448 



April 23, 1857. 



The LORD WROTTESLEY, President, in the Chair. 

 The following communication was read : 



" On the Magnetic Induction of Crystals." By Professor JULIUS 

 PLUCKER, of Bonn, For. Memb. R.S., Hon, M.R.I., &c. 

 Received March 26, 1857. 



[Abstract.] 



The author commences by referring to his discovery of the peculiar 

 action of magnets on crystalline bodies, and to the researches to 

 which he was thereby led. With reference to the form in which he 

 enunciated the law regulating the action of a magnet on a uniaxal 

 crystal that the optic axis is attracted or repelled by the poles of 

 the magnet he disclaims any intention of assigning a physical cause 

 to the phenomenon, or doing anything more than expressing the 

 results of observation, which are as if such a force existed. In the 

 case of crystals of a more complicated character, he was led, in the 

 first instance, to assume the existence of two magnetic axes, possessing 

 a similar character as to attraction and repulsion with the one axis of 

 optically uniaxal crystals. But finding that the proposed law did 

 not hold when the crystal was examined in all directions, and not 

 solely along peculiar axes, he abandoned, nearly two years ago, a 

 hypothesis respecting which serious doubts had arisen long before. 

 For the hypothesis of one or two axes acted upon by the magnet, 

 he substituted another similar hypothesis. In the case of uniaxal 

 crystals he now conceived an ellipsoid of revolution, consisting of 

 an amorphous paramagnetic or diamagnetic substance, and having 

 within the crystal its principal axis coincident with the principal 

 crystallographic axis. It is easy to verify that both crystal and 

 ellipsoid, the poles of the magnet not being too near each other, 



