2gd Theory of Crvjlallifat'wn. 



fjdering that AB is a horizontal line taken on the upper bafe 

 of the nucleus, bazr the fe&ion of the firft lamina of 

 fuperpofition, gfe n that of the fecond, &c. I call mixed 

 decrements thofe which exhibit this new kind of exception 

 from the fnnpleft laws. 



Thefe decrements, as well as the intermediary ones, rarely 

 exift any where elfe, and it is particularly in certain metallic 

 fubftances that I have difcovered them. Having tried to ap- 

 ply the ordinary laws to a variety of thefe fubftances, 1 lound 

 fo great errors in the value of the angles, that I at firft 

 believed they were inconliftent with theory. But after I 

 had conceived the idea of giving to this theory the extent of 

 which I have jttft fpoken 1 arrived at refults fo correct, that 

 I no longer entertained any doubt of the exiftence of the 

 laws on which thefe refults depend. 



RefeFtions on the preceding Refults. 



All the rretamorphofes to which cryflals are fubjected 

 depend on thofe laws of ftrufture juft explained, and others 

 of the like kind. Sometimes the decrements take place at 

 the fame time on all the edges ; as in the dodecaedron hav- 

 ing rhombufes for its planes, as before mentioned ; or on all 

 the angles, as in the o£taetiron originating from a cube. 

 Sometimes they take place only on certain edges or certain 

 angles. Sometimes there is an uniformity between them, fo 

 that it is one fingle law by one, two, three ranges, &c. which 

 a£ls on the different edges, or the different angles ; as is ob- 

 ferved in the two folids of which I fhall fpeak hereafter. 

 Sometimes the law varies from one edge to the other, or 

 from one angle to the ether; and this happens above all 

 when the nucleus has not a fymmetrical form ; for example, 

 when it is a parallelopipedon, the faces of which differ by 

 their refpettive inclinations, or by the meafure of their 

 angles. In certain cafes the decrements on the edges con- 

 cur with the decrements on the angles to produce the fame 

 cryftalline form. It happens aifo fometimes that the fame 



edge. 



