32 On the Magneto-optic Properties of Crystals, 



regard to its axis. There is no difference in construction, 

 save in the superior workmanship of nature, and there is no 

 difference at all as regards deportment. Surely these consi- 

 derations suggest a common origin for the phenomena exhi- 

 bited by both. 



We have the same action in the case of the compressed 

 dough, formed from the powdered carbonate of iron and bis- 

 muth. A plate of the former, three-quarters of an inch 

 square and one-tenth of an inch in thickness, stands between 

 the conical poles, brought within an inch of each other, ex- 

 actly axial ; between the same poles, two inches apart, it 

 stands equatorial. A plate of compressed bismuth dough 

 stands, between the near points, equatorial, between the di- 

 stant points, axial. 



Any hypothesis which solves these experiments must em- 

 brace crystalline action also ; for the results are not to be 

 distinguished from each other. But in the above cases an 

 optic action is out of the question. With the similarity of 

 structure between beryl and the sand-paper model, above 

 described, — with the complete identity of action which they 

 exhibit, before us ; is it necessary, in explanation of that ac- 

 tion, to assume the existence of a force which, in the case of 

 the crystal, is all but inconceivable, and in the case of the 

 model is not to be thought of? In his able strictures on the 

 theory of M. Becquerel*, M . Pliicker himself affirms, that 

 we have no example of a force which is not associated with 

 ponderable matter. If this be the case as regards the optic 

 axis force, if the attraction and repulsion attributed to it be 

 actually exerted on the mass of the crystal, how is it to be 

 distinguished from magnetism or diamagnetism ? The as- 

 sumption of Mr. Faraday appears to be the only refuge here; 

 the abandonment of attraction and repulsion altogether. 



In the first section of this memoir it has been proved, by 

 the production of numerous exceptions, that the law of M. 

 Pliicker, as newly revised, is untenable. It has also there 

 been shown, that the experiments upon which Mr. Faraday 

 grounds his hypothesis of a purely directive force, are refer- 

 able to quite another cause. In the second section an attempt 

 has been made to connect this cause with crystalline structure, 

 and to prove its sufficiency to produce the particular pheno- 

 mena exhibited by crystals. In the third section we find the 

 principle entering into the most complicated instances of these 

 phenomena, and reducing them to cases of extreme simpli- 

 city. The choice therefore rests between the assumption of 

 three new forces which seem but lamely to execute their mis- 

 * Poggendorff's Annakfi, vol. lxxyii. p. 57S, 



