62 PHYSICAL PROPERTIES OF MINERALS. 



it on a shovel heated nearly to redness, the whole takes on 

 a bright glow. In some varieties, the light is emerald green , 

 in others, purple, rose, or orange. A massive fluor, from 

 Huntington, Connecticut, shows beautifully the emerald 

 green phosphorescence. 



Some kinds of white marble, treated in the same way, 

 give out a bright yellow light. 



After being heated for a while, the mineral loses its 

 phosphorescence ; but a few electric shocks will, in many 

 cases, to some degree, restore it again. 



ELECTRICITY AND 3IAGNETISM. 



Electricity. — Many minerals become electrified on 

 being rubbed, so that they will attract cotton and other light 

 substances ; and when electrified, some exhibit positive, and 

 others negative electricity, when brought near a delicately 

 suspended magnetic needle. The diamond, whether polished 

 or not, always exhibits positive electricity, while other gems 

 become negatively electric in the rough state, ana! positive 

 only in the polished state. Friction with a feather is suffi- 

 cient to excite electricity in some varieties of blende. Some 

 minerals, thus electrified, retain the power of electric attrac- 

 tion for many hours, as topaz, while others lose it in a few 

 minutes. 



Many minerals become electric when heated, and such 

 species are said to be pyro-electric, from the Greek pur, fire, 

 and electric. 



If a prism of tourmaline, after being heated, be placed on 

 a delicate frame, which turns on a pivot like a magnetic 

 needle, on bringing a magnet near it, one extremity will be 

 attracted, the other repelled, thus indicating the polarity al- 

 luded to. The same is better shown if the ends of the crystal 

 be brought near the poles of a delicately suspended magnetic 

 needle. The prisms of tourmaline have different secondary 

 planes at the two extremities, or, as it is expressed, are hemi- 

 hedrally modified (page 37.) 



Several other minerals have this peculiar electric property, 

 especially boracite and topaz, which, like tourmaline, are 

 hemihedral in their modifications. Boracite crystallizes in 



Will electricity restore the phosphorescent property when it is lost by 



eating a mineral ? What two modes are there of exciting electricity 



n minerals'? What is said of the diamond as compared with other 



gems ? What is a pyro-electric ? What is said of tourmaline ? wha 



cf topaz and boracite ? 



