334 Dr. Tyndall on the Polarity of Bismuth, 



tions, together with the fact of having in my possession a piece 

 of bismuth, whose peculiar structure suggested the possibihty of 

 submitting the question of diamagnetic polarity to an additional 

 test, are the inducements in which the present brief inquiry 

 originated. 



2. In December 1847 a paper on 'Diamagnetic Polarity* was 

 read before the Academy of Sciences in Berlin by Professor Pog- 

 gendorff, the result arrived at by the writer being, that a bis- 

 muth bar, suspended horizontally and occupying the equatorial 

 position between two excited magnetic poles, was transversely 

 magnetic, — that side of the bar which faced the north pole pos- 

 sessing north polarity, and that side which faced the south pole 

 possessing south polarity ; the excitation being thus the oppo- 

 site of that of iron, and in harmony with the original conjecture 

 of Faraday. 



3. The method adopted by the writer was as follows : — the 

 bismuth bar was suspended within a helix of copper wire, the 

 coils of which were perpendicular to the axis of the bar. The 

 helix was placed between the opposite poles of a magnet, so that 

 the axis of the hehx was perpendicular to the line joining the 

 poles. The bismuth took up the usual equatorial position, its 

 length thus coinciding with the axis of the helix. On sending 



algebraic when negative quantities are concerned) least capacity for diamag- 

 netic induction perpendicular to the optic axis. I am not aware that this 

 particular conclusion has been verified by any experimenter," &c. Since 

 the above was written the diflferential action of calcareous spar has been 

 estabhshed ; and up to a day or two ago, when I subjected Mr. Thomson's 

 paper to a more cai-eful perusal, my impression was that his theory and my 

 experiments perfectly harmonized. I now fear that there must be some 

 misapprehension on my part as regards his meaning, for not only do the 

 results of my investigation seem to be at variance with his conclusion, but 

 the conclusion appears to be at variance with the experiment of Professor 

 Faraday quoted by himself. This is written with some diffidence, as the 

 manner in which Mr. Thomson has stated Mr. Faraday's experiment proves 

 that he clearly comprehends the particular result obtained. The repulsion 

 which a sphere of calcareous spar endures at any particular point may, I 

 think, be taken as a coiTect measure of its * tendency ' to move from that 

 point ; but it has been proved that the repulsion of a sphere of calcai*eous 

 spar when the optic axis is parallel to the lines of force (parallel to the 

 axis of the soft iron core w hich repels it) being represented bv the number 

 67, the repulsion experienced when the optic axis is perpendicular to the 

 lines of force will be represented by the number 61 (see Phil. Mag. for 

 Sept. p. 176). The * tendency' to pass from stronger to weaker places of 

 force is therefore stronger when the optic axis is parallel to the lines of force 

 than when it is perpendicular to them, which is a conclusion precisely the 

 reverse of that expressed by Mr. Thomson. I would here, however, repeat 

 my conviction, either that 1 misunderstand Mr. Thomson, or that through 

 some excusable inadvertence, perhaps through a typographical error, each of 

 the words 'parallel ' and * perpendicular' occupies the place which should 

 be occupied by the other. — J. T. 



