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Ch. 2: Minerals: Physical Properties

Ch. 2: Minerals: Physical Properties Page of 515 Ch. 2: Minerals: Physical Properties Text size:minus plus Restore normal size   Mail page  Print this page
PHYSICAL PROPERTIES OF MINERALS.                   85
dimly, as through a cloud; and translucent when the light that passes through it is so obscured that the objects can be no longer discerned. Some minerals are only thus trans lucent on the thinnest edges, and in others even these trans mit no light, and the body is named opaque or untranspa rent. These degrees pass gradually into each other, and cannot bo separated by any precise line; and' this is also the case in nature, where some minerals pass through the whole scale, as quartz, from the fine transparent rock-crys­tal to opaque dark-black varieties. Such minerals may be described generally as pellucid. This change often arises from some mixture in their composition, especially of me­tallic substances. Perfect opacity is chiefly found in the metals or their compounds with sulphur, though even these seem to transmit light when reduced to laminae of sufficient thinness.
Double Refraction.—When a ray of light passes ob­liquely from one medium into another of different density, it is bent or refracted from its former course. The line which it then follows forms an angle with the perpendicu­lar, which in each body bears a certain proportion to that at which the ray fell upon it; or, as definitely stated, the sine of the angle of refraction has a fixed ratio to the sine of the angle of incidence, this ratio being named the index of refraction. This simple refraction is common to all transparent bodies, whether crystalline, amorphous, or fluid ; but some crystals produce a still more remarkable result. The ray of light which entered them as one is divided into two rays, each following different angles, or is doubly re­fracted. In minerals of the tesseral system this property does not exist, but it has been always observed in minerals belonging to the other systems, though in many only after they have been cut in a.particular manner, or have been otherwise properly prepared. It is most distinctly seen in
Ch. 2: Minerals: Physical Properties Page of 515 Ch. 2: Minerals: Physical Properties
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