Crystal

Crystal (from the Greek krustallos, clear ice) was the term originally applied to the clear, colourless form of quartz (q.v.), still known as rock-crystal, which occurs in six-sided prisms with six-sided terminal pyramids, and which, until the seventeenth century, was supposed to be the result of extreme cold acting upon water or some other substance. The term was afterwards extended to other bodies, some colourless, others coloured but still translucent, and others even opaque or metallic, until it came to indicate the definite geometrical form, and not the ice-like transparency.

Nicholas Steno, a Dane, recognised in 1669 that the angles of inclination of the faces of rock-crystal were constant, though the faces themselves varied in shape and size. Bartholinus, Huyghens, and Newton studied the property of double refraction (q.v.) exhibited by Iceland spar, the clear form of calcite (q.v.), and, to a less extent, by quartz; and Robert Boyle (1672) recognised that artificially prepared chemical compounds or "salts," when precipitated from solution, and metallic substances, such as bismuth, on consolidation from fusion, assume regular forms similar to natural crystals. Rome de l'Isle (1772), however, first showed the geometrical derivation of one form from another by the replacement of edges or angles and the constancy of the angles for each substance, assuming six "primitive forms w - viz. the cube. regular octahedron, regular tetrahedron, rhombohedron, octahedron with rhombic base and double six-sided pyramid. The contact-goniometer, a graduated semicircle with a movable radius, invented by Carangeau, assisted him in his work and enabled him to figure 500 forms as against 40 described by Limueus about twelve years earlier. The abbe" Rene Just Hauy, in 1784, by studying the laws of the natural cleavage of crystals, was led to the laws of symmetry and of whole numbers. He showed that when one edge or face of a crystal was replaced or truncated by a plane, all other geometrically similar edges or faces are similarly modified, and that the forms of crystals can be explained by supposing them to be built up of a definite number of layers of "integrant molecules," or brick-like bodies, and one, two, three, or some other definite number of layers to be removed in the modification of any face. Hauy also showed a close connection between angles and chemical composition, as exemplified in the four minerals baryte, witherite, celestine, and strontianite, the sulphate and carbonate respectively of barium and strontium, then classed together under the one name "heavy spar." In 1809 Wollaston, by his invention of the reflecting goniometer (q.v.), a graduated circle with a vernier and movable axis, in using which the angle is measured by obtaining reflections of signals in succession in the two faces forming it, rendered much greater accuracy possible. In the same year Weiss published an epoch-making dissertation referring all crystalline forms to four classes according to their axes, or lines round which they are uniformly disposed. These classes were - (1) The cube, regular octahedron, regular tetrahedron, and rhombic dodecahedron, with three equal axes at right angles; (2) the octahedron and right (upright) prism, with three axes at right angles, but only two equal; (3) the rhombohedron, six-sided prism, and double six-sided pyramid, with four axes, three equal, equally inclined and in one plane, and a fourth perpendicular to that plane; and (4) the octahedron and four-sided prisms of which the base is not square, with three unequal axes at right angles. Mohs, who arrived at these four classes independently, in 1820, named them the Cubic, Pyramidal, Rhombohedra], and Prismatic Systems, and in 1822 asserted, as the result of more precise measurement, the existence of two other systems, in one of which two of the axes are inclined, whilst in the other all three axes are so. In 1819 Sir David Brewster divided crystals into three classes according to their action upon polarised light (q.v.): (1) Those that are isotropic or singly-refracting, having no such action, which are those of the Cubic System; (2) those of the Pyramidal ahd Hexagonal Systems, which are anisotropic or doubly-refracting in all directions save one, the optic axis, and are hence termed uniaxial; and (3) those which have two optic axes, or are biaxial. Further study of these optical characters confirmed Mohs' conclusion that there are six systems of crystals, the two last to be discriminated being known as Monoclinic or Oblique and Triclinic or Anorthic. The correspondence of thermal and electrical properties with crystalline form has, since Brewster's time, still further deenonstrated that the classification of Weiss and Mohs is no arbitrary grouping of mere abstract geometry. Fuchs in 1815, and Mitscherlich in 1822, showed that certain chemical elements and groupings of elements, or radicals, were vicarious, or could replace one another in a compound without appreciably affecting the crystalline form, though in basing upon this his doctrine of isomorphism (q.v.) the latter observer recognised the occurrence of minute differences in the angles.

Though exceptionally, as in dolomite and diamond (q.v.), crystals have curved faces, we now define a crystal as a solid bounded by plane surfaces which co-exist according to a definite geometrical law of symmetry, and exhibiting a tendency to split indefinitely in directions parallel to such surfaces or at fixed angles with them.

Crystals are not confined absolutely to the dead mineral world, as they occur also in the cells of many plants, forming, for instance, no less than 80 per cent. of the dried tissue of the stems of some Cacti. These crystals consist of calcium sulphate, carbonate, phosphate, or most commonly oxalate, and occur either in spherical aggregations or in needle-like forms.