Magneto-optic Properties of Crystals. 3 



It has, however, lately undergone considerable modification 

 at the hands of M. Pliicker himself. In a letter to Mr. Fa- 

 raday, which appears in page 450, vol. xxxiv. of this Maga- 

 zine, he expresses himself as follows:- — 



" The first and general law I deduced from my last expe- 

 riments is the following one : — ; There will be either repulsion 

 or attraction of the optic axes by the poles of a magnet, ac- 

 cording to the crystalline structure of the crystal. If the 

 crystal is a negative one, there will be repulsion ; if it is a po- 

 sitive one, there will be attraction*'" 



This law applies to crystals possessing two optical axes, 

 each of the said axes being attracted or repelled according as 

 the crystal is positive or negative. It will simplify the subject 

 if we regard the line bisecting the acute angle inclosed by the 

 two axes as the resultant of attraction or repulsion ; for the 

 sake of convenience, we shall call this the middle line. In 

 positive crystals, therefore, the middle line, according to the 

 above law, must stand axial \ in negative crystals, equa- 

 torial. It is also evident, that the plane passing through the 

 optical axes must, in the one class of crystals, stand from pole 

 to pole, in the other class at right angles to the line joining 

 the poles. 



In explaining this new modification of the law, M. Pliicker 

 lays particular emphasis upon the fact, that the attraction or 

 repulsion is the result of an independent force, connected in 

 no way with the magnetism or diamagnetism of the mass of 

 the crystal ; and this view is shared by Mr. Faraday, who, in 

 expressing his concurrence with M. Pliicker, denominates the 

 force in question an " optic axis force f." 



The experiments described in our first paper upon this 

 subject J, furnish, we conceive, sufficient ground of dissent from 

 these views. * In the case of five crystals of pure carbonate of 

 lime (Iceland spar), we found the law of Pliicker strictly veri- 

 fied, all five crystals being diamagnetic; on replacing, how- 

 ever, a portion of the carbonate of lime by carbonate of iron, 

 nature herself being the chemist in this case, the crystal was 

 no longer diamagnetic, but magnetic; in every other respect 

 it was physically unchanged ; its optical properties remained 

 precisely as before, the crystal of carbonate of lime and the 

 crystal of carbonate of lime and iron being both negative. In 

 the one case, however, the optical axis was attracted ; in the 

 other, the said axis was repelled ; the attraction being evi- 

 dently caused by the passage of the crystal from the diamag- 

 netic into the magnetic state. 



* Phil. Mag., vol. xxxiv. p. 450. t Phil. Trans. 1849, p. 32. 



| Phil. Mag., vol. xxxvi. p. 178. 



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