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NEW YORK STATE MUSEUM 



When heated moderately in a closed tube, the water of crystal- 

 lization is given off and the fragments of plaster, when water is- 

 added, have the power of recomhining with the water in such a 

 way as to form the original compound. If heated to a tempera- 

 ture of 343°C.; however, gj^psum is converted into anhydrite 

 (CaSO^) which will not harden or set with water. 



The principal varieties of gypsum are, 



1 (Selenite, in transparent plates or crystals 



2 Fibrous gypsum or satin spar 



3 Alabaster, a compact white gypsum used extensively for orna- 

 ments 



4 Rock gypsum, a dull variety containing more or less im- 

 purities 



5 Earth}' gypsum, or plaster earth. 



When pulverized, all these forms become a white or gray 

 powder. Its specific gravity varies from 2.314 to 2.328. It is 

 sparingly soluble in water, as shown by the following table by 

 Marignac.^ 



One part of gypsum 

 Temperature dissolves in 



0°O 415 parts of water 



18°0 386 



24^0 378 



32°C 371 



38°C 368 



41°C 370 



53°C 375 



72°C 391 



86°C 417 



100°C 452 



It occurs ver}^ abundantly both in a solid state and in solution. 

 As plaster rock, it occurs in large masses and is usually found 



^Annales de Chimie, Paris. Ser. 5. 1:274-81. Quoted by Grimsley^ 

 Special Report on Kansas Gypsum. 



