92 University Geological Survey of Kansas. 
supersaturated and CaSO,, 2H.O, or gypsum, crystallizes out. ~ 
In other words, the plaster of Paris dissolves, and becomes 
hydrated, then crystallizes out as gypsum; and every particle 
of the plaster goes through these steps. 
ORIGINAL THEORY. 
The theories given above are the only ones so far found in the 
literature which attempt to explain the set of plaster. Before 
finding the above references to the process of set in plaster, the 
writer made independent investigations to determine the cause 
of the set in calcined gypsum on the addition of water, with 
the following results : 
Fia. 10. 
Figure 10. Uncalcined gypsum earth from central Kansas, 500. 
Figure 11. Calcined gypsum earth from central Kansas, one-half hour after water was added 
to it, < 600. 
Under the microscope the ground gypsum and the gypsum 
earth before calcination agree in being composed of rather large 
masses of varying size, as shown in Figure 10. The former 
consists of more or less broken crystals, while the gypsum earth 
shows crystals of more regular shape. After calcination these 
larger crystals are found to be broken into fine granules of nearly 
uniform size and shape, as shown in the drawing made from 
calcined gypsum earth from central Kansas, in Figure 12. As 
the material is heated, the water is changed into steam through- 
