File:Birds and nature (1901) (14745393851).jpg

Identifier: birdsnature9101unse (find matches)
Title: Birds and nature
Year: 1900 (1900s)
Authors:
Subjects: Birds Natural history
Publisher: Chicago, Ill. : A.W. Mumford, Publisher
Contributing Library: University of Illinois Urbana-Champaign
Digitizing Sponsor: University of Illinois Urbana-Champaign

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the stone acquires to pick up and holdbits of tissue paper, straws, etc. Some-times the friction from merely rubbingthe stone between the fingers will be suf-ficient to produce this electrical condi-tion, while many Brazilian Topazes, ifsimply pressed between the fingers, es-pecially in the direction of the prismaticaxis, become electric. The electricalcondition often persists from twenty-fourto thirty hours. The crystals of Topaz belong, to theorthorhombic system of crystallization.They are usually elongated in the direc-tion of the prism and have sharp, brightfaces. They vary much in size and of-ten are large. One crystal weighingtwenty-five pounds was found in Siberia. A well-marked characteristic of all To-paz crystals is their tendency to cleaveacross the prism parallel with its base.Such a cleavage plane can be seen cut-ting across the crystal shown in the up-per right-hand corner of the accompany-ing plate. This cleavage is so markedand the cleavage plane so bright and flat 216
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453 COPYRIGHT 1901, BY A. W. MUMFORD, CHICAGO TOPAZ. Topaz with Mica and Feldspar (Russia.) Topaz (Japan.) Topaz in Rhyolite (Utah.) Topaz (Brazil.) Waterworn Topaz (Brazil.)—Loaned by Foote Mineral Co. LlBRAW that in cutting Topaz for a gem a cleav-age surface is used as the upper face ofthe gem and the other faces formedaround it. Owing to this easy cleavagethe owner of a cut Topaz should be care-ful not to let the stone drop, as it mightbe cracked or broken. The name Topaz is derived from theGreek name topazios, which is that ofan island in the Red Sea. The gemknown to the ancients as topaz, however,was not our Topaz, but the mineral chry-solite. Topaz usually occurs in gneissor granite, with tourmaline, mica, beryl,etc. In Brazil it occurs in a talcose rockor in mica slate. It is sometimes in suf-ficient abundance to form an essentialrock constituent. When so occurring,however, it has not the transparent gemquality, but is white and opaque. Muchof the Brazilian Topaz occurs as r

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