Dr. Tyndall on Diamagnetism and Magnecrystallic Action. 175 



dissented from, and it is there proved that the action of the cry- 

 stal, so far from being independent of the magnetism or dia- 

 magnetism of its mass, is totally changed by the substitution of 

 a magnetic constituent for a diamagnetic. Our experiments led 

 us to the conclusion, that the position of the crystal of carbo- 

 nate of lime was due to the superior repulsion of the mass of the 

 crystal in the direction of the optic axis. This view, though 

 supported by the strongest presumptive facts, has remained up 

 to the present time without direct proof; if, however, a differ- 

 ence of repulsion, such as that we have supposed, actually exists, 

 it may be expected to manifest itself upon the torsion-balance. 



But the entire repulsion of calcareous spar is so feeble, that 

 to discover a differential action of this kind requires great nicety 

 of experiment. I returned to this subject three different times ; 

 twice I failed, and despaired of being able to establish a differ- 

 ence with the apparatus at my command. But the thought 

 clung to me, and after an interval of some weeks I resolved to 

 try again. 



The spheres of calcareous spar were placed upon the beam, 

 and the latter was exactly balanced. The index above was so 

 placed, that when the beam came to rest, the attached glass fibre 

 exactly coincided with a fine black line drawn upon the Bristol 

 board underneath. Two dots were placed upon the glass cover, 

 about the fiftieth of an inch asunder, and the fibre was observed 

 through the interval between them. The beam was about 

 4 inches below the cover, and parallax was thus avoided. On 

 exciting both cores the balls receded, the index above was softly 

 turned against the recession, till the fibre was brought once more 

 into exact coincidence with the fine black line, and the torsion 

 necessary to effect this was read off upon the graduated circle 

 above. 



The repulsion of the spheres was measured in four different 

 directions : — 



1 . The optic axes were parallel to the axes of the iron cores. 



2. The spheres were turned through an arc of 90°, so that 

 the optic axes were at right angles to the cores. 



3. The spheres were turned 90° in the same direction, so that 

 the other ends of the axes faced the cores. 



4. The spheres were turned 90° further, so that their axes 

 were again at right angles to the cores, but with the opposite 

 surface to that in (2) facing the latter. 



The following are the respective repulsions : — 



Repulsion. 



1st position 28*5 



2nd position 26*5 



3rd position 27*0 



4th position 24*5 



