Slate. Is a blue fossil, very soft when dug out of the quarry, and is easily cleft, cut, or sawed into thin long squares, to serve in lieu of tiles, for the covering of houses. Sometimes also it is used for tables, and other purposes. The ancients were unacquainted with the use of slate, and covered their houses with shingle, as we read in Pliny. Besides the blue-slate, we have in England a greyish slate, called also Horsham-stone, from a town in Sussex of that name, where the greatest quantities of it art; found. The blue slate is a very light, lasting, and beau-tiful covering, to judge of the goodness of slate, Mr. Celepress recommends an examination of its sonorous quality; if it emits a good clear sound, the stone is firm and good. Another method of proving its goodness is by weighing it, then letting it lie six or eight hours under water, and wiping it very clean; if it weighs then more than it did before, it will not endure without rotting the lath and timber. .The clay-slate, which varies in colour from light blue to purple, is the most generally used. The blackish-grey kind is used in schools by scholars learning arithmetic, and for writing; the splinter kind is made into pencils, and several varieties are used as whetstones. Some kinds of hard slate, or schistus, have iron p\ rites, and mica embedded in them ; others are soft, and used for designing as black chalk.