119. TRIPOLI is a kind of clay of yellowish grey, brown, or white colour, sometimes striped or spotted, and of an earthy texture.
It feels harsh and dry to the touch; is soft, scarcely adheres to the tongue, and will not take a polish from the nail.
This substance obtained its name from having formerly been imported into Europe from Tripoli, on the north coast of Africa. It is, however, now found in several parts of Germany; and a granulated kind has been discovered in England.
Tripoli is used for the polishing of metals and stones. For this purpose, it is mixed with sulphur, in the proportion of two parts of tripoli to one of sulphur. These are well rubbed together on a marble slab, and are applied to the stone or metal with a piece of leather.
When tripoli is combined with red ironstone, it is used for the polishing of optical glasses. It is sometimes made into moulds, in which small metallic or glass figures and, medallions are cast; and a kind of tripoli is found near Burgos, in Spain, which is used as an ingredient in the manufacture of porcelain.
In Derbyshire,, and several parts of Staffordshire, is dug a kind of tripoli which has the name of rotten stone. This is considered to be a produce of limestone shale, which has undergone a decomposition by exposure to the air and moisture. It is used for most of the same purposes as tripoli.
CLAY SLATE FAMILY.
120. CLAY SLATE, or ROOFING SLATE, is a kind of stone of foliated texture, and greyish, black, brown, green, or bluish colour.
It breaks into splinters, does not adhere to the tongue, yields generally a clear sound when struck, and is nearly thrice as heavy as water.
Vast and extensive beds of slate occur in different parts of the world; and this mineral sometimes constitutes even a principal portion of mountains. In our own country there are many important quarries of it, particularly in Westmoreland, Yorkshire, Wales, and Derbyshire.