ORDER X. PllISMATIC ARSENICAL-PYRITES. 451 



tinued beyond the face of composition. &. Face 

 of composition parallel, axis of revolution perpen- 

 dicular to a face of P -f oo ; the composition often 

 takes place parallel to both faces, or is repeated in 

 parallel layers. Massive: composition columnar, 

 individuals of various sizes, generally straight and 

 divergent or irregular. The faces of composition 

 are irregularly streaked. Individuals joined m a 

 granular composition are often very small, or even 

 impalpable, and strongly connected ; their fracture 

 is uneven. 



OBSERVATIONS. 



1. Two subspecies, the common Arsenical pyrites and the 

 argentiferous Arsenical pyrites, have been distinguished 

 among the varieties of the present species, which, how- 

 ever, differ chiefly in a small proportion of silver which the 

 latter contains, and which has probably been the only rea- 

 son of this distinction having been introduced. The latter 

 contains only a few varieties in small imbedded acicular 

 crystals and massive nodules, while all the rest rank under 

 the head of common Arsenical pyrites. 



2. According to STROMEYER, the prismatic Arsenical- 

 pyrites consists of 



Iron 36-04. 



Arsenic 42-88. 



Sulphur 21-08. 



It is considered by BERZELIUS to be Fe As 2 -f Fe S 4 , 

 which corresponds to 33-5 of iron, 46-5 of arsenic, and 20-0 

 of sulphur. Before the blowpipe upon charcoal it emits 

 copious arsenical fumes, and melts into a globule, which is 

 nearly pure sulphuret of iron. It is soluble in nitric acid, 

 with the exception of a whitish residue. 



3. Prismatic Arsenical-pyrites is a pretty common mine- 

 ral both in beds and veins. It is accompanied by ores of 



