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 flint; 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 18 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 12 or of 16 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 12 or 10 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.

There 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 figures: these crystals are very common about mines; but the common spar, 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 effervescence violently only on its touching them; 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 nately 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 uses.

The origin and formation of crystals, as to the time and manner of them, deserve a very nice inquiry; since many of the more compound stonelike bodies are formed chiefly either of crystal, or of spar, a body in many things resembling it. The original formation and coalescence of those bodies of which spar is the basis, we know may have been but of yesteryear, since we have evident proofs that spar is concreting to this day, and that sparry bodies are forming every moment. This is evident from the sparry stalactites in the arches of modern buildings, particularly in one lately built as the new bridge at Westminster; the roofs of the arches of which were filled with these spars within a year after they were built. It is also demonstrable that the spars are not formed of matter exuding from the stone, since brick arches abound equally with them; and the brick vault which supports part of the grand terrace at London, was some time ago so full of them that there was not room to walk. These observations sufficiently demonstrate the growth of spar; but the vegetation of crystal remained dubious till Dr Hill showed by some experiments that crystal, as well as spar, is dissolved in every kind of water, even such as appears to be most pure and clear. This is also probable from an observation of Neuman's, who tells us, that he has seen leaves, stalks of plants, hay, straw, hogs bristles, &c., inclosed in sprigs of crystal. From the regular forms in which these natural crystals are found, the regular arrangement of salts into different figures takes the name of crystallization, and both are probably owing to the same cause. Henckel gives us a remarkable account of the formation of crystal out of human urine. He once filled a large round glass vessel half way up with the recent urine of a young lad, and tying a bladder over the mouth of the vessel, let it in a stove for four years together, never stirring it during that whole time. At the end of this time he found a number of small white stones growing to the inside of the glass; they were of the size of an oat-feed, of a prismatic figure, and tolerably pellucid: they stuck so fast to the sides of the glass that they could not be washed off by the shaking about of the urine; and when taken out had no saline taste, and were not soluble even in hot water.

Among the largest and finest crystals we have any account of, were those found in the mountains of Grünfels between immense strata of stone. They were all as pure and limpid as the clearest water, and the largest of them measured near three feet in length, and little less in circumference; its weight was 250 pounds. Others were of 130, 100, and so on, till they came to 10 pounds weight; but these were the smallest found in that place. Neuman, however, tells us of a piece mentioned in the Brellaw collections, which weighed upwards of 400 weight. In the Imperial collection at Vienna, there is a pyramidal crystal vase two ells in height, cut wholly out of one piece. It is usual with the largest crystals of the German mountains to be full of cracks and flaws, and to be so constructed internally as to shew all the prismatic colours; but the above mentioned ones were quite free from these blemishes, and resembled columns of the purest glass, only much clearer than any glass can be made. Crystal is also found in many parts of Britain and Ireland. About Bristol it is found of an amethystine tinge. In Silesia and Bohemia, in Germany, it is found stained with the colours 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 frequently cut; and lustres, vases, and toys, are made of it as of other beautiful stones. For this purpose it is to be chosen perfectly clear and transparent. It is to be tried by aqua fortis, or by drawing it along a pane of glass. The genuine crystal will not be affected by the acid, and will cut glass almost like a diamond. When any piece of workmanship of natural crystal is become foul and dark, the following method is to be used for recovering its brightness without hurting the polish. Mix together six parts of common water, and one part of brandy; boil these over a brisk fire, and let the crystal be kept in it, in a boiling state, a quarter of an hour; then take it out, and rub it carefully over with a brush dipped in the same liquor; after this it is to be wiped with a napkin, and by that means its surface will be perfectly cleaned, and rendered as bright as at first, without any injury to the points of the cutting, or the polish of the planes or facets, which would probably have happened had the cleaning been attempted by mere rubbing with a cloth.

Natural crystal may be reduced by calcination into a state proper for making glass with alkaline salts, and thus becomes a very valuable frit. The method of doing it is as follows: calcine natural crystal in a crucible; when it is red-hot throw it into cold water. Repeat this eight times, covering the crucible that no dust or ashes may get in among the crystal. Dry this calcined mass, and reduce it to an impalpable powder.

Colouring Crystal, for the imitation of gems. See Doublet.

Crystal is also used for a fictitious body, cast in glass-houses, 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 Venetian crystal, made at Moran near Venice. See Glass.

Island Crystal. See Island Crystal.

Crystal Dew. See Dew.

Dissociative Crystal. See Dissociative.

Rock-Crystal. See Rock-Crystal.