

SECTION VI. 



SYMMETRY. 



AMONG the definitions will be found an explana- 

 tion of what is meant by similar edges , angles and 

 planes of a crystal. 



It has been discovered, by the observation of a 

 great number of secondary forms of crystals, that 

 when a modification takes place on any one edge or 

 angle of any primary form, a similar modification 

 generally/ takes place on all the similar edges or 

 angles. And this has been observed to occur so fre- 

 quently, as to induce the conclusion of its being the 

 effect of a general law, which the Abbe Haiiy has 

 called the law of Symmetry. This law however does 

 not act universally with regard to all such similar 

 edges or angles as are included under the definitions 

 already given. The tourmaline will present an in- 

 stance of deviation from this law. The primary form 

 of the tourmaline is a rhomboid, and the three edges 

 terminating in , fig. 36, are similar to the three ter- 

 minating in 6; the six lateral edges, as well as the 

 six lateral solid angles, are also respectively similar. 



Yet it is found that the three edges terminating in 

 , are sometimes truncated, while those terminating 

 in by are not. It is also observed that sometimes only 

 the alternate three of the lateral solid angles are mo- 

 dified, the three others remaining entire; but the 

 tourmaline is pyro-electric, that is, capable of becom- 

 ing electric by heat, and many other substances which 



