COLLECTION OF MINERALS. 280 



which ornament its crystals. Amongst the re- 

 gular forms it most commonly assumes, we ob- 

 serve that of the cube exactly traced in a great 

 number of specimens. The most beautiful groups 

 in the collection are from Derbyshire and Nor- 

 thumberland, and were presented by M. Heuland. 

 On the upper shelves is a specimen of the concre- 

 tionnee variety, which is formed of bands and 

 zones like calcareous alabaster : in England it is 

 cut into slabs and cups of different forms. The 

 acid contained in this substance is also used for 

 engraving on glass, on account of the corrosive 

 property it possesses. 



The sulfate of lime, which we see in the 

 ninth case, is the mineral commonly called gyp- 

 sum, or plaster-stone. In fact, plaster is only a 

 mixture of this substance with carbonate of lime : 

 this mixture sometimes exists in nature, as at 

 Montmartre, which is almost entirely composed 

 of gypsum. The lenticular variety, so called on 

 account of its rounded form, is usually found 

 there ; on the lower shelves may be seen many 

 beautiful specimens of it. The thin and trans- 

 parent plates, which are detached from these 

 crystals, are vulgarly called specular stones, and 

 asses' looking-glasses. The ancients used them 

 instead of window-glass: according to Pliny, 

 the temple of fortune, which was built of this 



