T4 PHYSICAL PROPERTIES OF MINERALS. 
ways exhibits positive electricity, while other gems become 
negatively electric in the rough state, and positively only in the 
polished state. Some minerals, thus electrified, retain the power 
of electric attraction 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. 
A prism of tourmaline, on being heated, becomes polar; one 
extremity will be attracted, the other repelled, by a pole of a 
strong magnet. ‘The prisms of tourmaline have different second- 
ary planes at the two extremities. 
Several other minerals have this peculiar electric property, 
especially boracite and topaz, which, like tourmaline, are hemt- 
hedral in their modifications. Eocege crystallizes in cubes, 
with only the alternate solid angles similarly replaced (figs. 39, 
40, page 25). Hach solid angle, on heating the crystals, be- 
comes an electric pole; the angles diagonally opposite are dif- 
ferently moditied and have opposite polarity. Pyroelectricity 
has beeu observed also in crystals that are not hemihedral, and 
in many mineral species. In some cases the number of poles is 
more than two. In prehnite crystals a large series occur dis- 
tributed over the surface. 
MAGNETISM.—The name Lodestone is given to those specimens 
of an ore of iron, called magnetite which have the power of at- 
traction like a magnet; it is common in many beds of magnetite. 
When mounted like a horse-shoe magnet, a good lodestone will 
lift a weight of many pounds. This is the only mineral that 
has decided magnetic attraction. But several ores containing 
iron are attracted by the magnet, or, when brought near a 
magnetic needle, will cause it to vibrate; and moreover, the 
metals nickel, cobalt, manganese, palladium, platinum and os- 
mium, have been found to be slightly magnetic. _ 
Many minerals become attractable by the magnet after being 
heated that are not so before heating. This arises from a 
change of part or all of the iron to the magnetic oxide. 
TASTE anp ODOR. 
Taste belongs only to the soluble minerals; the kinds are— 
1. Astringent—the taste of vitriol. 
2. Sweetish-astringent—the taste of alum. 
3. Suline—taste of common salt. 
