48 



TERMINOLOGY. 



tered. In the second, we see a Cube whose 

 edges are indeed all similarly replaced, but 

 only the half of whose angles are thus af- 

 fected ; viz. one of the two which are op- 

 posite each other, at the extremities of the 

 same axis, Fi.g. 115.* 



In addition to the two cases just men- 

 tioned, it is necessary to add, that in sev- 

 eral other instances crystals are met with, 

 where the modifications have not taken place at once upon all the 

 edges, or all the angles, whose position with regard to each other is 

 identical. But it is noticeable that such exceptions to the law of 

 symmetry are by no means constant, or subject to any general rule. 

 And, besides, it is rarely difficult to find other crystals of the same 

 species, in which all the faces required by symmetry are present ; 

 a fact which tends strongly to justify the opinion that these devia- 

 tions are due to accidental causes, and therefore insufficient to form 

 an objection against the principle of agreement between the symme- 

 try of modifications and that of the structure of the primary forms. 



.51. PASSAGE OF ONE FORM INTO ANOTHER. 



The proportions existing between the extent of the pri- 

 mary and secondary planes are very variable. Some- 

 times, the change effected by the modification is so small 

 as scarcely to be perceptible; at others, the new planes 

 are equally extended with the primary ; and, again, they 

 are produced so as wholly to obliterate the original faces, 

 and thus to give origin to new forms. 



* The exceptions in the instances mentioned above were supposed by 

 the Abbe Hatty to be dependent upon electricity. These crystals are 

 electric by heat, and give two kinds of electricity in two opposite points. 

 From whence it is imagined that this anomaly of form is a result of elec- 

 tricity, and the conjecture appears to be strengthened by the observation 

 that among the crystals of Sphene, there are those which are electric 

 and those which are non-electric; in the first case, the two summits are 

 different, in the last, they are similar. 



