64 ELEMENTARY LESSONS ON [CHAP. I. 



and of other crystals mentioned in the next para- 

 graph. 



66. Pyro-electricity. There are certain crystals 

 which, while being heated or cooled, exhibit electrical 

 charges at certain regions or poles. Crystals thus 

 electrified by heating or cooling are said to be pyro- 

 electric. Chief of these is the Tourmaline, whose 

 power of attracting light bodies to its ends after being 

 heated has been known for some centuries. It is alluded 

 to by Theophrastus and Pliny under the name of Lapis 

 Lyncurius. The tourmaline is a hard mineral, semi- 

 transparent when cut into thin slices, and of a dark 

 green or brown colour, but looking perfectly black and 

 opaque in its natural condition, and possessing the power 

 of polarising light It is usually found in slightly irregu- 

 lar three-sided prisms which, when perfect, are pointed 

 at both ends. It belongs to the " hexagonal " system 

 of crystals, but is only hemihedral, that is to say, has 

 the alternate faces only developed. Its form is given 

 in Fig. 35, where a general view is first shown, the two 

 ends A and B being depicted in separate plans. It will 

 be noticed that these two ends are slightly different 

 from each other. Each is made up of three sloping 

 faces terminating in a point. But at A the edges 

 between these faces run down to the corners of the 

 prism, while in B the edges between the terminal faces 

 run down to the middle points of the long faces of the 

 prism. The end A is known as the analogous pole, 

 and B as the antilogous pole. While the crystal is 

 rising in temperature A exhibits + electricity and B ; 

 but if, after having been heated, it is allowed to cool, 

 the polarity is reversed ; for during the time that the 

 temperature is falling B is + and A is - . If the 

 temperature is steady no such electrical effects are 

 observed either at high or low temperatures ; and the 

 phenomena cease if the crystal be warmed above 150 

 C. This is not, however, due, as Gaugain declared, to 



