CHAP, v.] ELECTRICITY AND MAGNETISM. 305 



the conical poles of the electro-magnet (Fig. 127) turns 

 equatorially so as to put its ends into the regions that 

 are magnetically weaker. There is no reason to doubt 

 that in a magnetic field of uniform strength a bar of 

 bismuth would point along the lines of induction. 



343. Magne - Crystallic Action. In 1822 

 Poisson predicted that a body possessing crystalline 

 structure would, if magnetic at all, have different 

 magnetic powers in different directions. In 1847, 

 Pliicker discovered that a piece of tourmaline, which 

 is itself feebly paramagnetic, behaved as a diamagnetic 

 body, when so hung that the axis of the crystal was 

 horizontal. Faraday repeating the experiment with a 

 crystal of bismuth, found that it tended to point with 

 its axis of crystallisation along the lines of the field 

 axially. The magnetic force acting thus upon crystals 

 by virtue of their possessing a certain structure, he 

 named magne - cry stallic force. Pliicker endeavoured to 

 connect the magne -crystallic behaviour of crystals with 

 their optical behaviour, giving the following law : there 

 will be either repulsion or attraction of the optic axis 

 (or, in the case of bi-axial crystals, of both optic axes) 

 by the poles of a magnet ; and if the crystal is a 

 " negative " one (i.e., optically negative, having an extra- 

 ordinary index of refraction less than its ordinary index), 

 there will be repulsion, if a " positive " one, there will 

 be attraction. Tyndall has endeavoured to show that 

 this law is insufficient in not taking into account the 

 paramagnetic or diamagnetic powers of the substance as 

 a whole. He finds that the magne-crystallic axis of 

 bodies is in general an axis of greatest density ', and that 

 if the mass itself be paramagnetic this axis will point 

 axially ; if diamagnetic, equatorially. In bodies which, 

 like slate and many crystals, possess cleavage, the planes 

 of cleavage are usually at right angles to the magne- 

 crystallic axis. 



344. Diamagnetism of Flames. In 1847 Ban- 



x 



