Many substances have been shown to present two or more solid °phases,* each char acterized by specific form, optical properties, melting point, specific gravity, etc., and, while in general each "phase* has certain tempera ture limits beyond which it cannot exist it is possible for two modifications to exist under the same conditions when once formed, though one is the less stable and sometimes passes gradually or suddenly to the more stable modi fication.
Polymorphism mu.it not be confounded with °chemical isomerism" in which the differences between two substances with the same per centage composition are due either to the dif ferent number of atoms or the different linking of the atoms in the chemical molecule.
The distinction between polymorphism and chemical isomerism can sometimes be made by melting or dissolving the substance. In the resultant amorphous condition all differences due strictly to crystal structure disappear.
In chemical isomeres on the contrary, fusion or solution does not destroy the differ ences. states that in such fusions or solutions of polymorphic substances crystals of either modifications grow further on being placed in contact with the amorphous liquid mass, but such a solution or fusion will in con tact with a chemical isomere usually grow only its own kind of crystal.
Adding one chemical isomere to the other raises or lowers the melting or freezing points, whereas adding one polymorphous form to the other does not, there results always the melting of the more stable form.
Polymorphism must also be distinguished from “polysymmetry° or the tendency of crys tals of a substance of a certain grade of sym metry to unite to apparently simple forms of higher symmetry. For instance, substances which are orthorhombic, monoclinic or triclinic often unite to apparently hexagonal forms, and although twin lamellae usually reveal the com posite structure, at times repeated twinning may yield a structure in no way distinguishable from that of a hexagonal crystal.
In such a case, however, the physical prop erties are those of the original less symmetrical material and intermediate states usually can be found, whereas in the polymorphous substance the aphasesa differ in their physical characters. . When a fused compound capable of assum ing different °phases° is cooled, the tempera ture falls steadily until the transformation temperature is reached when the rate suddenly changes and coincidently the physical prop erties and crystalline form change. For in stance, according to Lehmann, the ordinary orthorhombic crystals of • ammonium nitrate .melt at about 168 degrees. If such a melted mass is gradually cooled there form eight-rayed skeleton crystals, isotropic in polarized light, which at 127 degrees suddenly become doubly refracting, increase in size and become rhombo hedra. At 87 degrees acicular orthorhombic crystals develop regularly about the rhombo
.hedral crystals. These phenomena are ob tained in reverse order if the cooled mass is reheated gradually.
Morphotropic Relations between Poly An interesting empirical relation ex ists between polymorphic modifications which may be expressed as follows: If comparable °set-ups° have been chosen then the products of each crystal volume by the corresponding specific gravity are in simple mathematical ratio. By crystal volume is meant the volume of the unit pyramid. For example: Crystal Vol. Sp. Gr. Product Ratio Marcasite 2.094 4.86 10.18 2 Pyrite 1 5.10 5.10 1 Senarmontite 1 5.25 5.25 1 Valentinite 9.179 5.72 52.50 10 Calcite 0.7399 2.713 2.0072 3 Aragonite 0.4489 2.95 1.324 2 Octahedrite 0.71084 3.84 2.7296 1 Brooltite 0.6667 4.065 2.7101 1 Rutile 0.6440 4.239 2 . 730 1 .aueEnantiomorphism—Pasteur in 1848 showed that racemic acid decomposed by solution into two varieties of tartaric acid and that while the solution of one possessed the power to rotate the plane of polarization to the right the solu tion of the other rotated this plane a corre sponding amount to the left, and that the crys tals of these two varieties were enantiomorphic mirror images of each other (like right and left hand). Further study led to the discovery that the same relations existed in other so-called optically active compounds.
Such a result is attributed to enantiomor phous chemical molecules and resultant enantio morphous crystal structures, the structure of the racemic acid corresponding to an inter penetration of the two enantiomorphous struc tures.
Structure Indicated by inter esting relation between chemical composition and crystalline form was shewn by Tschermak in 1903. If the repetition of equivalent direc tions in the crystal corresponds to repetition of corresponding atoms or groups this is likely to show even in the simpler formate.. Thus many rhombohedral minerals show a three-fold arrangement in the formula which may be written as A.B whereas in tetragonal minerals there is a tendency to the four-fold A413 in isometric to either three-fold or four-fold, and in hexagonal the six-fold, A.B may be as sumed, A corresponding sometimes to one member as 0, Cl, ILO, and sometimes to a pair. Examples are: Rlimabolutdrol..Proustite.. 3 Ag S. As.... Hematite Wes Tetragonal Zircon.... 04Zrs Cassiterite Oar* "somatic Eulytite... BiaSiO4. Senarmontite Hcsagosal Beryl 6(SiOs) Beath. Os Sbs See CRYSTALLOGRAPHY; CRYSTAL; CRYSTALLO CHEMICAL ANALYSIS; PHYSICAL CRYSTALLOG RAPHY; MINERALOGY.
Bibliography.— Groth-Marshall,
tion to Chemical Crystallography' (New York 1906); Groth, P., (Chemische Krystallog raphie) (3 vols., Leipzig 1910) Pope, W. J.,