78 SYMMETRY. 



are endued with a similar property, appear equally 

 subject to a similar interference with the general law 

 of symmetry. 



But other substances which are not pyro-electric, 

 as, for example, iron-pyrites, afford instances of de- 

 viation from that kind of symmetrical modification 

 by which the cube of galena is affected. Pyro-elec- 

 tricity is not therefore the only disturbing cause which 

 influences the deviation from the general law of sym- 

 metry. 



With all the known exceptions however to this 

 law, there are still so many substances influenced by 

 it, that the character it confers on crystals is generally 

 serviceable for determining the class of primary form 

 to which any secondary crystal belongs. 



