456 Prof. Pliicker and Dr. Beer on the Magnetic Axes of Crystals, 



observed during the earlier experiments. It was magnetic^ and 

 its optic axis was repelled. The ciystal was broken off at one 

 end, and the other was bounded by the base of the hexagonal 

 prism. It was of a white colour, but with a transparent layer 

 in the middle, which, however, was so thin, that even if we 

 regard it as diamagnetic, it would not be sufficient to bring 

 about the position of the crystal. 



" The deportment observed by Messrs. Tyndall and Knoblauch 

 was exhibited by a solid milk-white crystal. It was magnetic, 

 and its axis was attracted. Three other crystals were afterwards 

 found which acted in a similar manner. The first was from 

 Prczbram, the second from Zillerfeld. The third was a trans- 

 parent hexagonal prism, which, contrary to all expectation, proved 

 to be decidedly magnetic. This prism, 14 millims. long, 7 mil- 

 lims. and 4 millims. in diameter, set itself when raised 150 mil- 

 lims. always axial. A plate 5 millims. thick, cut perpendicular to 

 the axis, when hung so that the said axis could swing horizon- 

 tally, set itself with the latter axial. A tolerably transparent 

 crystal from Brilon behaved in a similar manner. It showed a 

 good ring-system, and proved, like all transparent crystals of 

 the spar, in optical respects negative. 



" If we sum up the foregoing, we find that there are both dia- 

 magnetic and magnetic specimens of this crystal which have their 

 axes repelled by the poles of a magnet ; and, on the other hand, 

 magnetic and diamagnetic specimens which have their optic axes 

 attracted ; and hence we may presume that there are crystals 

 which exhibit no action whatever in this respect. The different 

 deportment of calcareous spar has been justly brought to bear, 

 by Messrs. Tyndall and Knoblauch, against the notion that the 

 attraction and repulsion of the axis is due to the optical nature 

 of the crystal alone. It can, however, with equal justice be ad- 

 duced against the conclusion, that the position of the optic axis 

 is determined by the magnetism or diamagnetism of the mass. 



"To explain this diversity of action in calcareous spar, the ques- 

 tion must tu'st be answered, whether in all cases where the dia- 

 magnetic spar is rendered impure by a mixture of iron, the latter 

 exists under the form of isomorphous carbonate of iron, or is 

 merely deposited in layers along the cleavages. Above all, how- 

 ever, must be taken into account the difference of the laws of 

 magnetic attraction and diamagnetic repulsion, or in other words, 

 the different coercive force of magnetic and diamagnetic sub- 

 stances. If we limit ourselves to chemically pure calcareous 

 spar, we obtain the undeniable result, that the crystal is negative 

 and that its optic axis is repelled.^' 



The next ciystals examined are nitrate of soda, which is nega- 

 tive, diamagnetic, and has its axis repelled; carbonate of lime 



