SHALE, in natural history, a species of SCHISTUS. It is a black flaty substance, or a clay hardened into a stony consistency, and so much impregnated with bitumen that it becomes somewhat like a coal. The acid emitted from shale, during its calcination, uniting itself to the argillaceous earth of the shale, forms alum. About 120 tons of calcined shale will make one ton of alum. The shale, after being calcined, is steeped in water, by which means the alum, which is formed during the calcination of the shale, is dissolved: this dissolved alum undergoes various operations before it is formed into the alum of the shops. Watson's Chemical Essays, vol. ii. p. 315. See ALUM.
This kind of slate forms large strata in Derbyshire; and that which lies near the surface of the earth is of a softer and more shivery texture than that which lies deeper. It is also found in large strata, generally above the coal, in most coal counties of this kingdom. Dr Short informs us, that the shale wafles the lead ore near it, by its strong acid; and that it corrodes and destroys all minerals near it except iron or coal, of whose vitriol it partakes.