Rock Crystal: a transparent colourless stone; of a regularly angular figure, which is generally that of an hexagonal column terminated by a pyramid of the same number of sides; hard, so as to strike fire freely with steel; becoming white, spake, and friable, by repeated ignition and extinction in water; not dissoluble by any acid, either in its natural state or when calcined; fusible, with vitrefactive fluxes, into a nearly colourless glass.

Rock crystal, and some other stones of the same general nature, introduced into medicine by the credulity of former times, not yet expunged from catalogues of the materia medica, and in some places dill made ingredients in officinal compositions; appear, from their indiffolu-bility in every known species of humid men-struum, to be incapable of exerting any action in the human body: unless that by the rigidity and hardness, which their particles retain, however sinely levigated, they may offend the sto-mach and intestines; or that by virtue of the calcareous earth, which they abrade plentifully from the marble instruments with which they are levigated, the prepared powder may act as an absorbent.

The colours of the precious stones appear to depend on a principle distinct from the stony matter which makes their basis. It is said, that the sapphire, emerald, amethyst, and cornelian, on being urged with a strong fire, become colourless, and nearly similar to common crystal: that the emerald, in parting with its tinging matter in the fire, emits visible flames (a): that the hyacinth and garnet melt, in a vehement fire, into a vitreous mass, of a brown-ish or blackish colour like that which ferrugi-neous calces communicate (b). From these kinds of experiments it has by some been inferred, that the coloured precious stones, though their stony basis is consessedly inactive, may, nevertheless, have some medicinal powers depending on the tinging metallic impregnation (b). But surely this reasoning does not take off the impropriety, or rather absurdity, of using, as medicines, these costly concretes, from a poslibility of their producing effects, which far cheaper substances are known to produce with certainty.