OF CRYSTALS BY THE POLES OF A MAGNET. 357 



I selected a dark brown, almost opake crystal of tourmaline, 

 having the form of a six-sided prism, which was about 36 millim. 

 long and 4'5 millim. in thickness, and placed the apices of the 

 poles so near together, that it could only just oscillate freely be- 

 tween them. The magnetic attraction caused the tourmaline to 

 assume such a position, that the axis of the prism, which is also 

 its optic axis, coincided with the line of the apices of the poles. 

 The more the latter were separated from each other,. the less in- 

 tense was the force with which the crystal assumed this position ; 

 and when their distance amounted to more than 80 millim., it ro- 

 tated 90°, as if it had become diamagnetic, so that its axis was now 

 perpendicular to the line of the apices of the poles. On the further 

 separation of the latter, the force which retained it in the position 

 just described increased ; and in this it continued distinctly to 

 remain after the apices of the poles had been entirely removed. 



The apices were again inserted and pushed forwards until the 

 tourmaline assumed an axial position (in the line of the apices). 

 When it was now raised or lowered, by winding or unwinding 

 the thread by which it was suspended, it turned round, at a cer- 

 tain elevation or depression, 90°, at the same time assuming an 

 equatorial position (perpendicular to the line of the apices of the 

 poles). As far as the limit to which this rotation extended, the 

 axial rectilinear force diminished until it finally vanished ; the 

 equatorial rectilinear force then came into play, increasing when 

 the crystal was further raised or lowered, and finally again dimi- 

 nished, being however distinctly perceptible when removed from 

 200 to 250 millim. from the line of the apices of the poles. 



In all these experiments, by opening and closing the circuit, 

 the tourmaline could be rotated 180°, and retained in the oppo- 

 site position, 



9. The passage of the tourmaUne from one position to the, 

 other still appeared to take place to exactly the same extent 

 when the power of the electro-magnet was either increased or 

 diminished. 



10. If we retain the hypothesis of a repulsive action exerted 

 by the poles of the magnet upon the axial direction, in accord- 

 ance with the experiments which have been detailed in the two 

 last paragraphs, we must necessarily admit that the force which 

 produces the repulsion diminishes more slowly with the increase 

 of the distance than the force of the magnetic attraction ema- 

 nating from the same poles. 



