340 plOcker on the theory of diamagnetism. 



netic, according to the greater or less distance from the poles, or 

 the greater or less intensity of the magnetic force ; it must some- 

 times be attracted and sometimes repelled. The experiment 

 shows this. Whether our hypothesis has exceptions, must be 

 decided by future experiments. 



28. The method of explanation now before us leaves nothing 

 to be desired, if in consequence of it the question might not be 

 started, whether bismuth, when with its strong coercive force 

 it is once excited by a magnet, does not retain its polarity when 

 removed from the magnet. On this point we have heretofore 

 possessed no data, but the experiment in No. 16, which was 

 made with reference to this subject, proves it. 



Whether a piece of bismuth or other diamagnetic body, with 

 stronger coercive force, remains a permanent diamagnetic for a 

 considerable time, is probably not to be proved on account of 

 the feebleness of the force; for even hardened steel retains 

 manifestly only a small portion of the magnetic excitation which 

 it experiences in the neighbourhood of a strong magnetic pole. 

 It is sufficient here to have proved experimently that bismuth, 

 far from every diamagnetic induction, nay even against the 

 latter, retains a sensible amount of diamagnetism for a short 

 time. 



Part II. 



[I repress here the enumeration of the known facts regarding 

 the diamagnetic deportment of crystals. In the following I will 

 confine myself strickly to the more mathematical treatment.] 



3. These facts are completely determined. The action of a 

 magnet upon crystals is modified by some cause which lies in 

 the crystallization, or even inverted by this cause. From the 

 experiments described, and many other similar ones, the follow- 

 ing law is derived. 



The optic axis of uniaxal crystals is attracted or repelled by 

 the poles of a magnet according as the crystal is positive or 

 negative. 



By this it appears as proved that the same final cause which 

 produces the modification of light within a crystal, also modifies 

 the action of the magnet upon it. The only direction in which 

 no double refraction takes place, which, according as the crystal 



