results of his observations on crystals of quartz. He found that
although the faces of different crystals vary considerably in
shape and relative size, yet the angles between similar pairs of
faces are always the same. He further pointed out that the
crystals must have grown in a liquid by the addition of layers of
material upon the faces of a nucleus, this nucleus having the
form of a regular six-sided prism terminated at each end by a
six-sided pyramid. The thickness of the layers, though the
same over each face, was not necessarily the same on different
faces, but depended on the position of the faces with respect to
the surrounding liquid; hence the faces of the crystal, though
variable in shape and size, remained parallel to those of the
nucleus, and the angles between them constant. Robert Hooke
in his Micrographia (London, 1665) had previously noticed the
regularity of the minute quartz crystals found lining the cavities
of flints, and had suggested that they were built up of spheroids.
About the same time the double refraction and perfect
rhomboidal cleavage of crystals of calcite or Iceland-spar were
studied by Erasmus Bartholinus (Experimenta crystalli Islandici
disdiaclastici, Copenhagen, 1669) and Christiaan Huygens
(Traité de la lumière, Leiden, 1690); the latter supposed, as did
Hooke, that the crystals were built up of spheroids. In 1695
Anton van Leeuwenhoek observed under the microscope that
different forms of crystals grow from the solutions of different
salts. Andreas Libavius had indeed much earlier, in 1597,
pointed out that the salts present in mineral waters could be
ascertained by an examination of the shapes of the crystals
left on evaporation of the water; and Domenico Guglielmini
(Riflessioni filosofiche dedotte dalle figure de’ sali, Padova, 1706)
asserted that the crystals of each salt had a shape of their own
with the plane angles of the faces always the same.
The earliest treatise on crystallography is the Prodromus Crystallographiae of M. A. Cappeller, published at Lucerne in 1723. Crystals were mentioned in works on mineralogy and chemistry; for instance, C. Linnaeus in his Systema Naturae (1735) described some forty common forms of crystals amongst minerals. It was not, however, until the end of the 18th century that any real advances were made, and the French crystallographers Romé de l’Isle and the abbé Haüy are rightly considered as the founders of the science. J. B. L. de Romé de l’Isle (Essai de cristallographie, Paris, 1772; Cristallographie, ou description des formes propres à tous les corps du règne minéral, Paris, 1783) made the important discovery that the various shapes of crystals of the same natural or artificial substance are all intimately related to each other; and further, by measuring the angles between the faces of crystals with the goniometer (q.v.), he established the fundamental principle that these angles are always the same for the same kind of substance and are characteristic of it. Replacing by single planes or groups of planes all the similar edges or solid angles of a figure called the “primitive form” he derived other related forms. Six kinds of primitive forms were distinguished, namely, the cube, the regular octahedron, the regular tetrahedron, a rhombohedron, an octahedron with a rhombic base, and a double six-sided pyramid. Only in the last three can there be any variation in the angles: for example, the primitive octahedron of alum, nitre and sugar were determined by Romé de l’Isle to have angles of 110°, 120° and 100° respectively. René Just Haüy in his Essai d’une théorie sur la structure des crystaux (Paris, 1784; see also his Treatises on Mineralogy and Crystallography, 1801, 1822) supported and extended these views, but took for his primitive forms the figures obtained by splitting crystals in their directions of easy fracture of “cleavage,” which are aways the same in the same kind of substance. Thus he found that all crystals of calcite, whatever their external form (see, for example, figs. 1-6 in the article Calcite), could be reduced by cleavage to a rhombohedron with interfacial angles of 75°. Further, by stacking together a number of small rhombohedra of uniform size he was able, as had been previously done by J. G. Gahn in 1773, to reconstruct the various forms of calcite crystals. Fig. 1 shows a scalenohedron (σκαληνός, uneven) built up in this manner of rhombohedra; and fig. 2 a regular octahedron built up of cubic elements, such as are given by the cleavage of galena and rock-salt.
 | |
| Fig. 1.—Scalenohedron built up of Rhombohedra. |
Fig. 2.—Octahedron built up of Cubes. |
The external surfaces of such a structure, with their step-like arrangement, correspond to the plane faces of the crystal, and the bricks may be considered so small as not to be separately visible. By making the steps one, two or three bricks in width and one, two or three bricks in height the various secondary faces on the crystal are related to the primitive form or “cleavage nucleus” by a law of whole numbers, and the angles between them can be arrived at by mathematical calculation. By measuring with the goniometer the inclinations of the secondary faces to those of the primitive form Haüy found that the secondary forms are always related to the primitive form on crystals of numerous substances in the manner indicated, and that the width and the height of a step are always in a simple ratio, rarely exceeding that of 1 : 6. This laid the foundation of the important “law of rational indices” of the faces of crystals.
The German crystallographer C. S. Weiss (De indagando formarum crystallinarum charactere geometrico principali dissertatio, Leipzig, 1809; Übersichtliche Darstellung der verschiedenen natürlichen Abtheilungen der Krystallisations-Systeme, Denkschrift der Berliner Akad. der Wissensch., 1814–1815) attacked the problem of crystalline form from a purely geometrical point of view, without reference to primitive forms or any theory of structure. The faces of crystals were considered by their intercepts on co-ordinate axes, which were drawn joining the opposite corners of certain forms; and in this way the various primitive forms of Haüy were grouped into four classes, corresponding to the four systems described below under the names cubic, tetragonal, hexagonal and orthorhombic. The same result was arrived at independently by F. Mohs, who further, in 1822, asserted the existence of two additional systems with oblique axes. These two systems (the monoclinic and anorthic) were, however, considered by Weiss to be only hemihedral or tetartohedral modifications of the orthorhombic system, and they were not definitely established until 1835, when the optical characters of the crystals were found to be distinct. A system of notation to express the relation of each face of a crystal to the co-ordinate axes of reference was devised by Weiss, and other notations were proposed by F. Mohs, A. Lévy (1825), C. F. Naumann (1826), and W. H. Miller (Treatise on Crystallography, Cambridge, 1839). For simplicity and utility in calculation the Millerian notation, which was first suggested by W. Whewell in 1825, surpasses all others and is now generally adopted, though those of Lévy and Naumann are still in use.
Although the peculiar optical properties of Iceland-spar had been much studied ever since 1669, it was not until much later that any connexion was traced between the optical characters of crystals and their external form. In 1818 Sir David Brewster found that crystals could be divided optically into three classes, viz. isotropic, uniaxial and biaxial, and that these classes corresponded with Weiss’s four systems (crystals belonging to the cubic system being isotropic, those of the tetragonal and hexagonal being uniaxial, and the orthorhombic being biaxial). Optically biaxial crystals were afterwards shown by J. F. W. Herschel and F. E. Neumann in 1822 and 1835 to be of three kinds, corresponding with the orthorhombic, monoclinic and
