OF CRYSTALS BY THE POLES OF A MAGNET. 361 



of them, which was from 26 to 30 millira. broad and long and 

 6 millim. thick, when suspended in the same manner and oscil- 

 lating freely, assumed the same position as a diamagnetic mass ; 

 but on further separating the poles, or on shortening or elon- 

 gating the silkworm thread, it rotated 90°, and remained as 

 if it had become magnetic, the axis being turned perpendicularly 

 to the line of the apices of the poles. 



22. The experiments described above point out uniformly that 

 a repulsive force is exerted by the poles of the magnet upon the 

 axial direction of the calcareous spai', and that, when by the 

 abbreviation of the dimensions in this direction, an attraction 

 towards the axial direction arises from the diamagnetic repulsion 

 of the substance of the crystal, the poles being sufficiently sepa- 

 rated, this attraction is less than the repulsion. 



23. Whilst the colourless and transparent calcareous spar is 

 diamagnetic, a white opake crystal of calcareous spar is magnetic, 

 and one of them presented the same phaenomena as tourmaline. 



24. Rock-crystal is diamagnetic like calcareous spar, and like 

 it exhibited the repulsion of the axial direction ; but this repul- 

 sion is less intense. When a plate cut perpendicularly to the 

 axis (which exhibits the rotation of the plane of polarization) is 

 about three times as long and broad as thick, on suspending it 

 with the axis horizontal, it assumed the same position as a dia- 

 magnetic body, and no longer rotated 90° on separating the 

 poles, which decidedly occurred with plates the dimensions of 

 which were less contracted in the direction of the axis. 



25. In a Soleil's apparatus for exhibiting the conjugate hy- 

 perbolae with polarized light, two similar prisms are ground out 

 of rock-crystal ; their height amounts to 50 millim., and their 

 total base is an almost regular octagon, the two opposite sides 

 of which are 26 millim. apart, and cut at right angles by the optic 

 axis. The two prisms are cemented together to form a single 

 prism, in such a manner that the axial directions in the two halves 

 are at right angles to each other. When the entire prism is 

 suspended so that its axis (the axis of the prismatic form) coin- 

 cides with the direction of the silkworm thread, and thus the 

 optic axes can oscillate horizontally, when raised or lowered, so 

 that at one time the lower, at another time the upper half gets 

 between the apices of the poles, it successively assumes two dif- 

 ferent positions, either of which passes into the other by a rota- 

 tion of 90°, whereby each time the direction of the optic axis of 



