666 CONSTANTS OF MAGNETISATION. 



u H 2 (/-'). If A is the density of the body, p its radius of gyra- 

 tion, and n the number of oscillations per second, we have 



H 2 



We only obtain the difference of the coefficients k and k '; 

 another experiment would be necessary to determine one of them 

 in absolute value. 



Bismuth* has the character of symmetry in question. Its 

 crystalline shape is that of a rhombohedron of 87 40' with a 

 cleavage perpendicular to the axis. It is diamagnetic and uniaxial. 

 Moreover, the direction of the axis puts it in stable equilibrium 

 parallel to the field; we have then, in numerical strength, k<k. 



Iceland spar* often seems magnetic, but it only possesses this 

 property when it contains traces of carbonate of iron. In the 

 pure state it is diamagnetic, and, contrary to what is the case for 

 bismuth, the axis of crystallisation is in equilibrium perpendicularly 

 to the field that is when k>k'. 



When the three coefficients , /', and k" are unequal, their 

 difference in pairs may be determined by means of the couples 

 D, D', and D" in respect of. the same direction along which the 

 principal axes have been successively placed. 



The number of oscillations , ', and ", for instance, observed in 

 the three cases, will give, assuming k>k'>k", 







k' - k" _k - k" _k - k' _7T 2 A/>2 



~ 2 ~ ' 2 := ~^2~ = H 2 



The equation of condition which follows from this has been 

 verified by Pliicker with formiate of copper^ which crystallizes in 

 the klinorhombic system. 



1228. ELLIPSOID OF INDUCTION. The magnetic properties of 

 anisotropic bodies may be represented by an ellipsoid* analogous 

 to the ellipsoid of polarization, by which Fresnell represented the 

 double refraction of light ; we thus find correlated properties in 

 the two orders of phenomena. 



* FARADAY. Experimental Researches, xxn. Series. 1848. 



