74 PHYSICAL PEOPEPwTTES 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 powoi 

 of electric attraction for many hours, as topaz, while others lose 

 it in a few minutes. 



M any minerals become electric when heated, and suKb species 

 are said to be pyro-electric, from the Greek pui , fire, and 

 electric. 



A prism of tourmaline, on being heated, becomes polar ; ono 

 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 hemi- 

 hedral in their modifications. Boracite crystallizes in cubes, 

 with only the alternate solid angles similarly replaced (figs. 39, 

 40, page 25). Each solid angle, on heating the crystals, be- 

 comes an electric pole ; the angles diagonally opposite are dif- 

 ferently modified and have opposite polarity. Pyroelectricity 

 has been observed also in crystals that are not hetnihedral, 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. 



7. TASTE and 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. Saline — taste of common salt. 



