MAGNECRYSTALLIC FORCE — MAGNETO-OPTIC FORCE. 
35 
convince me that the force active in his experiments, and that in my results with 
bismuth, &c., is the same*. 
2608. A small rhomboid of Iceland spar was raised to the highest temperature in 
the magnetic field which a spirit-lamp could give (2570.) ; it was at least equal to 
the full red heat of copper, but it pointed as well then as before. A short thick tour- 
maline was heated to the same degree, and it also pointed equally well. As it cooled, 
however, it became highly magnetic, and seemed to be entirely useless for experiments 
at low temperatures ; but on digesting it for a few seconds in nitromuriatic acid, a 
little iron was dissolved from the surface, after which it pointed as well, and in ac- 
cordance with Plucker’s law, as before. A little peroxide upon the surface had been 
reduced by the flame and heat to protoxide, and caused the magnetic appearances. 
2609. There is a general and, as it appears to me, important relation between 
Plucker’s magneto-optical results and those I formerly obtained with heavy glass 
and other bodies (2152, &c.). When any of these bodies are subject to strong induc- 
tion under the influence of the magnetic or electric forces, they acquire a peculiar 
state, in which they can influence a polarized ray of light. The effect is a rotation 
of the ray, if it be passed through the substance parallel to the lines of magnetic 
force, or in other words, in the axial direction ; but if it be passed in the equatorial 
direction, no effect is produced. The equatorial plane, therefore, is that plane in 
which the condition of the molecular forces is the least disturbed as respects their 
influence on light. So also in Plucker’s results, the optic axis, or the optic axes, if 
there be two, go into that plane under the same magnetic influence, they also being’ 
the lines in which there is the least, or no action on polarized light. 
2610. If a piece of heavy glass, or a portion of water, could be brought before- 
hand into this constrained condition, and then placed in the magnetic field, I think 
there can be no doubt that it would move, if allowed to do so, and place itself 
naturally, so that the plane of no action on light should be equatorial, just as 
Plucker shows that a crystal of calcareous spar or tourmaline does in his experi- 
ments. And, as in his case, the magnetic or diamagnetic character of the bodies, 
makes no difference in the general result ; so in my experiments, the optical effect is 
produced in the same direction, and subject to the same laws, with both classes of 
substances (2185. 2187-). 
2611. But though thus generally alike in this great and leading point, there is still 
a vast difference in the disposition of the forces in the heavy glass and the crystal ; 
* The optic axis is the direction of least optic force ; and by Plucker’s experiments, coincides with what 
I consider in my results as the direction of minimum magnecrystallic force. It is more than probable that, 
wherever the two sets of effects (whether really or only nominally different) can be recognized in the same 
body, the directions of maximum effect, and also those of minimum effect, will be found to coincide. — No- 
vember 23, 1848. 
F 2 
