ELECTEICITT 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 arranged A -. ict as one by the addition of one Nicol 

 prism. The opaque vnblende 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 is that of the asteriated 

 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 o* 

 zinc blende is sufficient to elicit light. 



Fluorite is the most convenient mineral for showing phos 

 phorescence by heat. On powdering it, and throwing it on a 

 plate of metal heated nearly to redness, the whole takes on a 

 brignt 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, gfr ^ 

 out a bright yellow light. 



After being heated for a while the mineral loses its phos« 

 phorescence ; but a few electric shocks will, in many cases, to 

 some degree restore it again. 



6. ELECTRICITY, and MAGNETISM. 



Electricity. — Many minerals become electrified on being 

 rubbed, so that they will attract cotton and other light sub- 

 stances ; and when electrified some exhibit positive and others 

 negative electricity, when brought near a delicately suspended 

 magnetic needle. The diamond, whether polished or not, al- 



