GRIMSLEY. | Technology of Gypsum. 93 
out the crystal mass, and, expanding, breaks the crystals into 
finer particles. There is thus a physical change as well as a 
chemical one. In the plaster which has not been sufficiently 
burned the grains are coarser and more irregular. 
Fic. 12. Calcined gypsum earth from central Kansas, 
x 500. 
Under the microscope, when water is added to the calcined 
plaster, small needle-like prisms are seen forming and shooting 
out here and there. As these become more and more abundant, 
they unite with one another and rapidly form a solid mass, in 
which the individual crystals can scarcely be distinguished. 
Open spaces are left in the mass apparently filled with water, 
and finally these are closed, and a firm solid mass results. The 
network formed by these crystals at first, is imperfectly shown 
in the drawing in Figure 11. 
The treatment of uncalcined gypsum with water in the same 
way shows very little change, Figure 13. Gypsum crystallized 
from solution by evaporation shows crystals which are not needle 
shaped, but they are broader and show considerable irregularity. 
They are more or less twinned and they do not interpenetrate, 
but form a loose mass which readily crumbles. This crystalliza- 
