OF CRYSTALS BY THE POLES OF A MAdNET. 369 



of the apices of the poles ; but when the suspension was in X, it 

 occurred after an elevation ofW millim. only. 



39. A binaxial crystal of the prismatic form, and ^he substance 

 of which is magnetic, u hen suspended freely between the apices 

 of the poles, must necessarily be differently acted upon from a 

 uniaxial crystal of about the same external form. Thus the force 

 with which a uniaxial crystal is moved into the equatorial posi- 

 tion, remains exactly the same in whatever way it may be sus- 

 pended, provided that its longitudinal direction can oscillate in 

 the horizontal plane. This was confirmed by direct experi- 

 ment, on three times repeating with the same tourmaline the 

 experiment described in paragraph 8, so that, the apices of the 

 poles remaining undisturbed, the tourmaline was suspended in 

 three different directions, lying in a plane at right angles to the 

 axis of the prism. Each time an elevation of exactly 24 millim. 

 was requisite to turn the tourmaline round 90°, so that it should 

 assume an equatorial position. 



This cannot be the case in a binaxial crystal, whatever may 

 be the position of the two optic axes. We will suppose in such 

 a crystal that the middle line Y coincides with the axis of the 

 prism. Whatever may then be the direction in the plane X Z 

 in which we suspend the crystal, the magnetic action always 

 fixes it with the same force in the axial position, whilst the re- 

 pulsive force exerted by the poles against the two axes tends 

 to place it equatorially but with different force. Thus when 

 the crystal is suspended in the direction Z, and hence the 

 two optic axes can oscillate horizontally, the forces from each of 

 the poles of the magnet acting upon the two axes tend to pro- 

 duce rotations in the reverse direction. However, when the 

 crystal is suspended in the direction of the axis X, and thus the 

 planes of the two axes constantly remain vertical during the 

 oscillation of the crystal, the forces emanating from each pole of 

 the magnet accumulate, and the resulting force is obviously 

 greater than in the previous method of suspension. Moreover, 

 it is clear that the moment of rotation increases in proportion 

 as the direction of the silk thread is removed from the direction 

 Z and approximates the direction X. 



40. The above considerations led me to the experiment with 

 the staurolite mentioned in paragraph 34 ; and I had no doubt 

 that what had previously embarrassed me as being an inex- 



VOL. V. PART XIX. 2 C 



