PARALLELISM OF EDGES. 395 



Fig. 363. 



Mr. Monteiro had undertaken to describe a new 

 crystal of carbonate of lime, as exhibited in fig. 363, 

 the planes o of which were so imperfect, .that they 

 could not be subjected to the goniometer, and con- 

 sequently the law of decrement by. which they were 

 produced, could not be determined by the ordinary 

 methods. Two parallelisms of their edges, however, 

 enabled Mr. M. to determine that law geometrically, 

 without knowing the inclination to any other plane, 

 of the secondary planes in question. 



The planes /, /', were found to correspond with 

 mod. / of the rhomboid, and were observed to be stri- 

 ated, as those planes frequently are, in the direction 

 of their oblique diagonals; this direction being pa- 

 rallel to the superior edges of the primary rhomboid. 



The position of the planes /, and /', relatively to the 

 edge of the primary form being thus known, it was 

 readily perceived that the plane e corresponded with 

 mod. e of the same rhomboid, and the planes o, with 

 some particular modification belonging to class o. 



SD 



