PHYSICAL PROPERTIES OF MINERALS. 123 



. 100. MAGNETISM. 



Some minerals act upon the magnetic needle, if they are 

 brought within the sphere of its attraction. Others become 

 magnets themselves. These phenomena are made use of 

 as characters, under the name of Magnetism. 



The only minerals hitherto known, which exercise a considerable 

 action upon the magnetic needle, are the Native Iron and Octahe- 

 dral Iron-ore. A few other ores of iron also act upon it, but with 

 much less energy. 



Instead of a needle, the horse-shoe magnet may be employed in 

 examining minerals for this property, but in such cases the mineral 

 should be reduced to the condition of a powder. 



.101. ELECTRICITY. 



Several minerals produce electric phenomena ; some of 

 them by friction, others by pressure, others by communica- 

 tion, and others by heat. Some are electrics ; others are 

 conductors of electricity. 



Vitreous or positive electricity is produced by friction, in a large 

 number of earthy minerals, as Quartz, Topaz, Emerald, &c. and 

 even in some salts. In the same way, the combustibles, as Sulphur, 

 Amber and Coal, present the phenomena of negative or resinous 

 electricity. As conductors of electricity, the native metals may be 

 mentioned. 



Heat produces electric phenomena, in Topaz, Mesotype, Preh- 

 nite, Tourmaline, &c. The opposite extremities of the crystals 

 assume opposite kinds of electricity, and therefore possess electric 

 axes.* Boracite exhibits four electric axes, coinciding with the 

 axes of the Cube, which is the form of its crystal. This difference 



* In cut and polished pieces of transparent Tourmaline, electrical ex- 

 citement is produced by natural fluctuations of temperature in the air; 

 or, in other words, this substance may be said to be spontaneously 

 electric. 



