114 GYPSUM. 



powder, which, after being mixed with a little water, be- 

 comes on drying, hard and compact. This ground gypsum 

 is plaster of Paris, and is used for taking casts, making 

 models, and for giving a hard finish to walls. Alabaster u 

 cut into vases and various ornaments, statues, &c. It owes 

 its beauty for this purpose to its snowy whiteness, translu- 

 cency and fine texture. It is moreover so soft as to be cut 

 or carved with common cutting instruments. Gypsum is 

 ground up and used for improving soils. 



anhydrite. — Anhydrous Sulphate of Lime. 

 Trimetric. In rectangular prisms, cleaving easily in three 

 directions, and readily breaking into 

 square blocks. The figure is a side view 

 of a crystal; M : a = 124° 10 ; M : a = 

 153° 50 ; M : e = 135° 35'. Occurs 

 also fibrous and. lamellar, often contort- 

 ed ; also coarse and fine granular and 

 compact. 



Color white or tinged with gray, red, or blue. Luster 

 more or less pearly. Transparent to subtranslucent. H = 

 2-5—3-5. Gr=2-9— 3. 



The crystallized varieties have been called muriacite. 

 Vulpinite is a siliceous variety containing 8 per cent, of 

 silex, and a little above the usual hardness, (3*5.) 



Composition : lime 41 2, sulphuric acid 58*8. It is a sul- 

 phate of lime like gypsum, but differs in containing no water. 

 Whitens before the blowpipe, but does not exfoliate like 

 gypsum, and finally with some difficulty becomes covered 

 with a friable enamel. No action with acids. 



Dif. Differs from gypsum in being harder and not ex- 

 foliating when heated ; from carbonate of lime and the 

 zeolites which it sometimes resembles, in the non-action of 

 acids, and its action before the blowpipe. Its square forms of 

 crystallization and cleavage are also good distinguishing 

 characters. 



Obs. A fine blue crystallized anhydrite occurs with gyp- 

 sum and calcareous spar in a black limestone at Lock- 

 port. Foreign localities are at the salt mines of Bex in Swit- 



What is plaster of Paris, and how is it used ? For what is alabastel 

 used? How is gypsum employed in agriculture 1 How does anhydrite 

 differ in composition from gypsum ? Mention o'her distinguishing 

 characters. 



