356 PLUCKER ON THE THEORY OF DIAMAGNETISM. 



the analogy between light and magnetism, as regards the latter, 

 the classification must still continue to exist. The magnetic axes 

 and the optic axes must both be regarded ; the former as well as 

 the latter are either positive or negative. In a similar manner 

 different axes have been observed in crystals, for the different 

 colours which either do or do not lie in the same plane. The 

 uniaxial crystals do not enable us to decide here, and I have 

 been compelled to turn to the biaxial crystals in order to seek 

 the universal response. As this investigation is not yet com- 

 plete, for the present I refrain from drawing any inferences 

 from it. 



34. The experimental investigations are rendered complicated 

 in a high degree by the circumstance that foreign substances 

 are generally mixed up with the crystals. Thus, for instance, 

 Faraday^s crystals of arsenic were diamagnetic, mine, evidently 

 because they contained iron, magnetic at all distances from the 

 poles. But the crystals of both set so that the direction per- 

 pendicular to the principal plane of cleavage coincided with the 

 line which united the poles. If the iron entered as chemical 

 constituent into the ultimate particles of the crystal, then we 

 must expect the axis of the crystal in one case to be attracted, 

 and in the other case repelled. Inasmuch, however, as the 

 opposite takes place, I believe I am justified in concluding that 

 the iron was a foreign body in the crystals which I examined. 



35. From our way of viewing the subject, we ought to expect 

 that in organic substances phsenomena must exhibit them- 

 selves analogous to those observed in the case of crystals. Thus, 

 for instance, we should imagine that an organic web, saturated 

 with a magnetic substance, would not in magnetic respects 

 prove the same in all directions, but that, on the contrary, in 

 the direction of the length the capillary canals filled with the 

 magnetic substance would cause the magnetism to exhibit itself 

 more forcibly in this direction than in any other. 



36. In certain crystals it might, indeed, be doubted whether 

 the observed phaenomena were due to the crystalline form and 

 its possible action upon the aether, and not, on the contrary, to an 

 admixture of foreign magnetic substances uniformly distributed 

 between the ultimate particles of the crystals. I reckon here 

 many crystals, which in their state of purity contain no iron, 



