Page:Collected Physical Papers.djvu/115

Rh I subjoin a representative list of crystals belonging to the different systems, which would be found to produce double refraction of the electric ray.


 * Tetragonal System.—Idocrase, scapolite.
 * Orthorhombic System.—Barytes, celestine, cryolite, andalusite, hypersthene.
 * Hexagonal System.—Calcite, apatite, quartz, beryl, tourmaline.
 * Monoclinic System.—Selenite, orthoclase, epidote.
 * Triclinic System.—Labradorite, microcline, amblygonite.

Effect due to Pressure.—A piece of glass, when strongly compressed, becomes double refracting for light. An analogous experiment may be shown with electric radiation. Here is a piece of slate which from the nature of its formation had been subjected to great pressure. I interpose this piece with the plane of stratification inclined at 45°, and the spot of light flies off the scale. I now carefully rotate the piece of slate; there is no depolarisation effect when the plane of stratification is parallel to either the polariser or the analyser. Thus the existence of strain inside an opaque mass can easily be detected, and what is more, the directions of maximum and minimum pressures can be determined with great exactitude.

An effect similar to that produced by unannealed glass may be shown by this piece of solid paraffin, which was cast in a mould, and chilled unequally by a freezing mixture. One of these blocks was cast two years ago, and it has still retained its unannealed property. This effect may even be shown without any