THE GEOLOGY OF CEMENTS 189 
New York; by L. H. Cole (Gypsum in Canada, Can. Dept. 
Mines, 1913, pp. 94-99) for those in British Columbia; 
by G. E. Pilgrim (Mem. &.S. India, 1908, xxxiv, p. 104), and 
by J. V. Harrison (Econ. Geol., 1924, p. 270) for the gypsum 
of Persia, and by Murray Stewart (Rec. G.S. India., 1919, 
1, pp. 63, 67) for that of the Salt Range of the Punjab. 
The setting of gypsum is a delicate process requiring 
careful control of the temperature and amount of water, as 
too much leaves the crystals loose and the plaster friable. 
The water present should be not more than about a thousandth 
of that required to dissolve the material. The temperature 
in burning plaster of Paris should not rise above 260° F., 
at which the dihydrate (CaSO,, 2H,0), is converted into hemi- 
hydrate (2CaSO,, H,0) ; it readily dissolves in water, forming 
a supersaturated solution and crystals of dihydrate. Their 
formation sets free water, which dissolves more hemihydrate ; 
it releases more water as it crystallizes into dihydrate; 
and thus a small excess of water works through the material 
until all has set. 
The value of plaster of Paris in preparing casts and orna- 
mental mouldings is due to its expansion on solidification, 
so that the material is forced into the smallest crevices of the 
mould. The burning of gypsum mixed with alum or borax 
at 400° F. produces ** hard plaster.”