PLUCKEB ON THB THEORY OF DIAMAGNETISM. 355 



the molecular currents do not take place in all directions indif- 

 ferently. 



31. This is not the place to enter into the detail of the expe- 

 riments which 1 have undertaken with the intention of imitating 

 the different phaenomena which are exhibited by crystals (par- 

 ticularly by biaxial ones) between the magnetic poles. One 

 difficulty still remains, which springs from the experiment 

 alluded to in 9. 



32. Faraday w^as the first who discovered that in certain 

 crystals a definite direction was attracted by the poles of a 

 magnet ; as I was the first to observe the repulsion of such a 

 direction, nothing was more natural than to ascribe both classes 

 of phaenomena to the same cause. Of the well-established fact, 

 that in crystals certain directions, dependent on their structure, 

 occur, which are either attracted or repelled by the poles of a 

 magnet, I have, I conceive, in the foregoing given a sufficient 

 mechanical explanation. The great question, however, still 

 remains to be decided as to what cause we are to attribute 

 these phaenomena, which in fact would lead us to pronounce 

 upon the nature of magnetism itself. Is it the form or position 

 of the ultimate particles of the crystal which modifies the action 

 of the magnet; or is it rather that magnetism, like light, is due 

 to the motion of an aether, the elasticity of which is subjected 

 to modifications within the crystal ? 



33. Adopting the great English experimenter's view, that the 

 attracted line is perpendicular to the plane of most eminent 

 cleavage, and assuming, further, that in other crystals the same 

 direction is repelled, we arrive of necessity to the conclusion — 

 whether it be that one surface of cleavage is present or several 

 such surfaces, and we must regard the resultant action, — that in 

 uniaxial crystals a single direction exists, which is either at- 

 tracted or repelled by the magnetic poles, and that this axis 

 must coincide with the optic axis round which the crystal is 

 symmetrical. In the uniaxial negative crystals which I have 

 examined the optic axis is repelled, and in the positive attracted. 

 I have thought myself justified, in accordance with this view, 

 in extending the classification of crystals under positive and 

 negative to the opake ones, such as bismuth and antimony. 

 But even should subsequent observations compel us to give up 



