Volltext Seite (XML)
QTXAKTZ. 249 like fig. 270 or 271. 'When the fig. 277. fig. 278. incident light is plane polarized and the emergent light circu larly polarized, the spirals re semble fig. 278 or 277, accord ing as the slice is taken from a crystal like fig. 270 or 271. When the emergent light is polarized by reflexion, the incident hght being circularly polarized and the emergent light plane polarized, crystals 270, 271 produce spirals resembling figs. 278, 277 respectively. When the incident light is plane polarized and the emergent light circularly polarized, crystals 270, 271 produce spirals resembling figs. 277, 278 respectively. The purple variety of quartz, called amethyst, is frequently composed of alternate layers having opposite optical properties, and exhibits traces of this structure in a kind of rippled fracture. When a slice of amethyst or of a twin crystal, bounded by planes perpendicular to the axis, is placed in a polarizing apparatus, at such a distance as to be distinctly seen by the observer, the portions of the individuals of which it consists, may be dis- twguished from each other, when they have opposite optical properties, by a difference of colour. When the planes of polarization and analyzation are perpen dicular to each other, the centre of the rings is not dark, as is the case with any other uniaxal crystal. Using homogeneous {'ght, the centre becomes dark on turning the analyzing plane, m the direction which enlarges the rings, through an angle pro portional directly to the thickness of the slice, and inversely as the square of the length of the undulation of the light employed. |he slice of quartz being 0'04 inch thick, the centre becomes dark for different colours, on turning the analyzing plate through the following angles Hod .... i9 0, oo Blue .... 32 0, 3l Orange ... 21 -40 Indigo ... 36 -13 Yellow ... 24 -00 Violet .... 40 ‘88 Green .... 27 '86 Colourless, white, violet-blue, rose-red, clove-brown, apple- green. Streak white, n = 7’0. G = 2'5...2 - 8; for the pure varieties o = 2-65. Infusible before the blowpipe. Exposed to the flame of the nxyhydrogen blowpipe it melts so that it may be drawn into me threads, and is volatilized. With soda fuses with intu mescence into a clear glass. Insoluble in all acids except hy- rofluoric acid. Two pieces rubbed together in the dark emit a Phosphorescent light and a faint einpyreumatic odour.
