268 Proceedings of the British Association 



beam, which was six inches long, two spoon-shaped hollows were work- 

 ed out, in each of which a ball of the substance to be experimented 

 with might be placed- Two cones of soft iron, surrounded by helices 

 of copper wire, were placed at right angles to the beam when horizon- 

 tally suspended, the one cone facing the ball at one end, and the other 

 cone facing the ball at the other end. The silver wire was carried up- 

 ward through a tube three feet In length, and was connected at the top 

 with a torsion head. When the cones were excited, by sending an 

 electric current through the surrounding helices the balls were repelled. 

 The index of the torsion head was then gently turned against the repul- 

 sion until the balls were brought within yMh of an inch of the ends of 

 the respective cones. 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 cones 

 were excited by currents which varied from 10° to 57^, and the corres- 

 ponding repulsions were determined. Spheres of the following dia- 

 magneiic substances were used: — I. Bismuth of commerce; 2. Chem- 

 ically pure bismuth obtained by dissolving the material of commerce 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 commerce; 4. Spheres from a crystal of native sulphur 



' ■ obtained in Sicily; 5, Calcareous spar from Clitheroe [?] ; 6. Calca- 



reous spar from Andreasberg, in the Hartz mountains, Germany. In 



* all these cases the diamagnetism of the spheres followed precisely the 



same law as the magnetism of the sphere of soft iron r — it was exactly 

 proportional to the exciting current. 



These results cannot be reconciled with the statement that diamag- 

 netism increases with the increasing power of the magnet in a much 

 quicker ratio than magnetism. The experiments of Piiicker might be 

 accounted for in many ways, but such explanations, being necessarily 

 conjectural, may be omitted here. It is known that crystalline bodies 

 suspended between poles of a magnet exhibit phenomena which are 

 absent in the case of amorphous bodies. A certain line through the 

 crystal will take up a certain determinate position ; and if this 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 al- 

 ways turn until the optic axis Is perpendicular to the line joining the 

 poles, in which position it will come to rest. This Aict was discovered 

 by Piiicker, 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 compan- 

 ionship with Prof. Knoblauch, of Marburg, this hypothesis of a new 

 force is rejected ; and it is there shown that the poshion of the optic 

 axis, so far from being independent of the magnetism and diamagnet- 

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

 stituted for a diamagnetic. Thus, for instance, carbonate of iron dif- 

 fers from carbonate of lime only in the fact that in the former case 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 



4 



