OF CRYSTALS BY THE POLES OF A MAGNET. 365 



yet on the average) is at the same time the central Une between 

 their optic axes, equatorial. These phaenomena, owing to the 

 diamagnetic action, of themselves prove nothing ; only it was re- 

 quisite that they should not furnish a different result. But an 

 experiment made with a crystal of staurolite, which was mag- 

 netic and became axially placed when in the line of the apices 

 of the poles, but equatorially when raised or lowered, was con- 

 clusive. This experiment was also performed, even before that 

 on the topaz, with the most decided result ; but subsequently, 

 to my astonishment, with somewhat doubtful effect ; and at the 

 conclusion of these experimental investigations it was repeated 

 in a new point of view, so that I shall return to it more fully in 

 a subsequent paragraph (40). 



A crystal of lepidoUte was so strongly magnetic, that when 

 raised it could not be made to rotate ; just so with a beautifully 

 perfect crystal of hornblende (a thick hexagonal prism with acute 

 extremities). The former did not exhibit the least polarity, 

 whilst in the latter it was very decided. 



35. It must moreover be remarked, that all binaxial as well as 

 all uniaxial crystals, which in the above experiments assumed an 

 axial or equatorial position of a certain dimension, were rotated 

 180°, and retained in this position by opening and closing the 

 circuit. 



36. The phaenomena detailed in paragraphs 30 and 35 are 

 perfectly explained by the assumption, that a repulsive force is 

 exerted upon both the optic axes by the poles of the magnet, 

 and then, in accordance with the second experiment upon the 

 topaz (33), we must add to this supposition, that the repulsion 

 exerted upon both the optic axes is of equal intensity. The two 

 experiments with mica (30, 31), as also the first with topaz (32), 

 do not allow of the assumption of a repulsive force exerted 

 merely upon the central line, instead of upon both axes. 



In the second experiment with the plate of mica, the repul- 

 sion exerted upon the axes overcame the magnetic, and in the case 

 of the experiment with the topaz and sugar the diamagnetic 

 force, which tended each time to bring the crystal, in conformity 

 with its form, into a position differing by 90°. 



37. Two other experiments, suggested by a theoretical com- 

 bination, were performed, which are conclusive as regards bin- 

 axial crystals, and deserve special attention. However, before 

 describing them, for the purpose of facihtfiting the survey of the 



