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248 OXIDES, EABTHS, AND ACIDS. polarities resembling drops of liquid. The light reflected from r is reddish, that from as greenish. When breathed upon r be comes dark yellow, z violet. In many instances the faces z are dull. 2. Twin- face £. The axes of the two rhombohe- drons^ make with each other an angle of 84 34. 3. Twin-faee r. The axes of the two rhomhohedrons make with each other an angle of 103° 34'. Cleavage, r, z, b, interrupted. Fracture conchoidal... splin tery. Transparent... trand^^ Lustre vitreous; in some varieties inclining to resinous on surfaces of fracture. Ihe optical properties are peculiar, the wave surface being a sphere and a prolate spheroid the axis of which is a little less than the radius of the spherical surface. The indices of refrac tion ot rays corresponding to Fraunhofer’s lines are t h 1-54990 1-56085 1-66328 1-56831 1-65894 1-66365 1-66772 If a plate of quartz hounded by planes perpendicular to the axis ot therhombohedron be placed in a polarizing apparatus, the diameters of the coloured rings will he seen to vary on turnin'* the analyzer (the tourmaline plate, Nicol’s prism or reflector of blackened glass nearest to the eye) round its axis; and the direction in which it must he turned to enlarge the rings is found to depend upon the situation of the hemihedral faces, when they occur. With a slice of crystal in which either of the fqces v, x, y, in the zone bz , is situated as in figs. 270, 272, 269, the axis of the polarizing instrument being horizontal,’ tho upper part of the analyzer must he turned to tho observer’s left hand to make the rings dilate. With a slice of a crystal in which either of the faces v, x, y, in the zone bz, is situated as m figs. 271, 273, 274, the upper part of tho analyzer must be turned to the observer’s right hand to make the rings dilate. When the emergent light is polarized by transmission, the incident light being circularly polarized and the emergent light plane polarized, coloured spirals are seen having the form of hg. 277 or 278, according as the slice is taken from a crystal B . C . D . E . F . Gr H . f* 1-54090 1-54181 1-64418 1-64711 1-54965 1-55425 1-65817
