182 On the Deportment qf' Crystalline Bodies. 



amount of the magnetic force in their various directions. On 

 this assumption the equatorial position is readily explained ; 

 and the necessity of the parallelogram, when hung edgeways, 

 to set itself axial, is also manifest. 



As may be expected, when the parallelogram is made very 

 long in comparison to its width, the long diameter of our hy- 

 pothetical egg is overpowered by the united action of a number 

 of short ones, and the oblong stands axial. 



We have succeeded in obtaining analogous results with 

 ivory, which, though diamagnetic, can be so cut that it stands 

 almost axial. The anomaly is explained by reference to the 

 structure of the tooth, which modifies, in certain directions, 

 the diamagnetic power. By attending to these circumstances, 

 we have been able, with these two substances, gutta percha 

 and ivory, to imitate almost all the experiments which we have 

 made with both classes of crystals. 



If we suppose the shorter diameter of an ellipse to coincide 

 with the straight line formed by the intersection of any two 

 surfaces of cleavage, and the ellipse to rotate around this dia- 

 meter, an oblate spheroid will be the result. Conceive lines 

 drawn through the centre of this figure and terminated by the 

 surface, to represent the amount of magnetic or diamagnetic 

 force in the direction of these lines, and we have an hypothesis 

 of magnetic or diamagnetic action within the crystal, sufficient, 

 not only to account for every fact noticed in this paper, but 

 for numerous^ others, the discussion of which we refer to a 

 future occasion. 



Extending this principle to the intersections of the three 

 surfaces of cleavage, we obtain a resultant which falls in the 

 direction of the principal axis of the crystal, or of the optical 

 axis. The position of that resultant between the poles will 

 depend solely upon the magnetism or diamagnetism of the 

 crystal, and in nowise upon the fact of its being negative or 

 positive, as asserted by Professor Pliicker. 



It is highly improbable that our representative spheroid 

 will be of a constant shape in all crystals : in the case of gutta 

 percha we assumed it formed by the rotation of a semi-ellipse 

 round its longer axis, or what is commonly called prolate ; in 

 the case of Iceland spar it is oblate ; in common iron it would 

 be a sphere, as here the magnetic force appears to act equally 

 in all directions. Every crystal will doubtless modify it in a 

 manner peculiar to its own substance and structure. Future 

 experiments will perhaps enable us, in many cases, to deter- 

 mine the numerical values of the long and short diameters of 

 these spheroids. 



From these considerations it would follow, that M. PlUcker> 



