CHARACTERS DEPENDING ON ELECTRICITY.

79

By friction. Light is readily evolved from quartz, by the fric-tion of one piece against another, and merely the rapid motion of afeather across some specimens of sulphuret of zinc, will often elicitfrom this mineral, light, more or less intense. Friction, however,will evolve light from a few only of the mineral species.

By heat. Fluor spar is very beautifully phosphorescent at thetemperature of about 300° F. Different varieties give off light ofdifferent colors ; the chlorophane variety, a splendid emerald greenlight; others a purple of different shades. This may be observedin a dark place, by throwing the pulverized mineral on a shovelheated below redness. Some varieties of carbonate of lime emit ayellow light when treated in the same manner. At a certain tem-perature, the fluor spar loses the power of phosphorescence ; it maybe restored, however, by subjecting it to an electrical discharge.

Light of the sun. The only substance in which an exposure tothe light of the sun produces very apparent phosphorescence, isthe diamond—some specimens seem to be destitute of this power.

The phosphorescence of minerals is a character of little import-ance in distinguishing them: it is, however, deserving of attentionin the descriptive part of the science.

CHAPTER II.ELECTRICITY.

95. The means of developing electricity in minerals, are frictionand heat.

1. By friction. There is no line of distinction among the mine-ral species, separating them into those of resinous and those of vit-reous electricity. The same mineral in its different varieties, oftenpresents both kinds, and frequently, the two are exhibited by thesame specimen. This character is, therefore, of no importance norinterest to the mineralogist.

2. By heat. The thermo-electrical character of the mineral spe-cies, are also of little importance in distinguishing minerals ; hutthey are so peculiar that they deserve some attention, and shouldnot be neglected in the descriptions of the species.

The effect of the application of heat, is the development of electricpolarity. This property belongs in a remarkable degree to turma-line and boracite. These minerals usually occur in liemihedralcrystals, the one under the form of a three or six sided prism, (se-condary to a rhombohedron,) differently terminated at its extremi-ties, the other in that of a cube, with its opposite solid angles dis-similarly replaced. M. Becquerel remarks concerning the turma-