Magneto-optic Properties of Crystals, 2 1 



Application of the 'principle of Elective Polarity to Crystals. 



We shall now endeavour to apply the general principle of 

 elective polarity to the case of crystals. This principle may 

 be briefly enunciated as follows : — 



If the arrangement of the component particles of any body be 

 such as to present different degrees of proximity in different 

 directions, then the line of 'closest proximity, other circumstances 

 being equal, will be that chosen by the respective forces for the 

 exhibition of their greatest energy. If the mass be magnetic, 

 this line will stand axial ; if diamagnetic, equatorial. 



From this point of view, the deportment of the two classes 

 of crystals, represented by iceland spar and carbonate of iron, 

 presents no difficulty. Their crystalline form is the same ; 

 and as to the arrangement of the particles, what is true of one 

 will be true of the other. Supposing, then, the line of closest 

 proximity to coincide with the optic axis ; this line, according 

 to the principle expressed, will stand axial or equatorial, as 

 the mass is magnetic or diamagnetic, which is precisely what 

 the experiments with these crystals exhibit. 



Analogy, as we have seen, justifies the assumption here 

 made. It will, however, be of interest to inquire, whether 

 any discoverable circumstance connected with crystalline for- 

 mation exists, upon which the difference of proximity de- 

 pends; and, knowing which, we can pronounce with tolerable 

 certainty, as to the position which the crystal will take up in 

 the magnetic field. 



The following experiments will perhaps suggest a reply. 



If a prism of sulphate of magnesia be suspended between 

 the poles with its axis horizontal, on exciting the magnet the 

 axis will take up the equatorial position. This is not entirely 

 due to the form of the crystal; for even when its axial dimen- 

 sion is shortest, the axis will assert the equatorial position ; 

 thus behaving like a magnetic body, setting its longest dimen- 

 sion from pole to pole. 



Suspended from its end with its axis vertical, the prism will 

 take up a determinate oblique position. When the crystal has 

 come to rest, let that line through the mass which stands 

 exactly equatorial be carefully marked. Lay a knife-edge 

 along this line, and press it in the direction of the axis. The 

 crystal will split before the pressure, disclosing a shining sur- 

 face of cleavage. This is the only cleavage the crystal pos- 

 sesses, and it stands equatorial. 



Sulphate of zinc is of the same form as sulphate of magnesia, 

 ad its cleavage is discoverable by a process exactly similar 



