G22 ADDITIONS. 



plane of polarization rotated in the same direction; that is, all to the 

 right or all to the left ; but when a ray passes through the heavy glass, 

 the power of rotation exists only in a plane perpendicular to the mag- 

 netic line, and its direction as right or left-handed is reversed by 

 reversing the magnetic polarity. 



In this case, we have optical properties, which do not depend on 

 crystalline form, affected by the magnetic force. But it has also been 

 found that crystalline form, which is so fertile a source of optical pro- 

 perties, affords indications of magnetic forces. In 1847, M. Plucker, 

 of the University of Bonn, using a powerful magnetic apparatus, simi-. 

 lar to Faraday's, found that crystals in general are magnetic, in this 

 sense, that the axes of crystalline form tend to assume a certain posi- 

 tion with reference to the magnetic lines of force. The possession of 

 one optic axis or of two is one of the broad distinctions of the different 

 crystalline forms : and using this distinction, M. Pliicker found that a 

 crystal having a single optic axis tends to place itself with this axis 

 transverse to the magnetic line of force, as if its optic axis were repelled 

 by each magnetic pole ; and crystals with two axes act as if each of 

 these axes were repelled by the magnetic poles. This force is inde- 

 pendent of the magnetic or diamagnetic character of the crystal ; and 

 is a directive, more properly than an attractive or repulsive force. 



Soon afterwards (in 1848) Faraday also discovered 11 an effect of 

 magnetism depending on crystalline form, which at first sight appeared 

 to be different from the effects observed by M. Plucker. He found 

 that a crystal of bismuth, of which the form is nearly a cube, but more 

 truly a rhombohedron with one diagonal a little longer than the others, 

 tends to place itself with this diagonal in the direction of the lines of 

 magnetic force. At first he conceived 12 the properties thus detected 

 to be different from those observed by M. Plucker ; since in this case 

 the force of a crystalline axis is axial, whereas in those, it was equato- 

 rial. But a further consideration of the subject, led him 13 to a convic- 

 tion that these forces must be fundamentally identical : for it was easy 

 to conceive a combination of bismuth crystals which would behave in 

 the magnetic field as a crystal of calcspar does ; or a combination of 

 calcspar crystals which would behave as a crystal of bismuth does. 



And thus we have fresh examples to show that the Connexion of co- 

 existent Polarities is a thought deeply seated in the minds of the pro* 



' Taylor's Scientific Memoirs, vol. v. " Researches, Art. 2454, Ac. 



18 Art. 2469. 1S Art. 2593, 2601. 



