BAILEY. | Chemistry of Gypsum. 157 
made after heating a long time at this temperature. However 
this may be, in a series like that shown under the head of 
‘‘Manufacture of Plaster,’’ it is easy to observe that a high per 
cent. of calcium sulphate means a high per cent. of water, while 
a low value for calcium sulphate corresponds to a smaller quan- 
tity of water; so the analyses in the set are comparable the one 
with the other. 
In regard to the water remaining in the manufactured prod- 
uct, according to the best researches, this should correspond 
to the amount in the formula, (CaSO,),, H,O, or 6.2 percent. 
Now, it actually happens that the per cent. of water calculated 
on the commercial product is usually higher than this, as na- 
ture makes the combination more carefully than man can dehy- 
drate the material. For instance, a plaster containing 66.91 
per cent. of calcium sulphate ought to have associated with it 
only 4.42 per cent. of water, but an analysis shows that there 
is really 6.41 per cent. In fact, even if the plaster is quite im- 
pure from containing quite large quantities of carbonates and 
of insoluble residue, the amount of water is hardly ever below 
four per cent. 
NECESSITY OF A CAREFUL SAMPLING OF A DEPOSIT. 
In the case of the deposits of gypsite, especially, the greatest 
difficulty is often experienced in getting an average sample of 
the bed. It is probable that the only way to get it accurately 
would be to take samples all over the surface, then at a stated 
depth beneath the surface, so as to get the composition at that 
depth, and again at another depth below the previous one, and 
so on to the bottom of the deposit. A sample taken one month 
will probably differ in composition from one taken the next 
month, as the workmen are farther down in the deposit. The 
necessity of careful sampling can be illustrated by a case in 
which specimens taken from the same prospect hole at different 
depths showed a range from about 41 per cent. to 70 per cent. 
of anhydrous calcium sulphate, with a corresponding increase in 
the amount of water. As the material is clay, or partially de- 
composed minerals, sand, and calcium and magnesium carbo- 
