MOLECULAR ARRANGEMENT. 257 



repulsion outweighed the magnetic properties of the 

 crystal.* 



The relation of structure to physical phenomena of 

 essentially different characters is remarkable. Savart, 

 when making crystalline plates of quartz and carbonate 

 of lime vibrate, succeeded in determining a relation 

 between the acoustic figures that are produced in them, 

 and the particular mode of the crystallization of the 

 substance. He found that the direction of the optical 

 axis is constantly connected with that of the principal 

 forms of the acoustic figures. 



Mitscherlich has remarked that crystals do not expand 

 uniformly by heat, but that this dilatation is greater in 

 one direction than in another ; and that this difference 

 is connected with their crystalline form. M. de Senar- 

 mont has shown that conductibility for heat, which is 

 equal in all directions for the crystals of the regular 

 system, acquires in others a maximum or a minimum 

 value, according to directions parallel to the crystallo- 

 graphic axes ; so that the isothermic surfaces, which are 

 spheres in the former case, are, in the other, ellipsoids 

 elongated or flattened in the same direction. The optical 

 axes do not altogether coincide with the principal axes 

 of conductibility for heat ; but this appears to be due 

 merely to slight differences in the rate of progression, 

 or the refrangibility of the luminous and calorific rays. 



Wiedmann, by employing a fine point through which 

 he made electricity arrive upon a surface that he had 

 powdered with licopodium or red lead, succeeded in de- 

 termining, by means of the form assumed by this light 

 powder, the conductibility of crystals in different di- 

 rections. 



On a surface of glass, the powder which disperses 

 itself around the points, in consequence of electric re- 



* For a detailed account of the experiments of Faraday, Plucker, 

 Becquerel, Tyndale, and Knoblauch, see De La Rive's Treatise on 

 Ekctjitity in Theory and Practice, 



