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Ch. 2: Minerals: Physical Properties

Ch. 2: Minerals: Physical Properties Page of 515 Ch. 2: Minerals: Physical Properties Text size:minus plus Restore normal size   Mail page  Print this page
PHYSICAL PROPERTIES OF MINERALS.                   99.
ing water; while calc-spar and many silicates are only phosphorescent at from 400° to 700° Fahr. Electricity produces it in some minerals, as in green fluor spar and calcined barytes. In others it is excited when they are struck, rubbed, split, or broken ; as many varieties of zinc-blende and dolomite when scratched with a quill, pieces of quartz when rubbed on each other, and plates of mica when suddenly separated.
Friction, pressure, and heat also excite electricity in minerals. To observe this property; delicate electroscopes are required, formed of a light needle, terminating at both ends in small balls, and suspended horizontally "on a steel pivo't by an agate cup. Such an instrument can be nega­tively electrified by touching it with a stick of sealing-wax, excited by rubbing, or positively when the wax is only brought so near as to attract the needle. When the in­strument is in this state the mineral, if also rendered elec­tric by heat or friction, will attract or repel the needle ac­cording as it has acquired electricity of an opposite or similar kind; but if the mineral is not electric, it will at­tract the needle in both conditions alike. Most precious stones become electrical from friction, and are either posi­tive or negative according as their surface is smooth or rough. Pressure even between the fingers will excite dis­tinct positive electricity in pieces of transparent double-refracting calc-spai-. Topaz, arragonite, fluor spar, car­bonate of lead, quartz, and other minerals show this property.
Heat or change of temperature excites electricity in many crystals, as in tourmaline, calamine, topaz, calc-spar, beryl, barytes, fluor spar, diamond, garnet, and others, which are hence said to be thermo or pyroelectric. Some acquire polar pyro-electricity, or the two electricities appear in op­posite parts of the crystal, which are named, its electric
Ch. 2: Minerals: Physical Properties Page of 515 Ch. 2: Minerals: Physical Properties
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