Gypsum 



Shells. 



Shells of dead marine animals, brought up by the 

 rising tide and left, can be usefully employed if they 

 are crushed very finely. In some places there are 

 depots of shells which can be converted into lime. 

 They contain 95 per cent, of carbonate of calcium. 



Gypsum. 



Sulphate of calcium is found in two different 

 forms. 



1st. White, crystallised, without water in 

 crystallisation. This is " anhydrite " [CaS04], 



2nd. White or colourless, crystallised with two 

 molecules of water under crystallisation. This is 

 gypsum, CaS04+2H20. In the mass it forms 

 alabaster, which can easily be distinguished from 

 calcide because it can be scratched with a nail. 

 Gypsum is soluble in 450 times its volume of water. 

 It is the presence of gypsum which makes the water 

 of Paris cloudy, when it comes from the main, and 

 makes it unsuitable for consumption. 



Plaster of Paris is CaSO^, obtained by burning 

 gypsum, which, during the operation, loses its 

 2 molecules of water. When using equal quantities 

 of sulphate of calcium either gypsum or plaster can 

 be employed. Burned plaster when pure contains 

 90-92 per cent. CaSO^ and 8-10 per cent, water ; 

 gypsum plaster when pure contains 79 per cent, 

 sulphate of Ca and 21 per cent, water. Generally 

 it contains some impurities, when of course these 

 percentages will be reduced. 



It is found in many places all over the world, 

 especially in the United States, in France near Paris, 



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