56 Gypjum or Selmlte. [Book VI. 



calcareous earth, and conftitmes a fubftance which has 

 very little folubility. This is GYPSUM, felenite, or 

 plaifter of Paris, which exifts in confiderable quantities 

 in nature. Gypfums are found in folid mafles, very 

 foft, and eafily fcraped with a knife. They are dif- 

 tinguiihed from the combinations of calcareous earth 

 with carbonic acid, by not effervefcing with acids j 

 and from other earthy bodies, by being by heat 

 changed into a white powder, which when mixed with 

 a large proportion of water, fuddehly concretes into a 

 ftotiy mafs. They are moft commonly found in the 

 ftrata of clay, fometimes in thofe of fand, under the 

 appearance of a whitifh coloured mafs, but the fmaH 

 pieces 'are tranfparent, and fometimes have a red 

 tinge. They are often compofed of fmall fhining par- 

 ticles, like the grains of fugar, and when under this 

 appearance they are particularly called gypfum. When 

 hard, fo as to admit of being cut into toys and figures 

 they are called alabafter. The fecond form under 

 which they are found, is that of a fibrous ftruclure 

 of oblique cryfbils, which are parallel to themfelves, 

 but which crofs the mafs from the upper to the under 

 furface. In this ftace they are called tibrarias or fibrous 

 ftone by Dr. Hill. Thcfe differ much in fize and 

 regularity of concretion. The third fpecies is com- 

 pofed of clear t-ranfparent plates, like glafs, in clofe 

 contact with each other, but which may be feparated. 

 They fplit with a knife into fine plates, very flexible, 

 though they cannot be bent without producing flaws. 

 They are called glacies marina, and fometimes Muf- 

 covy glafs, but improperly, as that is a different fub- 

 ftance. 



The fourth fpecies is in the form of ffparate oblong 

 cryftals, which are called felenites ;>jome are long and 

 regular, like cryftals of fait. The firth fpecies of gyp- 



