Online Encyclopedia

HORNBLENDE

Online Encyclopedia
Originally appearing in Volume V13, Page 708 of the 1911 Encyclopedia Britannica.
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HORNBLENDE, an important member of the amphibole group of rock-forming minerals. The name is an old one of German origin, and was used for any dark-coloured prismatic crystals from which metals could not be extracted. It is now applied to the dark-coloured aluminous members of the mono-clinic amphiboles, occupying in this group the same position that augite occupies in the pyroxene group. The monoclinic crystals are prismatic in habit with a six-sided cross-section; the angle between the prism-faces (M), parallel to which there are perfect cleavages, is 55° 49'. The colour (green, brown or black) and the specific gravity (3.0-3.3) vary with the amount of iron present. The pleochroism is always strong, and the angle of optical extinction on the plane of symmetry (x in the figure) varies from o to 37°. The chemical composition is expressed by mixtures in varying proportions of the molecules Ca(Mg,Fe)s(SiO3)s, (Mg,Fe)(Al,Fe)2SiO6 and NaAl(SiO )s. Numerous varieties have been distinguished by special names: edenite, from Edenville in New York, is a pale-coloured aluminous amphibole containing little iron; pargasite, from Pargas near Abo in Finland, a green or bluish-green variety; common hornblende includes the greenish-black and black kinds containing more iron. The dark-coloured porphyritic crystals of basalts are known as basaltic hornblende. Hornblende occurs as an essential constituent of many kinds Buffon, as was his manner, enlarges on the cruel injustice done to these birds by Nature in encumbering them with this deformity, which he declares must hinder them from getting their food with ease. The only corroboration his perverted view receives is afforded by the observed fact that hornbills, in captivity at any rate, never have any fat about them. 2In The Malay Archipelago (i. 213), Wallace describes'a nestling hornbill (B. bicornis) which he obtained as " a most curious object, as large as a pigeon, but without a particle of plumage on any part of it. It was exceedingly plump and soft, and with a semi-transparent skin, so that it looked more like a bag of jelly, with head and feet stuck on, than like a real bird.' of igneous rocks, such as hornblende-granite, syenite, diorite, hornblende-andesite, basalt, &c.; and in many crystalline schists, for example, amphibolite and hornblende-schist which are composed almost entirely of this mineral. Well-crystallized specimens are met with at many localities, for example: brilliant black crystals (syntagmatite) with augite and mica in the sanidine bombs of Monte Somma, Vesuvius; large crystals at Arendal in Norway, and at several places in the state of New York; isolated crystals from the basalts of Bohemia. (L. J. S.) HORN-BOOK, a name originally applied to a sheet containing the letters of the alphabet, which formed a primer for the use of children. It was mounted on wood and protected with transparent horn. Sometimes the leaf was simply pasted against the slice of horn. The wooden frame had a handle, and it was usually hung at the child's girdle. The sheet, which in ancient times was of vellum and latterly of paper, contained first a large cross—the criss-crosse—from which the horn-book was called the Christ Cross Row, or criss-cross-row. The alphabet in large and small letters followed. The vowels then formed a line, and their combinations with the consonants were given in a tabular form. The usual exorcism—" in the name of the Father and of the Sonne and of the Holy Ghost, Amen "—followed, then the Lord's Prayer, the whole concluding with the Roman numerals. The horn-book is mentioned in Shakespeare's Love's Labour's Lost, v. 1, where the ba, the a, e, i, o, u, and the horn, are alluded to by Moth. It is also described by Ben Jonson " The letters may be read, through the horn, That make the story perfect."
End of Article: HORNBLENDE
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SIR GEOFFREY THOMAS PHIPPS HORNBY (1825-1895)

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