CALCIUM AND ITS COMPOUNDS 387 



the action shown in the equation, and for a considerable amount of 

 useful ventilation through walls of the building. 



Calcium Sulphate CaSO 4 , Various Forms. Calcium sul- 

 phate is a very common mineral. It occurs, as anhydrite CaS0 4 , 

 in salt deposits. Gypsum CaSO 4 ,2H 2 is found in masses, and 

 also in single crystals (selenite, Fig. 42, p. 94). Alabaster is 

 highly crystalline gypsum, tinted by impurities. 



Gypsum CaS0 4 ,2H 2 is the commonest form, and is the one 

 produced when calcium sulphate is precipitated. It is white 

 and much softer than calcite. It is only slightly soluble in water 

 (1 : 500 at 18). It is used as a fertilizer and in making plaster 

 of Paris and is the chief component of blackboard crayon or 

 " chalk." 



When gypsum is heated, the vapor pressure of the water it 

 gives off soon exceeds that of the moisture in the atmosphere, 

 and the compound begins to decompose: 



2[CaS0 4 ,2H 2 0] * (CaSO 4 ) 2 ,H 2 + 3H 2 O f . 



The hemi-hydrate which remains (plaster of Paris) gives a much 

 lower pressure of water vapor and is more stable. Plaster of 

 Paris is manufactured in large quantities by heating gypsum in 

 kilns. When moistened with water, it sets in about half an hour 

 to a solid mass of gypsum. The temperature used in making it 

 must not exceed 125, otherwise the hemi-hydrate is itself de- 

 composed, the plaster is " dead burnt," and it no longer sets 

 readily. The setting involves, simply, the reversal of the equation 

 given above. 



Plaster of Paris swells somewhat, in setting, and so fills out 

 completely every detail of a mould and applies itself closely to 

 the outline of an object on which it is spread. It is used in mak- 

 ing casts, and in surgical bandages where movable parts are to be 

 held rigidly in place. ^Stucco is made with sizing or glue instead 

 of pure water^j 



