CO PHYS10GJIAPHY. CLASS II. 



Pr 



pendicular, face of composition parallel to -f _. 



" 



(According to this law are formed the arrow-shap- 

 ed twins, consisting of lenticular crystals). Globu- 

 lar masses, generally formed of discernible indivi- 

 duals. Dentiform. Massive : composition granular, 

 passing into impalpable, sometimes scaly ; also co- 

 lumnar, often as thin as hair, long and generally 

 straight and parallel. Sometimes without cohesion 

 of the single particles in the state of powder. 



OBSERVATIONS. 



1. The forms, as given above, depend upon the deter- 

 minations of HAUY, and the subsequent labours of Mr 

 SORET on the crystallisations of this substance, attention 

 being given to the inclination of the axis. The connexion 

 among them appears more simple and conformable, to nature, 

 than in any of the representations that have hitherto been 

 given of the same subject ; yet the more accurate deter- 

 minations of the angles by means* of the reflective gonio- 

 meter have not yet been introduced ; and according to 

 these, it will be necessary in future to correct the angles. 

 According to PHILLIPS, the angle of P + os is = 111 20'. 



Within the species of prismatic Gypsum-haloide, two 

 species, Fraueneis or Sclenite, and Gypsum, used to be for- 

 merly distinguished, without, however, allowing deter- 

 mined limits to be indicated between them, and which on 

 that account, though not from reasons of Natural History, 

 have been again united by most mineralogists. Such va- 

 rieties as are pure, transparent, and, in short, most perfect- 

 ly formed, were comprehended within the first, less per- 

 fectly formed varieties within the second of the above- 

 mentioned species. The latter, or Gypsum, was divided 

 into several sub-species, comprehending almost exclusively 

 compound varieties pretty easily distinguishable from each 



