34 KANSAS UNIVERSITY QUARTERLY. 
belongs to Gypsum. If the gypsum be pure the per cent of water 
approximates quite closely to 20.9 per cent, the theoretical amount. 
If on the other hand it contains large quantities of calcium carbon- 
ate or of silica and insoluble material; the per cent of water some- 
times falls as low as 5.5. 
Calcium carbonate is seldom found higher than twelve per cent, 
while it frequently, in the rocks especially, is less than one per 
cent. The magnesium salts are usually present, but in very small 
quantity, and, indeed, so small that their presence or influence 
could usually be ignored. It is difficult to decide just what part 
the silica plays in increasing the efficiency of the cementing mate- 
rial. It seems quite probable, however, that within certain limits 
the silica would tend to form silicates, after the manufactured ma- 
terial has been mixed with water and begins to ‘“‘set.’’ This 
problem, however, can be more satisfactorily discussed in connec- 
tion with the process of manufacture and the composition of the 
commercial cements. 
