Sand has numerous uses. When mixed in due proportion with lime, it forms that hard and valuable cement called mortar. Melted with soda (200) and potash (205) it is formed into glass; white sand being used for the finer kinds, and coarse and more impure sand for bottle glass. A very pure kind of sand which is found in Alum Bay, on the west side of the Isle of Wight, and on some parts of the coasts of Norfolk, is in great request by glass─makers. Sand is also employed in the manufacture of earthenware; and its utility in various branches of domestic economy, but particularly for the scouring and cleaning of kitchen utensils, is well known. In agriculture sand is used by way of manure, to all soils of clayey lands: as it renders the soil more loose and open than it would otherwise be. The best sand for this purpose is that which is washed by rains from roads or hills, or that which is taken from the beds of rivers. There is a kind of sand which is naturally mixed with clay, and has the name of _Founder’s Sand_, from its being chiefly employed in the formation of moulds to cast metals in. At Neuilly, in France, there is a bed of perfectly transparent and crystalline sand. Each grain, when examined with a magnifying glass, is seen to consist of a perfect six─sided prism, terminated by two six─sided pyramids. The uses of the different kinds of _Sandstone_ will be enumerated in the account of the rocks (267, 268). 89. _LYDIAN STONE is a kind of flinty─slate, of greyish or velvet─black colour, not quite so hard as flint, opaque, and about twice and a half as heavy as water._ _It is usually massive, and, internally, has a glimmering appearance._ This mineral occurs in beds in primitive clay─slate (257); and is found in Bohemia and Saxony, and also in the Pentland hills near Edinburgh. It was first noticed in Lydia, whence it derived its name. It is sometimes used as a touchstone to ascertain the purity of gold and silver. This was its use among the ancients. The metal to be examined is drawn along the stone so as to leave a mark, and its purity is judged by the colour of the metallic streak. A good touchstone should be harder than the metals, or metallic compounds to be examined; if softer, the powder of the stone mixes with the trace of the metal and obscures it. A certain degree of roughness on the surface of the best stone is also requisite, that the metal to be tried may leave a trace or streak sufficiently distinct. It must not, however, be too rough, otherwise the particles of the metal will be hid amongst its inequalities, and no distinct trace will be formed. The touchstone should also be of black colour, as this tint shows the colour of the streak better than any other. 90. _FLINT is a peculiarly hard and compact kind of stone, generally of smoke─grey colour, passing into greyish white, reddish, or brown. It is nearly thrice as heavy as water, and when broken will split, in every direction, into pieces which have a smooth surface._