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Ch. 2: Minerals: Physical Properties

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A POPULAR TREATISE ON GEMS.
poles. Each pole is alternately positive and negative,—the one when the mineral is heating, the other when it is cool­ing. The poles that become positive during an increase of temperature are named analogue; those that become nega­tive in the same condition, antilogue poles, as shown in this table:
As already noticed, many polar electric minerals are also remarkable for their hemimorphic crystal forms. The num­ber and distribution of the poles likewise vary. In many monoaxial minerals, as tourmaline and calamine, there are only two poles, one at each end of the chief axis; whereas boracite has eight poles corresponding to the angles of the cube. In prehnite and topaz, again, two antilogue poles occur on the obtuse lateral edges of the prismP, and one analogue pole corresponding to the macrodiagonal chief section, or in the middle of the diagonal joining the obtuse edges. The power of retaining the electricity ac­quired by rubbing, for a longer time, varies in different minerals and gems; and as the latter are all electric, this property may sometimes be used as a distinguishing char­acter as to the length of retaining the electricity. Abbe Hatly found, in his experiments, that many precious stones lose their electric power after a few moments, whereas some will retain the same for twenty-four hours longer. The Brazilian topaz affected the needle, even after thirty-two hours.
Magnetism, or the power to act on the magnetic needle, is very characteristic of the few minerals in which it occurs, chiefly ores of iron or nickel. It is either simple, attracting
Ch. 2: Minerals: Physical Properties Page of 515 Ch. 2: Minerals: Physical Properties
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