SECT. XXXIV. MAGNECEYSTALLIC ACTION. 349 



The molecular structure of substances freely suspended between 

 the poles of a magnet has a decided effect upon the position they 

 assume. 



It has already been mentioned that the optic axis is a symme- 

 trical line in a doubly refracting crystal in which there is no 

 double refraction, and that in some crystals there are two such 

 symmetrical lines. Now, Professor Pliicker of Bonn discovered, 

 when such crystals are submitted to powerful magnetic influence, 

 that the single optic axis in the one, and the resultant or mean 

 line between the double optic axes in the other, set diametrically 

 or at right angles to the line of magnetic force ; and so powerful 

 did the Professor find the action of magnetism on crystalline 

 form, that the mineral cyanite, when suspended, arranges itself 

 so definitely with regard to terrestrial magnetism, that it might 

 be used as a compass needle. 



Dr. Faraday afterwards observed that amorphous substances, 

 cut in the form of a sphere, have no tendency to set or be 

 attracted or repelled in one direction in preference to any other ; 

 but if the sphere be formed of a crystallized substance, it is a 

 general fact that, whether it be paramagnetic or diamagnetic, it 

 is more powerfully attracted or repelled in one direction than in 

 any other a property named by Dr. Faraday magnecrystallic 

 action. For example, a sphere of calcareous spar, which is a 

 diamagnetic crystal, is most strongly repelled in the direction of 

 its principal optic axis, and least strongly in the direction of its 

 least axis. In a sphere of carbonate of iron, which has exactly 

 the same crystalline form and is highly paramagnetic, the line 

 which in carbonate of lime sets equatorially, in this case sets 

 axially, and more strongly in that direction than in any other. 

 The law according to which the attraction of the carbonate of iron 

 increases from the least to its greatest or principal optic axis, is 

 precisely the same as that according to which the repulsion of the 

 calcareous spar increases from the least to the principal optic axis. 

 These relations are not altered by the immersion of the spheres 

 in liquids of either magnetism. Dr. Faraday observed that a 

 line at right angles to the planes of principal cleavage in crystals 

 takes the axial position, and on that account he called it the 

 magnecrystallic axis. Its position was proved by MM. Tyndall 

 and Knoblauch to depend upon the general fact, that the mass is 

 most strongly repelled in the direction of the planes of principal 



