The mode of splitting these stones, as it is practised in some parts of France, is singular, and affords a proof of the extraordinary power of capillary attraction. The blocks are first cut into the form of cylinders, sometimes many feet in height. To split these horizontally into millstones, circular indentations are made round them, at proper distances, according to the thickness that is to be given to the stones; wedges of willow, that have been dried in an oven, are then driven into the indentations with a mallet. When these have been sunk to a proper depth, they are moistened with water; and, after a few hours, the several stones that have been marked out are found to be perfectly separated. 78. _ROCK CRYSTAL is an extremely beautiful kind of quartz, sometimes perfectly transparent, and sometimes shaded with grey, yellow, green, brown, or red. It occurs in the form of crystals with six sides, each terminated by a six─sided prism._ The name of this substance was considered by the ancients to signify ice, or water crystallized; and they imagined that crystal was produced from a congelation of water. Its uses are numerous. It is cut into vases, lustres, and snuff─boxes; and many kinds of toys of extremely beautiful appearance are made of it. When pure and perfectly transparent, it is much in request by opticians, who make of it those glasses for spectacles which are called _pebbles_, and who use it for various kinds of optical instruments. The best crystal is imported from Brazil and Madagascar, in blocks, not unfrequently from fifty to a hundred pounds in weight. This stone is wrought into the different shapes that are required, by sawing, splitting, and grinding. The sawing is effected by an extended copper wire fixed to a bow: the wire is coated with a mixture of oil and emery, and is drawn backward and forward until the operation is performed. But, as this process is a tedious one, particularly when the mass is large, a more expeditious, although less certain, method is sometimes adopted. The crystal is heated red hot, and a wet cord is drawn across, in the direction that the workman intends to split it. By the rapid cooling thus effected, in the direction of the cord, the stone easily splits by a single blow of the hammer, and generally in the direction required. The grinding is performed by means of emery: and the polishing effected by tin ashes and tripoli.