolve slowly against it with slight pressure, and the lower side of the knife rubs against two or three hard steel rings, which act as burnishers and keep up the fine wiry edge required in cutting cork.