Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/394

Rh 376 MINERALOGY The mineral phosphoresces vividly when the discharge passes through it ; it generally phosphoresces with a different colour after it has been thus recharged. Fluor- Fluorescence is the property whereby rays of light of a escence. refrangibility higher than those ordinarily seen by the human eye are rendered visible. The substance when placed in the violet end of the spectrum, and carried be yond it into the invisible rays, becomes luminous, through &quot; degrading &quot; the rays of extreme refrangibility. This property is well marked in those varieties of fluorite which are pale green by transmitted light, and deep purple by reflected light. Ozocerite and some petroleums also ex hibit the property. Electric, Magnetic, and Thermic Properties. Electric Electricity. Friction, pressure, and heat may all excite proper- electricity in minerals. To observe this property delicate minerals e l ectrosc P es are required, formed of a light needle termin ating at both ends in small balls, and suspended horizon tally on a steel pivot by an agate cup. Such an instrument, can be electrified negatively by touching it with a stick of sealing-wax excited by rubbing, or positively by merely bringing the wax 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 of a similar kind ; but if the mineral is not electric it will attract the needle in both conditions alike. Most precious stones become electric from friction, and are either positive or negative according as their surface is smooth or rough. All gems become positive when polished ; the diamond even when unpolished is positive. Pressure between the fingers will excite distinct positive electricity in pieces of transparent double-refracting calc-spar. Topaz, aragonite, fluor-spar, carbonate of lead, quartz, and other minerals show this property, but in a much smaller degree. Some bodies remain excited much longer than others, topaz for a very long time. 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 ; these are hence said to be thermo- or pyro-electric. Some acquire polar pyro-electricity, or the two electricities appear in opposite parts of the crystal, which are named its electric poles. Each pole is alter nately positive and negative, the one when the mineral is heating, the other when it is cooling. Hankel s investigations of these phenomena are specially noteworthy. As already noticed, many polar electric minerals are also remark able for their hemimorphic crystal forms. Tourmaline, calamine, and boracite are among the species thus affected. The polarity continues so long as the temperature is increasing, and becomes reversed when it commences to decline ; and when the heat is stationary it disappears. Pyro- Rose and Reiss name one of the poles the analogue electric pole, electri- and the other the antilogue electric pole. The former becomes city. positive while the crystal is heating, and negative while cooling ; the latter negative while heating, and positive while cooling. Becquerel found that in tourmaline at 30 C. electrical polarity was sensible ; it continued unchanged to 150, as long as the temperature continued to rise ; if the temperature remained Fig. 249. Fig. 250. stationary an instant, the polarity disappeared, but shortly mani fested itself reversed, when the temperature commenced to decline. If but one end of the crystal was heated the crystal was unpolarized, and when two sides were unequally heated each acquired an electri cal state independent of the other. In tourmaline the extremities of the prism are dissimilarly modified, and that end which presents the greater number of planes is the antilogue pole ; or, if the thus e 3 and d* of the anti- Fig. 251. number of planes is the same, the secondary rhombohedrons of the antilogue pole have (one or more of them) longer vertical axes than those of the analogue pole. Fig. 249 (tourmaline) is the antilogue pole (negative under increasing heat), and fig. 250 the analogue pole. The pyramid of the analogue end is more flattened by its facets than that of the antilogue end logue end are more acumi nating than e 1 and d&quot; of the analogue end. The same is the case with the other two crystals (figs. 251, 252). Pyro-electricity has been observed in the following substances : tourmaline, topaz, axinite, boracite, scolezite, prehnite, electric calamine, sphene, rhodizite, heavy spar, rock-crystal. Pyro-electricity is of two kinds, either terminally polar or centrally polar. In the former the extre mities are opposite poles. In the latter two sides of a prism are of the same name, and the opposite pole to each is intermediate be- Fig. 252. tween the two. The examples of the first kind are tourmaline, calamine, and scolezite, which are uniaxal ; axinite, binaxal ; boracite and rhodizite, with four axes. Calamine, like tourmaline, has the sharper extremity the antilogue end, and the more flattened the analogue. Compound crystals from Altenberg have both ends analogue, and the portion which lies between the twins antilogue electric ; the pyro-electric axis corresponds with the vertical axis of the prism, as in tourmaline. Boracite, which crystallizes in cubic forms, with the opposite solid angles differently modified, has four pyro-electric axes, corre sponding to the four octahedral axes. In fig. 253 of this species, Ant. Fig. 253. the plane which has its angles modified by v is the antilogue pole, and that with the unmodified angles the analogue pole ; and, generally, the antilogue pole has either more numerous or larger facets. Rhodizite re sembles boracite in its pyro-electricity. The species in which pyro-electricity of the second kind has been observed are prehnite and topaz. If fig. 254 represent a tabular crystal of prehnite, the poles will be situated as marked, the analogue being central, and the antilogue at either extremity of the shorter diagonal of the rhombic prism. Topaz has in a similar manner a central analogue pole, and an antilogue at either ex tremity of the shorter diagonal. In some instances there is a separate Set of similar poles near one or the other angle, as in fig. 255 ; this must be due to the crystals being of a composite nature. Magnetism. This property is very characteristic of the Magnot- few minerals in which it occurs, chiefly ores of iron or ism - nickel. Some magnetic iron ores possess polar magnetism, or are natural magnets ; while the common varieties of magnetite, meteoric iron, magnetic pyrites, precious garnet, and other minerals, are simply magnetic. Most minerals are only attracted by the magnet, but do not themselves attract iron. Minerals, as other substances, have also been divided into magnetic and diamagnetic. See MAGNETISM. The ordinary mode of testing whether a mineral is magnetic or not is to bring it near a pole of a delicately suspended magnetic