Publisher's Synopsis
This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1888 edition. Excerpt: ...crystallised and the colloid forms of silica. The following extract from M. Michel LeVy's paper will convey an idea of the microscopic characters of these globules: ' Between crossed Nicols one is surprised to see such regular globules, the centre of each appearing to be a pole of symmetry with four extinctions situated at right angles for every total revolution of the section; one is therefore forced to conclude that they are composed of a crystallised substance, and are, moreover, orientated in an unique manner. Sometimes the extinction is simultaneous over an entire globule, sometimes it is different for two or more segments; but the most curious peculiarity, exhibited by the concentrically-zoned globules, lies in the fact that two adjacent zones do not always undergo extinction simultaneously. Such a globule will undergo extinction in its central portion and will, at the same time, present a perfectly regular narrow border which is still illuminated; if then the observer continue to turn the section, this border will become dark while the spherical central nucleus will in its turn become clear in a homogeneous manner.' 1 H. Rosenbusch, Mikroskop. Physiogr. d. massif en Gesteine. Stuttgart, 1877, p. 301. 2 Ibid. p. 225. 3 H.'Mohl, Basalte u. PhonolitheSachsens. Dresden, 1873. 1 Bull. StK, CM. de France, y serie, t. v. 1877, no. 3. Magnetite. Crystalline system cubic. It usually occurs in the form of the octahedron, sometimes in that of the rhombic-dodecahedron, also granular and massive. Cleavage parallel to the faces of the octahedron. Colour black. Streak black. Strongly magnetic and often displays polarity. The chemical formula of magnetite is FeO, Fe2O3, or Fe3O4. The approximate percentage composition is Fe2O3 = 69. FeO = 31....
