CLASSIFICATION. 363 



the isothermal surface is an ellipsoid with three unequal 



In the remaining three classes, called the mono-clinic, 

 diclinic, and triclinic, the axes are more or less oblique, 

 as described above (vol. ii. p. 360), and at the same time 

 unequal. The complication of phenomena is therefore 

 greatly increased, and it need only be stated that there 

 are always two directions in which a ray is singly re- 

 fracted, but that in all other directions double refraction 

 takes place. The conduction of heat is unequal in all 

 directions, the isothermal surface being an ellipsoid of 

 three unequal axes. The relations of such crystals to other 

 phenomena are often very complicated, and hardly yet 

 reduced to law. Thus some crystals, called pyro-electric, 

 manifest vitreous electricity at some points of their sur- 

 face, and resinous electricity at other points when rising 

 in temperature, the character of the electricity being 

 changed when the temperature sinks again. This pro- 

 duction of electricity is believed indeed to be connected 

 with the hemihedral character of the crystals exhibiting 

 it. The crystalline structure of a substance again influ- 

 ences its magnetic behaviour, the general law being that 

 the direction in which the molecules of a crystal are most 

 closely approximated tends to place itself axially or equa- 

 torially between the poles of a magnet, according as the 

 body is magnetic or diamagnetic. Further questions arise 

 if we apply pressure to crystals. Thus doubly refracting 

 crystals with one principal axis acquire two axes when 

 the pressure is perpendicular in direction to the principal 

 axis. 



All the phenomena peculiar to crystalline bodies are 

 thus closely correlated with the formation of the crystal, 

 or will almost certainly be found to be so as investigation 

 proceeds. It is upon empirical observation indeed that 

 the laws of connexion are in the first place founded, but 



