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LXII. On the Magnetic Axes of Crystals, and their relation to 

 Crystalline Form and to the Optic Axes. By Prof. Plucker 

 and Dr. Beer*. 



Introduction. 



1. ^T^O the discovery of Faraday, that the magnet acts upon 

 A all bodies in nature, attaches itself the new fact, that 

 this action is modified in a peculiar manner when the body is 

 crystalhne. 



2. According to Faraday, bodies range themselves in two 

 great classes, the magnetic and the diamagnetic. I regard the 

 diaraagnetic phsenomena originally described by him just as I 

 do the magnetic, as being produced by induction, with this dif- 

 ference only, that by the induction of magnetic substances the 

 north pole excites a south pole, and the south pole a north pole ; 

 whereas by the induction of diamagnetic substances the north 

 pole excites a north pole, and the south pole a south pole. In 

 the language of the theory of Ampere, the meaning of this would 

 be, that in magnetic substances molecular currents are excited 

 which have the same direction as the inducing stream, whereas 

 in diamagnetic substances the molecular currents have an opposite 

 direction. Could we succeed in rendering the induced currents 

 in diamagnetic bodies permanent, as in the case of magnetic steel, 

 such a permanent diamagnet could in no way be distinguished 

 from a permanent magnet : magnetism and diamagnetism are 

 identical. The development only of both is different : magnetic 

 and diamagnetic induction are opposite. I have followed up 

 this view in a memoir presented to the Royal Society of the Ne- 

 therlands (December 1849), in which the existence of diamag- 

 netic polarity, already proved by MM. Reich, Weber, Poggendorff 

 and myself, is established by new and various experiments. 



3. In the same memoir experiments are adduced in proof of 

 the opinion, that the coercive force, which varies considerably in 

 different magnetic substances, must also be exhibited in cases of 

 diamagnetic induction, and that it is comparatively greater here 

 than in cases of magnetic induction. The coercive force mani- 

 fests itself particularly therein, that the polarity once conferred 

 does not vanish with the inducing stream. I have proved this 

 with bismuth. The coercive force manifests itself also by the 

 fact, that when a body possesses it in a high degree, to attain a 

 certain amount of magnetic or diamagnetic power the said body 

 must be brought to a stronger pole, or nearer to the same pole 

 than a body of less coercive force. And in tliis way the law 

 established otherwise by experiment, that the magnetic induction 

 decreases more slowly with the strength of the poles, and con- 



* From PoggendorfF's Annalen, vol. Ixxxi. p. 115. 



