in its original meaning, signified ice. It was afterwards applied to rock crystal, or crystallized siliceous earth; for the ancients, according to Pliny, regarded that body, as water which was congealed by the action of cold.
a species of stone belonging to the quartz or siliceous genus. It always appears, where there has been no interruption to its crystallization, in hexagonal prisms pointed at both ends. It is found of different kinds and colours. 1. Opaque or semi-transparent, and white or of a milk colour. 2. Opaque and red, or of a carnelian colour, from Oran in Barbary. 3. Opaque and black, from the same place. 4. Clear. The specific gravity of these kinds of crystals is from 2650 to 2700. Professor Bergman extracted from them about six parts of argilla and one of calcareous earth per hundred weight; but Mr. Gerhard found none so pure as to contain neither. 5. Clear and blackish brown; the smoky topaz, or rauchtopaz of the Germans. It is found at Egan in Norway, and at Lovila in Finland. These crystals are said to become clear by boiling them in tallow. 6. Clear and yellow; found in Bohemia, and sold instead of topazes. 7. Clear and violet-coloured; the amethyst from Saxony, Bohemia, and Dannemore in Uppland. The most transparent of these are called false diamonds, Bristol, Kerry stones, Alençon diamonds, &c. 8. Colourless rock crystal, properly so called, found in Bohemia, the province of Jemtland, and many other places. 9. Pyramidal crystal with one or two points. These have no prismatic shape, but either stand upon a base in cavities of quartz veins, have only a single pyramid, Crystal pyramid, and are of various colours; or they lie in clayey earths, and have both pyramids, but no prism. They are found at Blackenburg upon the Hartz, and at Morferloch in the Silverland in Transylvania.
The coloured transparent crystals derive their tinge from an exceedingly small portion of metallic oxide, but lose them entirely when strongly heated. They are called false gems; viz., the red from Oran in Barbary, false rubies; the yellow from Saxony, false topazes; the green from Dauphiny, very rare, false emeralds or prales; the violet from Vil in Catalonia, false amethysts; the blue from Puy in Valais in France, false sapphires. There are likewise opal or rainbow crystals, the various colours of which are thrown out in zones across the surface. They make a very fine appearance, though they never shine like the oriental opal.
M. Foureroy makes a remarkable difference between the crystals and quartz, by affirming that the former are unalterable in the fire, in which they neither lose their hardness, transparency nor colour; while the quartz loses the same qualities, and is reduced by it to a white and opaque earth. He classes the rock crystals,
I. According to their form, viz. 1. Inflated hexagonal crystals ending in pyramids of six faces, which have a double refraction, or show two images of the same object when looked through. 2. Hexagonal crystals united, having one or two points. 3. Tetradedral, dodecaedral, flattened crystals; and which, though hexagonal, have nevertheless their planes irregular. 4. Crystals in large masses, from the island of Madagascar, which have a simple refraction.
II. With regard to their colour, as being either diaphanous, reddish, smoky, or blackish.
III. With regard to accidental changes, some are hollow; some contain water within one or more cavities; some are called one within the other; some are of a round form, as the pebbles of the Rhine; some have a crust of metallic calcareous or of pyrites; some are found crystallized in the inside of a cavity; while some seem to contain amianthus or asbestos; and others contain shirils. The same author reckons among crystals the oriental topaz, the hyacinth, the oriental sapphire, and the amethyst. M. Daubenton has always looked upon this last as a quartz of a crystal.
When the rock crystals are semitransparent or intermixed with opaque veins, they are called by the Swedish lapidaries milk-crystals. When they are found in the form of round pebbles, which is occasioned by their being tossed about and rubbed against one another by floods or by the sea, they are called by the English lapidaries pebble-crystals. They come from the Indies, Siberia, and other places.
According to Bomare, the rock-crystals are generally formed upon or among quartz, which shows their great affinity, and are to be found in all parts of the world. The greatest quantity of them is brought from Mount Saint Gothard in Switzerland. Large pieces of these, weighing from 5 to 800 pounds, were found there at Grimmelberg; another of about 1200 pounds weight was found some years ago at Fribourg in the Valais; and a piece five feet long, four wide, and equally thick, was found in the island of Madagascar, where these natural productions are of the most extraordinary size and perfection.
In the imperial collection at Vienna, there is a pyra-
midal 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 show all the primitive 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 take great advantage of it, selling it under the name of occidental sapphire.
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.
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 metalline 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 rhombohedral figures: 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 aquafortis, 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 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 flocks, or that kept for medicinal uses.
With regard to the formation of crystals, it is certain that they must have been once in a soft state, since some are found to have water in their cavities. Professor Bergman obtained 13 regular formed crystals, by suffering the powder of quartz to remain in a vessel with fluor acid for two years. These were about the size of small peas, and were less hard than quartz. Mr Magellan informs us, that he received from Mr Achard Achard two crystals, one of the flary kind, and the other as hard and transparent as rock crystal. The first he procured by means of calcareous earth, and the latter from the earth of alum, both dissolved in water impregnated with fixed air, the water filtrating very slowly through a porous bottom of baked clay. The apparatus is described by the author in the Journal de Physique for January 1778; but though the process was attempted by Mr Magellan, and afterwards a second time by Mr Achard himself, neither of them were able to succeed. Mr Moreau, however, in the first volume of the Dijon Memoirs for 1785, affirms that he has produced a very small artificial crystal; and gives the proper method for succeeding in the process.
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 aquafortis, 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 five 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 faces, 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 frits, 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 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.
Iceland or Iceland Crystal, a transparent fissile stone, brought from Iceland, soft as talc, clear as rock crystal, and without colour, remarkable for its unusual refractions. It is a carbonate of lime.
It is there found in great abundance all over the country, but is particularly plentiful in a mountain, not far from the bay of Roezford, where the finest and most pellucid pieces are found on digging. The mountain lies in 65 degrees latitude, and has its whole outside made up of it; but though this makes a very bright and glittering appearance, it is not so fine as that which lies at a little depth, and is met with on opening the surface. This is generally taken up out of the earth in masses a foot long, and its corners very frequently are terminated in these large masses by a fort of crystals, very different in figure and qualities from the rest of the mass. The stone itself is of a parallelopiped figure; but these excrements are either single pyramids, affixed to columns like common crystal, or double pyramids with or without columns between. The stone itself is soft; these are hard, and cut glass; the stone calcines to lime in the fire; these run into glass: in short, the stone itself is true spar, and these are true crystal. Beside these, there sometimes grows out of the end of the larger masses a pure fine asbestos. This likewise is the case sometimes in the spar found about Bareges in France, and shows how nearly together the formation of bodies, wholly different from one another, may happen. The general figure of the stone is paralleloped; or, as some express it, rhomboid; and it retains this not only while whole, but also when broken to pieces; every fragment it naturally falls into, though ever so small, being truly of that shape. But it is remarkable, that in some places of this mountain the same sort of matter is found in form of triangular pyramids, all which have the same property of the double refraction with the parallelopeds of the same substance; so that the original error of supposing its qualities owing to its shape, is refuted by this, as well as by the trials made with other pellucid bodies of the same figure, which do not show this remarkable property.
The Iceland crystal is electrical, and when rubbed will attract straws, feathers, and other light substances, in the same manner that amber does.
The vast masses of white spar which are found in the lead mines of Derbyshire, though they are not externally of the paralleloped figure of the Iceland crystal, nor have anything of its brightness or transparency in the general lump; yet when they are broken they separate into rhomboidal fragments, and some of these are found to be tolerably pellucid; all those which are to have the property of the Iceland crystal; and being laid upon paper where a black line is drawn, they all show that line double, in the same manner as the real Iceland crystal does.
Iceland crystal bears a red heat without losing its transparency; and in a very intense heat calcines without fusion; steeped a day or two in water, it loses its natural polish. It is very soft and easily scratched with the point of a pin: it will not give fire on being struck against steel; and ferments and is perfectly dissolved in aquafortis. It is found in Iceland, from whence it has its name; and in France, Germany, and many other places. In England fragments of other spars are very often mistaken for it, many of them having in some degree the same property. It has none of the distinguishing characters of crystal; and is plainly a genus of spars, called from their figure parallelopedia, which, as well as some other bodies of a different genus, have the same properties. Bartholomew Huygens, and Sir Isaac Newton, have described the body at large, but have accounted it either a crystal or a talc; errors which could not have happened, had the criteria of fossils been at that time fixed; since Sir Isaac Newton has recorded its property of effervescing with nitric acid, which alone must prove that it is neither talc. Crystal, taic nor crystal, both those bodies being wholly unaffected by that menstrum. It is always found in form of an oblique parallelopiped, with six sides; and is found of various sizes, from a quarter of an inch to three inches or more in diameter. It is pellucid, and not much less bright than the purest crystal; and its planes are all tolerably smooth, though when nicely viewed they are found to be waved with crooked lines made by the edges of imperfect plates. What appears very singular in the structure of this body is, that all the surfaces are placed in the same manner, and consequently it will split off into thin plates, either horizontally or perpendicularly; but this is found, on a microscopic examination, to be owing to the regularity of figure, smoothness of surface, and nice joining of the several small parallelopiped concretions, of which the whole is composed; and to the same cause is probably owing its remarkable property in refraction.
The phenomena of this stone are very remarkable, were first suggested by Bartholin, and have been examined with great accuracy by Mr Huygens and Sir Isaac Newton.
1. Whereas in other pellucid bodies there is only one refraction, in this there are two; so that objects viewed through it appear double.
2. Whereas in other transparent bodies, a ray falling perpendicularly on the surface, passes straight through, without suffering any refraction, and an oblique ray is always divided; in Iceland crystal, every ray, whether perpendicular or oblique, becomes divided into two, by means of the double refraction. One of these refractions is, according to the ordinary rule, the fine of incidence out of air into crystal, being to the fine of refraction as five to three; but the other is perfectly new. The like double refraction is also observed in crystal of the rock, though much less sensibly. When an incident ray is thus divided, and each moiety arrives at the farther surface, that refracted in the first surface after the usual manner, is refracted entirely after the usual manner at the second; and that refracted in the unusual manner in the first is entirely refracted after the like manner in the second; so that each emerges out of the second surface parallel to the first incident ray. Again, if two pieces of this crystal be placed over each other, so that the surfaces of the one be parallel to the corresponding ones of the other; the rays refracted in the usual manner in the first surface of the first, are refracted after the usual manner in all the other surfaces; and the same uniformity appears in the rays refracted after the unusual manner; and this in any inclination of the surfaces, provided their planes of perpendicular refraction be parallel.
From these phenomena Sir Isaac Newton infers, that there is an original difference in the rays of light; by means whereof some are here constantly refracted after the usual manner; and others in the unusual manner. Were not the difference original, and did it arise from any new modifications impressed on the rays at their first refraction, it would be altered by new modifications in the three following ones; whereas, in fact, it suffers no alteration at all. Again, he hence takes occasion to suspect, that the rays of light have several sides, endued with several original properties:
for it appears from the circumstances, that these are two sorts of rays differing in their nature from each other, one constantly in all positions, refracted in the usual, and the other in the unusual manner; the difference in the experiment mentioned being only in the position of the sides of the rays to the plane of perpendicular refraction. For one and the same ray is refracted sometimes after the usual, and sometimes after the unusual manner, according to the position of its sides to the crystal: the refraction being alike in both, when the sides of the rays are posted the same way to both, but different when different. Every ray therefore may be considered as having four sides or quarters; two of which, opposite to each other, dispose the ray to be refracted after the unusual manner; and the other two in the usual. These dispositions, being in the rays before their incidence on the second, third, and fourth surfaces, and suffering no alteration; for what appears in their passage through them must be original and connate.
Farther Beccaria corrects the observations of Huygens and Newton concerning the refraction of rock or mountain crystal. The double refraction of the latter happens when a ray passes through two sides that are inclined to each other, and consequently illus coloured: whereas that of the Iceland crystal is made by the passage of a ray through two parallel sides, and therefore it illus colourless. He suggests, that there may be other substances in which there is a manifold refraction. Graveshane had a prism of Brazil pebble, which had a double refraction at each angle, but of a different kind from one another. Mr B. Martin prepared several prisms of Iceland crystal, which exhibited not only a double but a multiple refraction. A single prism produced a six-fold refraction; and by combining several prisms, a number of refractions was obtained equal to the product of those of the single prisms; i.e. a prism which afforded two images applied to one of six, produced a prism of twelve images, &c. He farther observes, with respect to Iceland crystal, that though the sides of its plane of perpendicular refraction be parallel to one another, a beam of light transmitted through them will not be colourless; in which property it differs from all other known substances.