230 CHEMISTRY. 



idea of the size of these formations, the accumulations of 

 constant dripping for ages, by the human figure at the foot 

 of one of them. 



318. Carbonate of Calcium in the Sea. Though rain-water 

 may be free from carbonate of lime, water which has per- 

 colated through the earth is never wholly free from it. 

 Though it may deposit it as it comes out of the springs 

 and runs along brooks and rivers to the ocean, yet even 

 when, it arrives there it retains some of it in solution, for 

 it has still dissolved in it some of the carbonic acid which 

 it derived from the soil. If it were not so, the shell-fish 

 would have no material for the formation of their external 

 skeletons, or houses, as they may more properly be called. 



319. Sulphate of Calcium, CaSO 4 . The common name of 

 this salt is gypsum. It has also the name of plaster of Paris, 

 which it received from the fact that it was first used in the 

 form of plaster in Paris, there being immense quantities of 

 it in the neighborhood of that city. It is a white and quite 

 soft mineral, occurring in various forms, some of them very 

 beautiful. One of its forms, alabaster, which is snowy 

 white, is cut into vases and ornaments of various kinds. 

 Sometimes it is crystallized in exceedingly thin leaves, laid 

 together so nicely that a multitude of them make a white 

 crystal clearer than the clearest glass. Then there is the 

 satin-spar, so called from the splendid lustre of its fibrous 

 arrangement. Gypsum is about one fifth water. This wa- 

 ter can be driven off by heat, and then this powdered an- 

 hydrous gypsum has the property of "setting" with wa- 

 ter; or, in other words, becoming with water a firm, coher- 

 ent, and dry mass. For this purpose it is moistened with 

 water to about the consistency of cream. In this state it 

 can be poured into moulds, or it can be put upon walls as 

 hard finish, the water disappearing as it hardens, partly by 

 evaporation, and partly by becoming a part of the solid, 



