473 



Tf we keep in itiiiul that for other substances the nionients of 

 inertia about different axes possibly do not differ from each other 

 to such a degree as this seems to be the case with ferromagnetic 

 substances, and that with different ratios of the moments of inertia 

 very different cases can occur for paramagnetic substances also, and 

 if in addition we take into consideration the possibility that according 

 to § 6 there exist substances which possess a molecular field, but 

 not a CuRiE-point, there seems room for the fnrtiier interpretation of 

 different cases met witli by du Bois, Honda, and Owen ^) in their 

 investigations regarding the susceptibility of elements, and which 

 carmot according to Oosterhuis be represented with the assumption 

 of a constant magnetic moment without molecular field. 



APPENDIX. 



The following proof of equation (18) of this Communication I 

 was kindly communicated to me by Prof. Lorentz. 



In this appendix the references to sections and equations relate 

 to the article V 14, "Elektronentheorie", in the Math. Encyklopadie. 

 Some notations in this appendix differ from those used in the fore- 

 going communication. 



In considering the energy of a magnetized body one can start 

 from two different points of view, viz. one can assume: 



A. that the molecular magnets really are permanent magnets 

 (hence that the magnetic moment is not due to rotations or to the 

 circulation of electric charges), or 



B. as in § 486^, that the magnetic moment is due to a rotation 

 or to the circulation of electric charges round a definite axis in 

 the molecule. 



In both cases the magnetic energy may be put equal to - IH'dS, 



the integration being taken over ttie whole space, including the space 

 within the magnets. H is not the same for the hypotheses A and B. 

 On the assumption A we have to imagine "magnetism" distributed 

 over the magnets, and to calculate the magnetic force due to this 

 magnetism. To this eventually the magnetic force due to electric 

 currents is to be added. On the assumption B we have to deal with 

 the force H, which, according to the equations of the electron theory, 

 is due to the circulating electricity. 



1) H. DU Bois and K. Honda. Proceedings. Jan. 1910, p. 547. K. Honda. 

 Ann. d. Phys. (4) 32 (1910), p. 1027. Science Reports Tohoku University Sendai 

 1 (1912), p. 1. M. Owen. Proceedings. Dec. 1911, p. 637. Ann. d. Phys. (4) 37 

 (1912), p. 657. 



