'^ 



THE 



LONDON, EDINBURGH and DUBLIN 



¥l[f LOSOPHICAL MAGAZrlfE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 





NOVEMBER \m\. — -» 



,1 j| 



1 1. 



LI. On the Polarity of Bismuth, including an Examination of the 



Magnetic Field. By John Tyndall, PLD."^ : |. 



1. ^T^HE polarity of bismuth is a subject on which philoso- 

 JL phers have differed and continue to differ. On the one 

 side we have Weber, Foggendorff, and Pliicker, each affirming 

 that he has established this polarity ; on the other side w^e have 

 Faraday, not affirraing the opposite, but appealing to an in- ^-S 



vestigation w^hich is certainly calculated to modify whatever con- 'j|. 



viction the results of the above-named experimenters might have 

 created. It will probably have occurred to everybody w^ho has 

 occupied himself experimentally with diamagnetic action, that 

 whenever the simple mode of permitting the body experimented : |r J 

 with to rotate round an axis passing through its own centre of ' ^ i 

 gravity, can be applied, it is preferable in point of delicacy to all > ^"q 

 others. A crystal of calcareous spar, for example, when sus- | «. §. 

 pended from a fine fibre between the poles, readily exhibits its j c , 

 directive action, even in a field of weak power ; while to establish \ c X 

 that peculiar repulsion of the mass which is the cause of the 1 ^^ 

 directive action, even with high power and with the finest torsion 

 balance, is a matter of considerable difficultyf. These considera- . 



=^3 



^:^ 





r 



* Communicated by the Author. 



t Phil. Mag. 4th series, vol. ii. )). 175. I have much pleasure in re- 

 ferring here to the following remark of Professor W. Thomson in his paper 

 " On the Theory of Magnetic Induction," which appears in the Phil. Mag. 

 for March last. " Thus," he writes, " a ball cut out of a crystal of pure | Z' 



calcareous spar which tends to turn -with its optic axis perpendicular to • *^. 



the lines of force, and which tends as a whole from places of stronger to I '% 



places of weaker force, would experience this latter tendency more strongly \ *i 



when the optic axis is perpendicular to the lines of force than when it is I '% 



parallel to them ; since, according to § 8 of the text, the crystal must have • | $ 

 the greatest inductive capacity, or (the language in the text being strictly 

 Phil Mag. S. 4. Vol. 2. No. 12. Nov, 1851. 2 A 



V 



