Fig. 8. 



to Magneci-ystallic Action. 133 



The disposition of this important experiment will be manifest 

 from fig. 8, where cd is the magnecrystallic axis of a sphere of 

 bismuth, or the line in which the 

 diamagnetic induction is least in- 

 tense; and s'li' the direction of 

 the principal cleavage, or that of 

 most intense diamagnetization. 

 Let n be the point of the bodkin, 

 say its north pole, the crystal will 

 be excited by the influence of this 

 pole, and the resultant action will 

 be the same as if it were exclu- 

 sively " diamagnetized" along the 

 line s'n'. At the end nearest to 

 the pole of the bodkin a repelled 

 pole r^ will be excited in the 

 bismuth ; at the most distant end 

 an attracted jjole ^ will be ex- 

 cited. Let the repulsive force 

 tending to separate n from n' 

 be represented by the line np, and let the attraction exerted 

 between s and n be represented by the line nq • the arrange- 

 nient is such that the force of s' acts more nearly in the direction 

 of the tangent than that of n' ; the latter may be decomposed 

 into two, one acting along the circle and the other across it : the 

 latter component exerts a pressure against the axis of suspension ; 

 the former only is effective in causing the pole n to move ; so 

 that the whole, or nearly the whole, of the attraction has to 

 compete with a comparatively small component of the repulsion. 

 The former therefore preponderates, and the pole n approaches 

 the crystal. It is manifest that as the angle which the line from 

 n to the centre of the crystal makes with the magneciystallic axis, 

 increases, the component of repulsion which acts in the direc- 

 tion of a tangent to the curve, augments also ; and that at a 

 certam point this component must become preponderant. Beyond 

 an angle of 30' it is to be presumed that Mr. Faraday did not 

 obtain the effect. Removing the crystal, and placing a small 

 magnet m the position of the line s' n', with its poles arranged 

 as in the figure, the same phfenomena would be produced*. 



As finally illustrative of the sufficiency of the principle of 

 polarity to explain the most complicated phtenomena of magne- 

 crystallic action, let us turn to the consideration of those curious 

 effects of rotation first observed by M. Pliicker, and which I 



* As tliere are no measurements given of the distances between the 

 crystal and the pole, it is of course impossible to do more than indicate 

 generally the theoretic solution of the experiment. 



