16 REPORT — 1851. 



other end. The silver wire was carried upward through a tube three feet in length, 

 and was connected at the top with a torsion head. When the cores were excited, 

 by sending an electric current through the surrounding helices, the balls were repelled. 

 The index of the torsion head was then gentlv turned against the repulsion until the 

 balls were brought within -i^th of an inch of the ends of the respective cores. The 

 torsion necessary to effect this, is evidently the expression of the repulsive force 

 exerted at this particular distance. 



The strength of the exciting current was measured by a galvanometer of tangents, 

 and it was regulated by means of a rheostat. The cores were excited by currents 

 which varied from 10° to 57°, and the corresponding repulsions were determined. 

 Spheres of the following diaraagnetic substances were used : — 1, bismuth of com- 

 merce ; 2, chemically pure bismuth, obtained by dissolving the material of com- 

 merce in nitric acid, precipitating it with distilled water, washing the precipitate for 

 six days successively, and reducing it by means of black flux ; 3, sulphur of com- 

 merce ; 4, spheres from a crystal of native sulphur obtained in Sicily ; 5, calcareous 

 spar from Clitheroe ; 6, calcareous spar from Andreasberg in the Harz mountains. 

 The diamagnetism of all these spheres followed precisely the same law as the mag- 

 netism of the sphere of soft iron ; it was exactly proportional to the exciting current. 



These results cannot be reconciled with the statement that diamagnetism increases 

 with the increasing power of the magnet in a much quicker ratio than magnetism. 

 The experiments of Pliicker might be accounted for in many ways, but such explana- 

 tions being necessarily conjectural may be omitted here. 



It is known that crystalline bodies, suspended between the poles of a magnet, ex- 

 hibit phsenomena which are absent in the case of amorphous bodies. A. certain 

 line through the crystal will take up a determinate position, and if the line be 

 forcibly moved away from this position, when the force is removed it will return 

 to it. Thus a crystal of pure carbonate of lime suspended by a silk fibre between 

 the poles with its optic axis horizontal, will always turn until the optic axis is per- 

 pendicular to the line joining the poles, in which position it will come to rest. This 

 fact was discovered by Pliicker, who referred it to the operation of a new force, which 

 was entirely independent of the magnetism or diamagnetism of the mass of the 

 crystal. 



In an investigation conducted by the author, in companionship with Prof. Knob- 

 lauch of Marburg, this hypothesis of a new force is rejected, and it is there shown 

 that the position of the optic axis, so far from being independent of the magnetism 

 and diamagnetism of the mass, is entirely changed if a magnetic constituent be sub- 

 stituted for a diamagnetic. Thus, for instance, carbonate of iron differs from carbo- 

 nate of lime only in the circumstance, that an atom of iron is substituted for an 

 atom of calcium. The crystalline form in both cases is identical, the optic axis of 

 carbonate of iron sets, nevertheless, from pole to pole, with an energy far surpassing 

 that with which the optic axis of carbonate of lime sets perpendicular to the line 

 joining the poles. 



But why is it that one direction in the crystal takes up a particular position ? 

 The torsion balance gives a prompt answer to this question. A sphere of cal- 

 careous spar was placed upon each of the spoon-shaped hollows of the beam, the di- 

 rection of the optic axis through each sphere being carefully marked. The spheres 

 were first placed so that the optic axes were parallel to the axes of the soft iron cores, 

 and, secondly, perpendicular to the same ; the repulsion in the former case was to 

 the repulsion in the latter in the ratio of 53 to 48. 



If a bismuth crystal be suspended between two poles, the plane of most eminent 

 cleavage will always set perpendicular to the line joining the poles, that is, equa- 

 torial. A cube formed from this crystal was placed on each end of the little beam ; 

 first, so that the planes of principal cleavage were parallel to the axes of the cores, 

 and secondly, perpendicular to them : — the repulsion in the former case was to the 

 repulsion in the latter in the ratio of 53 to 38. 



The diamagnetic mass in both these cases is repelled with a greater force in one 

 direction than in any other direction. When the crystal is suspended between two 

 poles, the line which marks the direction of maximum repulsion recedes as far as 

 possible from the poles, and hence sets equatorial. 



A result, the exact antithesis of the above, was observed with magnetic crystals. 



