A treatise on chemistry. Vol. 1, The non-metallic elements / by Sir H.E. Roscoe & C. Schorlemmer ...

  • Henry Enfield Roscoe
Date:
1881
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on chemistry. Vol. 1, The non-metallic elements / by Sir H.E. Roscoe & C. Schorlemmer ... Source: Wellcome Collection.

Provider: This material has been provided by University of Bristol Library. The original may be consulted at University of Bristol Library.

    762/792 (page 746)
    following table, which gives the coefficients of expansion of substances crystallizing in these two systems.^ Primary axis. Secondary axis. Quadratic System.—TmstonQ . 0-0004860 0-0004526 Zircon . . 0-0006264 0-0011054 Hexagonal Sijstem.—Clu^rtz . . 0-0008073 0-0015147 Tourmaline 0-0009369 0-0007732 Crystals belonging to the other systems possess co-efficients of expansion which differ for each of the three dii'ections of the axes. Hence whilst the angles of substances crystallizing in the regular system remain constant under change of temperature, those of crystals belonging to the other systems undergo small deviations with, alteration of temperature. The same relation is exhibited by crystals with, regard to the conduction of heat as holds good in the case of expansion. The conducting power of crystals of the regular system is the same in all directions. Those belonging to the quadratic and hexa- gonal systems conduct equally in two directions, and unequally in the third, whilst crystals belonging to the other systems conduct - differently in every direction. This difference in the conducting power of crystals can be well shown by covering a face of the crystal with a thin coating of wax, allowing the wax to solidify and then bringing the point of a hot needle or other pointed hot body against the wax coating. If the crystal conduct equally, the wax will melt in a circle of which the hot point is the centre. If the conduction be unequal, the melted wax will be seen to assume an oval form. Pyro-electric Action of Crystals.—Certain hemimorphous,- or tetratohedral crystalline forms, when heated exhibit a peculiar development of electricity, one end of the crystal, or fragment of crystal, becoming negatively electrified, whilst the other end exhibits positive electricity. Amongst these crystals tourmaline exhibits these properties in a very high degree. Boracite, cane- sugar, topaz, and silicate of zinc, are crystals which exhibit pyro-electrical reactions. 470 Optical Properties of Crystals.—The relations of crystals to light are of great importance as enabling us to determine crys- talline forms in cases in which the usual methods either give ' Pfaff. Pofig. Ann. civ. 171. ]3y this tiTiii is meant crystals exhibiting dissimilar modifications on each end of tlie crystal.