ELECTRICITY AND MAGNETISM. 73 
ray; and the two colors are seen side by side, at intervals of 
90° in the revolution of the mineral. 
For opaque minerals it is necessary to make a thin transparent 
section of the mineral and examine it with a polariscope, or with 
a microscope arrange? *~ wet asone by the addition of one Nicol 
prism. The opaque .r¢nblende of rocks is thus distinguished 
from pyroxene, and so in other cases. . 
3. Asterism.—Some crystals, especially the hexagonal, when 
viewed in the direction of the vertical axis, present peculiar re- 
flections in six radial directions. This arises either from pecu- 
liarities of texture along the axial portions, or from some im- 
purities. A remarkable example of it 1s that of the estertated 
sapphire, and the quality adds much to its value as a gem. 
The six rays are sometimes alternately shorter, indicating the 
rhombohedral character of the crystal. 
4. Phosphorescence.—Several minerals give out light either 
by friction or when gently heated. This property of emitting 
light is called phosphorescence. 
Two pieces of white sugar struck against one another give a 
feeble light, which may be seen in a dark place. The same 
effect is obtained on striking together fragments of quartz, and 
even the passing of a feather rapidly over some specimens c* 
zine blende is sufficient to elicit light. 
Fluorite is the most convenient mineral for showing phos 
phorescence by heat. On powdering it, and throwing it ona 
plate of metal heated’ nearly to redness, the whole takes on a 
brigut glow. In some varieties the light is emerald green; in 
others, purple, rose, or orange. A massive fluor, from Hun. 
tington, Connecticut, shows beautifully the emerald green phos- 
phorescence. 
Some kinds of white marble, treated in the same way, giv» 
out a bright yellow light. 
After being heated for a while the mineral loses its phos. 
phorescence ; Sian a few electric shocks will, in many cases, to 
some degree restore it again. 
6. ELECTRICITY, anp MAGNETISM. 
Execrriciry.—Many minerals become electrified on being 
rubbed, so that they will attract cotton and other light sub- 
Suances: ; and when electrified some exhibit positive ane others 
negative electricity, when brought near a delicately suspended 
magnetic needle. The diamond, whether polished or not, al- 
