on the Polarity of the Diamagnetic Force, 173 



I am indebted for the loan of two prisms of the self-same heavy 

 glass with which he made the discovery of diamagnetism. The 

 bismuth cylinders were withdrawn from the helices and the prisms 

 of glass put in their places. It was now necessary to have a 

 perfectly steady magnet, the expected result being so small as to 

 be readily masked by, or confounded with, a motion arising from 

 some extraneous disturbance. The feeble warmth developed in 

 the helices by a current from two cells I found able to create air 

 currents of sufficient power to defeat all attempts to obtain the 

 pure action of the prisms. To break up these currents I stuffed 

 all unfilled spaces of the box with old newspapers, and found the 

 expedient to answer perfectly. With a fresh battery, which 

 delivered a constant stream throughout the duration of an expe- 

 riment, the magnet was admirably steady*, and under these 

 favourable conditions the following results were obtained : — 



VII. 



Prisms of Heavy Glass. 

 Length 3 inches. Width 0'6. Depth 0*5. 



Current direct. Current direct. Current direct. 



Position 1. 664 Position 2. 662 Position 3. 660 



Thus in passing from position 1 to 3, or vice versa, a permanent 

 deflection corresponding to four divisions of the scale was pro- 

 duced. By raising or lowering the respective prisms at the 

 proper moments the amplitude of the oscillations could be con- 

 siderably augmented, and when at a maximum, could be speedily 

 extinguished by reversing the motions of the prisms. In six 

 different series of experiments made with this substance the same 

 invariable result was obtained. It will be observed that the 

 deflections are in all cases in the same direction as those pro- 

 duced by bismuth under the same circumstances. 



The following results were afterwards obtained with the same 

 prisms in the presence of M. De la Rive; the current was 

 "direct.'^ 



VIII. 



Position 1. 652 Position 2. 650 Position 3. 648 



On the negative result arrived at with this substance, it will be 

 remembered that M. v. Feilitzsch bases one of his arguments 

 against the conclusions of M. Weber. 



Calcareous spar was next submitted to experiment. Two cy- 

 linders of the transparent crystal were prepared and examined 

 in the manner already described. The results are as follows : — 



* It was necessary however to select a portion of the day when Albe- 

 marle Street was free from cabs and carriages, as the shaking of the entire 

 building, by the rolling of these vehicles, rendered the magnets unsteady. 



