44;8 Prof. Pliicker and Dr. Beer on the Magnetic Axes of Crystals, 



sequently also with the distance from thera^ than tlie diamagnetic 

 induction, is readily explained; a law whose validity receives 

 manifold support from the experiments detailed further on. 



4. With regard to the modification of magnetic and diamag- 

 netic induction brought about by crystallization, I must refer 

 back to the question as it stood on the publication of the pro- 

 gramme of this university, August 3, 18-1)9, a complete transla- 

 tion of which has found a place in the Annales de Chimie et de 

 Physique. It is there stated, that in crystals which belong to 

 the tesseral system, the common magnetic and diamagnetic action 

 is not modified; that in crystals with one optic axis, the latter 

 is attracted or repelled according as the crystal is positive or 

 negative, and that in crystals possessing two optic axes, the 

 attraction or repiilsion extends to both. Not without caution 

 was the last assertion written down. I was quite conscious that 

 a foreign admixture of iron might have been present in some of 

 the experiments from which I had made deductions ; and again, 

 a theoretic difiiculty presented itself in the fact, that the division 

 of crystals with two axes into positive and negative is not an 

 essential one, the diff"erence disappeai'ing in those crystals whose 

 axes, as in the case of sulphate of iron, are at right angles to 

 each other. In the place of the former hypothesis I then tried 

 to substitute another, that an action on FresnePs three axes of 

 elasticity takes place. I will here cite the words of the pro- 

 gramme itself: — 



" Rejecta aliquamdiu hypothesi a nobis de actione magnetis 

 in axes opticos posita (namque si invenire velis, ne anxius inter- 

 pretationem jam a te inventa explicantem retineas, vide) nobis 

 problema proposuimus annon, pro ilia hypothesi, phsenomena 

 explicari possent e sola cetheris in crystallis distributione a Fresnel 

 exposita. Etenim ilia linea media in crystallis positivis, in quibus 

 attrahitur, minimae, atqne in negativis, in quibus repellitur, 

 maximge elasticitatis est axis. Coi-jjus fingamus inter polos ita 

 suspensimi, ut circum gravitatis centrum ei libere rotandi data 

 sit facultas, atque circum idem centrum notam illam ellipso'i- 

 dicam superficiem descriptam, cujus radii vectores elasticitatum 

 quadratis sunt jequi. Nonne, fortasse quseris, tale corpus ita diri- 

 gitur ut minimus ellipsoidis axis cum linea concidat polos conjun- 

 gente ? Nonne, si ciixum verticalem tantum axem torqueri 

 potest, eum situm petit, quo elasticitatis minoris directiones ante 

 ceteras polls adverse sunt . . . . ? Itaque hsec hypothesis est 

 relinquenda." (P. 21.) 



Further experience since that time pronounces it a vain attempt 

 to endeavour to refer the difference of magnetic action in different 

 directions through the crystal to the different density of the 

 aether in these directions. I felt at that time the necessity of 

 undertaking the former observations anew, and of applying all 



