of the Genus Gypsum-haloide. 305 



globules, the specific gravity of which is = 2.64.* The lower 

 specific gravity might perhaps be accounted for by the delicacy 

 of the crystalline groups employed. In other respects, the 

 opinion that the variety described above, and the acicular 

 globules of pharmacolite, belong to the same species, is ground- 

 ed solely on the analogy of resemblance existing between the 

 former with crystals of gypsum, and between the latter and 

 the radiated groups so frequently observed in the same spe- 

 cies. That they both contain arsenic acid should not enter at 

 all into this comparison, as long as the species are not perfect- 

 ly established, though it was an experiment, proving this 

 substance to form one of the constituents of the hemi-prisma- 

 tic crystals, which suggested to me the propriety of comparing 

 them with the pharmacolite. However slight, therefore, the 

 reasons may be in themselves for uniting the two substances, 

 they are sufficiently strong to prevent us from establishing them 

 both as distinct species, as long as we are so much in want of 

 accurate information with respect to one of the varieties. 



The decomposed crystals, said to be " selenite already changed 

 into plaster •," do not, in fact, belong to, nor are they derivable 

 from the preceding species. They are white, opaque, and dull, 

 and cannot bear the slightest touch without crumbling into 

 pieces, like laumonite. From what I could collect in observ- 

 ing several crystals, most of them half fractured, their form 

 belongs to the prismatic system, and nearly resembles Fig. 8. 

 These also give a sublimate of arsenic, when mixed with 

 charcoal, and exposed in a glass tube to the heat of the spirit- 

 lamp. It is probable that, previous to their decomposition by 

 the loss of water, they have belonged to a distinct species, 

 which it would be very interesting to discover in nature. 



The " quarz" is nothing else but the second one of the 

 two species described in the beginning of this paper, the dia- 

 tomous gypsum-haloide. It forms crystalline coats, of anear- 



* Klaproth's Essays, Trmisl. vol. ii. p. 220. Klaproth says, " Its spe- 

 cific gravity, in the botryoidally aggregated crystals, I found to be =2.640. 

 Mr Selb, who probably weighed for the same purpose single or detached 

 crystals, states its specific gravity only at 2.530." Fence we may infer, 

 that the pharmacolite from Wittichcn, the variety analyzed by Klaproth, 

 sometimes occurs in crystals. 



VOL. III. NO. II. OCTOBER 1825. V 



