358 PLUCKER ON THE REPULSION OF THE OPTIC AXES 



11. Before examining other crystals in the same ^vay as 

 the tourmaline, it appeared to me best to investigate as com- 

 pletely as possible the remarkable phaenomena and their mo- 

 difications in this mineral, in which I first discovered them. 

 Two other crystals of tourmaline, resembling that which was 

 used in experiment 7? but heavier, one being 47 millim. in 

 length and about 3 millim. in thickness, the other of the same 

 length and about 5 miUim. in thickness, yielded in general 

 exactly the same results. All these tourmalines were attracted 

 into the immediate vicinity of one of the poles of a magnet, in 

 their entire mass. 



12. A rubellite, 9 millim. long and 6 to 7 milUm. thick, when 

 the apices of the poles were 13 millim. apart, became placed 

 axially in the line of the apices, but turned round 90° even 

 when raised about 20 millim. or lowered about 30 millim., at 

 the same time assuming an equatorial position. 



A red, transparent tourmaline, 30 millim. in length, did the 

 same. 



I then examined four smaller crystals of tourmaline from the 

 isle of Elba, 4 to 9 millim. long, the first half light and half 

 dark green, a second entirely light green, a third light green in 

 the middle but dark at both ends, and the fourth red. All these, 

 as also the two former, were strongly magnetic ; the first was 

 more strongly so at the dark than the light green end. They 

 exhibited all the phaenomena described in paragraph 7? except 

 that the third and smallest could not be made to assume the 

 axial position, because, when the apices of the poles were ap- 

 proximated with this intention, before it could attain this posi- 

 tion, it was drawn away by one of the apices of the poles. 



13. Lastly, I must mention a perfectly colourless tourmaline, 

 the long dimensions of which coincided with the direction of the 

 axis, but which in other respects was irregular. The mass of 

 this crystal proved to be diamagnetic throughout ; whence it ap- 

 pears that both the red and the green colour of this mineral are 

 produced by iron ; consequently the diamagnetic force emanating 

 from the poles of the electro-magnet must act upon the axial 

 direction in the same manner as the repulsive force, to produce 

 the equatorial position of the crystal. 



14. As regards the tourmaline, after the preceding remarks, I 

 considered myself justified in considering the following law de- 

 termined, viz. that its axial direction is repelled by the poles of 



