a black, smooth, glossy stone, used to examine the purity of metals. The ancients called it *lapis Lydus*, the Lydian stone, from the name of the country whence it was originally brought.

Any piece of pebble or black-flint will answer the purposes of the best lapis Lydus of Asia. Even a piece of glass made rough with emery is used with success, to distinguish true gold from such as is counterfeit; both by the metallic colour and the test of aquafortis. The true touchstone is of a black colour, and is not uncommon in many parts of the world.

**TOUCHWOOD.** See Boletus, Botany Index.