THE 

 LONDON, EDINBURGH axd DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



MARCH 1851. 



XXIV. On the Theoi-y of Magnetic Induction in Crystalline and 

 ■ Non-crijstalline Substances. By William Thomson, Professor 

 of Natural Philosophy in the Univei'sity of Glasgow^. 



POISSON, in his mathematical theory of magnetic induction, 

 founded on the hypothesis of " magnetic fluids " moveable 

 within the infinitely small " magnetic elements " of which he 

 assumes magnetizable matter to be constituted, does not overlook 

 the possibility of these magnetic elements being non-spherical 

 and symmetrically arranged in crystalline matter; and he re- 

 marks, that a finite spherical portion of such a substance would, 

 when in the neighbourhood of a magnet, act differently according 

 to the different positions into which it might be tm-ned with its 

 centre held fixed. But " such a circumstance not having yet 

 been observed t>" he excludes the consideration of the structm-e 

 which would lead to it from his researches, and confines himself 

 in his theoi-y of magnetic induction to the case of matter, consist- 

 ing either of spherical magnetic elements, or of non-symmetii- 

 cally disposed elements of any forms. It is easy to conceive the 

 modification which he would have introduced into his formulae 

 to make them applicable to a crystalline structure such as he 

 describes ; but, so far as I am aware, no writer has hitherto 

 attempted to make this extension of Poisson's mathematical 

 theoi-y of magnetic induction. Now, however, when a recent 

 di-scovery of Pliickcr's has established the very circumstance, the 

 observation of which was wanting to induce Poisson to enter 

 upon a full treatment of the subject, the importance of workmg 



* Communicated by the Author, having been read before the British 

 Association, August 1850. 



t " Memoire sur leMagnetisme enMouvement." {Mem. de l'Institut,\S23, 

 vol. vi. Paris, 1827.) l''or quotations from this and the two preceding 

 memoirs of Poisson, showing his theoretical anticipation of the discovery 

 of magnecrystaUic action, see the AjjpencUx to this article. 



Phil. Mag. S. 4-. Vol. 1. No. 3. March 1851. N 



