Royal Society. 307 



" In one experiment I obtained on a piece of heavy glass not com- 

 pressed, 3° of rotation to the right or to the left, according to the 

 direction of the current : on slightly compressing the glass, I had to 

 turn to the right the eyepiece to 4°, 5°, and even to 8° in order to 

 restore the image to its first condition. In closing the circuit, the 

 rotation produced in the same direction as that due to compression 

 was 3i° or 4°, while the rotation produced in the contrary direction 

 was from 2° to 1^°. On ceasing to compress the glass, I obtained 

 the same phrenomena as I had observed before the compression. 



" I have made in the same manner experiments with a piece of 

 flint-glass, which produced a rotation of 2° under the influence of the 

 magnet. When I applied the same magnet to pieces of compressed 

 flint-glass, I could not discover the slightest sensible rotation in 

 whatever direction I might make the current pass. Plates of quartz 

 cut perj^endicularly or parallel to the axis, and compressed in various 

 directions, did not acquire any rotatory power under the influence of 

 the magnet. I think that the peculiarity exhibited by compressed 

 heavy glass is of some interest, in as far as it appears likely to lead 

 to a more satisfactory explanation of the want of rotatory power 

 communicated by magnetism in crystalline bodies. 



" I shall conclude by communicating the negative results of some 

 experiments I attempted with a view to discover the action of dia- 

 magnetic bodies on each other, and of magnetism on gaseous bodies. 

 I suspended small needles of bismuth between the poles of a very 

 powerful electro-magnet, and with a good chronometer I counted 

 the number of their oscillations, either alone or in the vicinity of 

 pieces of bismuth of various shapes and sizes. I repeated these ex- 

 periments with all possible care, avoiding the slightest current of air, 

 reckoning the smallest oscillations, and those of the same extent in 

 the difi'erent cases. I never obtained any difl"erences beyond half a 

 second, which existed equally whether the pieces of bismuth were 

 near or not. The experiment therefore does not serve to show the 

 action of diamagnetic bodies on each other ; an action which natu- 

 rally ought to exist, but which perhaps is overpowered by the stronger 

 action of the magnet. 



" I afterwards counted the oscillations of a small needle of bismuth, 

 which I succeeded in suspending by a silk fibre {fil de cocon) inside 

 of a glass ball blown at the top of a barometer- tube. The ball was 

 placed between the poles of my electro-magnet. In this experiment 

 the bismuth needle was held sometimes in a nearly perfect vacuum, 

 at others in atmospheric air. The number of oscillations in both 

 cases was exactly the same. 



" We must therefore give up the idea of explaining diamagnetic 

 phenomena by a magnetic action, which would be stronger upon the 

 air than upon bismuth." 



X2 



