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is, confufedly heaped upon each other lU 

 their formation : that, lafily, though many 

 of them may compare in clearnefs with the 

 pureft rock cryftal, they are yet inferior in, 

 hardnelis, as they with difficulty cut glafs. 



I at firfl fufpeded that thefe zeoUtes were 

 a fimple modification of the fifth fpecies, 

 which, wherever it had a free fpace, had 

 formed itfelf into thefe brilliant cryflals, 

 either ifolated or aggregate. But this con- 

 jecture was not confirmed by obfervation. 

 It frequently happens that the white minute 

 fpheres which form the fifth fpecies, occupy 

 enly one half, or even lefs, of the contain- 

 ing cavities, without ever taking the form 

 of the fixth fpecies ; but it is conflantly to 

 be obferved that the tetrahedral prifms pro- 

 jed: farther beyond the convexity of the 

 fpheres, and have a greater tranfparency. 

 They muft therefore be confidered as two 

 diflindt fpecies. 



» 



This difference is filll more confirmed by 

 the action of fire and acids. The latter do 

 not a<^, at lead fenfibly, on the fixth fpecies, 



though 



