SULPHUR AND ITS COMPOUNDS. 37 



60. Gypsum. This is sulphate of lime. When crys- 

 tallized and free from all impurities it has a pearly lus- 

 tre, and the clearness of glass. About a fifth part of it 

 is water. This water is driven off in preparing it for 

 making casts, ornamental work for walls, etc. The man- 

 ner of using it for these purposes is described in 320, 

 Part II. Gypsum is a white and soft mineral, and ap- 

 pears in many forms, some of which are very beautiful. 

 One of these is the satin spar, so called from the splen- 

 did lustre of its delicate fibrous arrangement. Another, 

 called alabaster, which is generally snowy white, being 

 compact, with a fine grain, is cut into vases and orna- 

 ments of various kinds. Sometimes gypsum is composed 

 of exceedingly thin leaves, laid together so evenly that a 

 multitude of them make a crystal clearer than the clear- 

 est glass. The name selenite, which has been given to 

 this and some other varieties of gypsum, comes from se- 

 lene, the Greek word for moon. There is an anhydrous 

 sulphate of lime, that is, one which has no water incor- 

 porated with it, the term coming from two Greek words, 

 an, without, and udor, water. Gypsum occurs abundant- 

 ly in many parts of this country. Fine specimens are 

 found near Lockport, New York. As before noticed ( 

 1 6), in the Mammoth Cave of Kentucky alabaster appears 

 with its crystals arranged in various forms of flowers, 

 branches of shrubbery, vines, etc. 



Common limestone has sometimes been mistaken for 

 gypsum by persons who are ignorant of chemistry and 

 mineralogy. Professor Hitchcock relates a case of this 

 kind. A farmer supposed that he had found gypsum on 

 his farm, and his neighbors, believing it, bought large 

 quantities of the material for agricultural purposes. Aft- 

 er grinding up a considerable amount of it, some one ac- 

 cidentally discovered that it was limestone. The error 

 might have been avoided by simply testing the sub- 

 stance with acid. A drop would have occasioned an ef- 

 fervescence, showing that it was carbonate, and not sul- 



