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 importance: such substances are described as being “isomorphous.” Amongst minerals there are many examples of isomorphous groups, e.g. the rhombohedral carbonates, (q.v.), (q.v.); and amongst crystals of artificially prepared salts isomorphism is equally common, e.g. the sulphates and selenates of potassium, rubidium and caesium. The rhombohedral carbonates have the general formula R″CO3, where R″ represents calcium, magnesium, iron, manganese, zinc, cobalt or lead, and the different minerals (calcite, ankerite, magnesite, chalybite, rhodochrosite and (q.v.)) of the group are not only similar in crystalline form, cleavage, optical and other characters, but the angles between corresponding faces do not differ by more than 1° or 2°. Further, equivalent amounts of the different chemical elements represented by R” are mutually replaceable, and two or more of these elements may be present together in the same crystal, which is then spoken of as a “mixed crystal” or isomorphous mixture.

In another isomorphous series of carbonates with the same general formula R″CO3, where R″ represents calcium, strontium, barium, lead or zinc, the crystals are orthorhombic in form, and are thus dimorphous with those of the previous group (e.g. calcite and aragonite, the other members being only represented by isomorphous replacements). Such a relation is known as “isodimorphism.” An even better example of this is presented by the arsenic and antimony trioxides, each of which occurs as two distinct minerals:—

Claudetite and valentinite though crystallizing in different systems have the same cleavages and very nearly the same angles, and are strictly isomorphous.

Substances which form isodimorphous groups also frequently crystallize as double salts. For instance, amongst the carbonates quoted above are the minerals dolomite (CaMg(CO3)2) and barytocalcite (CaBa(CO3)2). Crystals of (q.v.) are monoclinic; and those of (q.v.), though closely related to calcite in angles and cleavage, possess a different degree of symmetry, and the specific gravity is not such as would result by a simple isomorphous mixture of the two carbonates. A similar case is presented by artificial crystals of silver nitrate and potassium nitrate. Somewhat analogous to double salts are the molecular compounds formed by the introduction of “water of crystallization,” “alcohol of crystallization,” &c. Thus sodium sulphate may crystallize alone or with either seven or ten molecules of water, giving rise to three crystallographically distinct substances.

A relation of another kind is the alteration in crystalline form resulting from the replacement in the chemical molecule of one or more atoms by atoms or radicles of a different kind. This is known as a “morphotropic” relation (Gr. , form,  , habit). Thus when some of the hydrogen atoms of benzene are replaced by (OH) and (NO2) groups the orthorhombic system of crystallization remains the same as before, and the crystallographic axis a is not much affected, but the axis c varies considerably:—

A striking example of morphotropy is shown by the (q.v.) group of minerals: successive additions of the group Mg2SiO4 to the molecule produce successive increases in the length of the vertical crystallographic axis.

In some instances the replacement of one atom by another produces little or no influence on the crystalline form; this happens in complex molecules of high molecular weight, the “mass effect” of which has a controlling influence on the isomorphism. An example of this is seen in the replacement of sodium or potassium by lead in the (q.v.) group of minerals, or again in such a complex mineral as tourmaline, which, though varying widely in chemical composition, exhibits no variation in crystalline form.

For the purpose of comparing the crystalline forms of isomorphous and morphotropic substances it is usual to quote the angles or the axial ratios of the crystal, as in the table of benzene derivatives quoted above. A more accurate comparison is, however, given by the “topic axes,” which are calculated from the axial ratios and the molecular volume; they express the relative distances apart of the crystal molecules in the axial directions.

The two isomerides of substances, such as tartaric acid, which in solution rotate the plane of polarized light either to the right or to the left, crystallize in related but enantiomorphous forms.

—An introduction to crystallography is given in most text-books of mineralogy, e.g. those of H. A. Miers and of E. S. Dana (see ). The standard work treating of the subject generally is that of P. Groth, Physikalische Kristallographie (4th ed., Leipzig, 1905). A condensed summary is given by A. J. Moses, The Characters of Crystals (New York, 1899).

For geometrical crystallography, dealing exclusively with the external form of crystals, reference may be made to N. Story-Maskelyne, Crystallography, a Treatise on the Morphology of Crystals (Oxford, 1895) and W. J. Lewis, A Treatise on Crystallography (Cambridge, 1899). Theories of crystal structure are discussed by L. Sohncke, Entwickelung einer Theorie der Krystallstruktur (Leipzig, 1879); A. Schoenflies, Krystallsysteme und Krystallstructur (Leipzig, 1891); and H. Hilton, Mathematical Crystallography and the Theory of Groups of Movements (Oxford, 1903).

The physical properties of crystals are treated by T. Liebisch, Physikalische Krystallographie (Leipzig, 1891), and in a more elementary form in his Grundriss der physikalischen Krystallographie (Leipzig, 1896); E. Mallard, Traité de cristallographie, Cristallographie physique (Paris, 1884); C. Soret, Éléments de cristallographie physique (Geneva and Paris, 1893).

For an account of the relations between crystalline form and chemical composition, see A. Arzruni, Physikalische Chemie der Krystalle (Braunschweig, 1893); A. Fock, An Introduction to Chemical Crystallography, translated by W. J. Pope (Oxford, 1895); P. Groth, An Introduction to Chemical Crystallography, translated by H. Marshall (London, 1906); A. E. H. Tutton, Crystalline Structure and Chemical Constitution, 1910. Descriptive works giving the crystallographic constants of different substances are C. F. Rammelsberg, Handbuch der krystallographisch-physikalischen Chemie (Leipzig, 1881–1882); P. Groth, Chemische Krystallographie (Leipzig, 1906); and of minerals the treatises of J. D. Dana and C. Hintze.

CRYSTAL PALACE, THE, a well-known English resort, standing high up in grounds just outside the southern boundary of the county of London, in the neighbourhood of Sydenham. The building, chiefly of iron and glass, is flanked by two towers and is visible from far over the metropolis. It measures 1608 ft. in length by 384 ft. across the transepts, and was opened in its present site in 1854. The materials, however, were mainly those of the hall set up in Hyde Park for the Great Exhibition of 1851. The designer was Sir Joseph Paxton. In the palace there are various permanent exhibitions, while special exhibitions are held from time to time, also concerts, winter pantomimes and other entertainments. In the extensive grounds there is accommodation for all kinds of games: the final tie of the Association Football Cup and other important football matches are played here, and there are also displays of fireworks and other attractions.

 CSENGERY, ANTON (1822–1880), Hungarian publicist, and a historical writer of great influence on his time, was born at Nagyvárad on the 2nd of June 1822. He took, at an early date, a very active part in the literary and political movements immediately preceding the Hungarian Revolution of 1848. He and Baron Sigismund Kemény may be considered as the two founders of high-class Magyar journalism. After 1867 the greatest of modern Hungarian statesmen, Francis Deák, attached Csengery to his personal service, and many of the momentous state documents inspired or suggested by Deák were drawn up by Csengery. In that manner his influence, as represented by the text of many a statute regulating the relations between Austria and Hungary, is one of an abiding character. As a historical writer he excelled chiefly in brilliant and thoughtful essays on the leading political personalities of his time, such as Paul Nagy, Bertalan, Szemere and others. He also commenced a translation of Macaulay’s History. He died at Budapest on the 13th of July 1880.