the name of a very large class of fossils; hard, pellucid, and naturally colourless; of regularly angular figures, composed of simple, not filamentous plates; not flexible nor elastic, giving fire with steel; not fermenting in acid menstrua, and calcining in a strong fire.

The orders of pure crystal are three; the first is perfect columnar crystals, with double pyramids, composed of eighteen planes, in an hexangular column, terminated by an hexangular pyramid at each end: the second order is that of perfect crystals, with double pyramids, without a column, composed either of twelve or of fifteen planes, in two hexangular pyramids, joined closely, base to base, without the intervention of any column: the third order is that of imperfect crystals, with single pyramids, composed either of twelve or ten planes, in an hexangular or pentangular column, affixed irregularly, at one end, to some solid body, and terminated, at the other, by an hexangular or pentangular pyramid.

These are all the general forms into which crystal, when pure, is found concreted: but under these there are almost infinite varieties in the number of angles, and the length, thickness, and other accidents of the columns and pyramids.

When crystal is blended with metallic particles at the time of its formation, it assumes a variety of figures wholly different from these, constituting a fourth order, under the name of metallic crystals: when that metal is lead, the crystal assumes the form of a cube; when it is tin, of a quadrilateral pyramid, with a broad base; when iron, the crystal is found concreted in rhomboidal crystals: these crystals are very common about mines; but the common spars, which are liable to be influenced in the same manner by the metals, and to appear in the very same form, are to be carefully distinguished from them. There is one very easy test for this purpose, which is, that all spars are subject to be dissolved by aqua fortis, and effervesc violently only on its touching it: but it has no such effects on crystal.

The pebble crystal is common enough in all parts of the world; but that which is formed of hexangular columns, affixed to a solid base at one end, and terminated by a hexangular column at the other, is infinitely more so: this is what we call sprig or rock crystal, and is the species described by most authors under the name of crystal of the shops, or that kept for medicinal use.

It is to be chosen the clearest, purest, and most transparent that can be had: it should be proved to be no spar, by means of aquafortis, or by drawing a point of it along a pane of glass, which it cuts in the manner of a diamond. It is found in vast abundance in many parts of England and Ireland; and in Germany it is yet more frequent. It is found about Bristol of amethystine tinge; in Silesea and Bohemia it is stained to the colour of the ruby, sapphire, emerald, and topaz, in which case jewellers make great advantage of it, selling it under the name of accidental sapphire, &c.

Crystal is also used for a fictitious body, cast in glasshouses, called crystal glass; being, in fact, no more than glass carried, in the composition and manufacture, to a greater perfection than the common glass.

The best kind of glass-crystal is that called Venice crystal, made at Moran, near Venice. See Glass.

Crystals, in chemistry, salts or other matters shot, or congealed, in the manner of crystal. See Chemistry.