149 Baron Beust on Twin-Crystals ofPUlUpsite. 



&& the whole quantity I had to operate upon did not exceed 

 220 milligrammes. It is however decidedly inferior to the spe- 

 cific gravity of the harmotorae properly so called. This va- 

 riety occurs in small crystals aggregated in globular shapes, 

 which again form together reniform coatings in the cavities of 

 an amygdaloid. The crystals are very easily separated by 

 the slightest pressure. 



7. The milky opaque crystals from the inside of cavities 

 of an amygdaloid from Vesuvius, 2.190. 



The latter were sometimes considered as harmotome, but 

 more frequently went by the name of abrazite. From the 

 exceedingly minute quantity of the crystals I could detach, 

 this specific gravity, like the preceding one, should again 

 be determined from a more considerable quantity. These 

 crystals, when exposed to the action of the atmosphere, are in- 

 clined to be decomposed, and though they preserve their ex- 

 ternal shape, t1>ey present, when broken across, a fibrous tis- 

 sue, analogous to the crystals, which once were blue copper, 

 but have been changed into malachite. 



So many different results have been obtained by chemists 

 on analyzing the varieties, formerly comprised under the name 

 of harmotome, that an exact indication of their physical pro- 

 perties now becomes imperiously called for, in order to inform 

 us what we should consider as a species, and what as a varie- 

 ty. It appears distinctly that there are differences in the spe- 

 cific gravity, which no doubt correspond to the chemical com- 

 position, in as much as some kinds, besides silica, alumina, 

 and water, contain baryta, others lime, and others lime and 

 potassa. The regular forms, that is to say the angles of the 

 crystals, have not been ascertained in any of them, chiefly 

 owing to the great difficulty of procuring specimens that will 

 allow of a minute examination. This nevertheless is unavoid- 

 ably requisite to fix our opinion on the subject. The ques- 

 tion is one of great importance, as it refers to the influence of 

 the substitution of one isomorphous substance for another in 

 a chemical mixture of the same description, on the determi- 

 nation of the mineralogical species. It is only the actual 

 examination of their physical properties that can decide on the 

 latter, whatever may be the results of chemical analysis. 



