OF CRYSTALS BY THE POLES OF A MAGNET. 367 



c, the crystal is magnetic, and its dimension in the direction 

 Y is greater than that in the direction X ; 



d, the crystal is diamagnetic, and its dimension in the direc- 

 tion Y is less than that in the direction X ; 



the magnetic attraction and diamagnetic repulsion of the mass 

 must previously be overcome before the repulsion of the axes 

 can be apparent. 



In the second and third normal positions of suspension, the 

 plane X Y of the two optic axes oscillates, so that it each time 

 constantly remains vertical, and each time it is forced by the 

 repulsion of the axes into the equatorial position. In the second 

 position of suspension, the direction X, in the third the middle 

 line Y, oscillate horizontally and become equatorial. The axial 

 action is increased by the magnetic attraction and diamagnetic 

 repulsion of the mass, when 



a, the crystal is magnetic, and its dimension X in the second 

 case and Y in the third case are less than Z ; 



b, the crystal is diamagnetic, and its dimensions X and Y are 

 greater than Z, These forces must moreover be overcome 

 before the axial repulsion can be perceived, when 



c, the crystal is magnetic, and its dimensions X or Y are 

 greater than Z ; 



d, the crystal is diamagnetic, and its dimensions X or Y are 

 less than Z. 



38. Arragonite crystallizes in right rhombic prisms, which by 

 truncation of the sides usually become six- sided. From one of 

 these crj'stals, which was perfectly transparent, I had a piece 

 ground at right angles to the axis and polished, and then, to en- 

 sure perfect certainty, determined the position of the two axes 

 by viewing them by polarized light. These axes, as is well 

 known, form with each other an angle of full 18° ; and the middle 

 line, subdividing this angle, coincides with the axis of the 

 prism. The planes of the two optic axes were perpendicular to 

 those two parallel lateral surfaces, the distance of which from 

 each other was extremely minute, and was 10 miUim. The 

 height of the prism was 12'5 millim., and the largest diagonal 

 of its terminal facets, which was at right angles to the plane of 

 the two optic axes, was 22 millim. The directions in which 

 these three dimensions were taken, coincide respectively with 

 the directions which we indicated in the last paragraph by X, 



