CHAP. I.] ELECTRICITY AND MAGNETISM. 



the crystal becoming a conductor at that temperature ; 

 for its resistance at even higher temperatures is still so 

 great as to make it practically a non-conductor. A 

 heated crystal of tourmaline suspended by a silk fibre 

 may be attracted and repelled by electrified bodies, or 

 by a second heated tourmaline ; the two similar poles 

 repelling one another, while the two poles of opposite 

 form attract one another. If a crystal be broken up, 

 each fragment is found to possess also an analogous and 

 an antilogous pole. 



67. Many other crystals beside the tourmaline are 

 more or less pyro-electric. Amongst these are silicate of 



}IOO 



i 

 AJ 



Fig. 35- 



Fig. 36. 



zinc (" electric calamine "), boracite, cane-sugar, quartz, 

 tartrate of potash, sulphate of quinine, and several others. 

 Boracite crystallises in the form shown in Fig. 36, which 

 represents a cube having four alternate corners trun- 

 cated. The corners not truncated behave as analogous 

 poles, the truncated ones as antilogous. This peculiar 

 skew- symmetry or hemihedry is exhibited by all the 

 crystals enumerated above, and is doubtless due to the 

 same molecular peculiarity which determines their sin- 

 gular electric property, and which also, in many cases, 

 determines the optical behaviour of the crystal in 

 polarised light. 



