Fie. 13. Fie. 14. 
Figure 13. Uncalcined ground gypsum in water, after standing three days. Blue Rapids. 
Figure 13. Gypsum crystallized from solution in water, after standing three days. Blue 
Rapids. 
tion is shown in Figure 14, which was obtained by evaporating 
a solution of uncalcined gypsum on a glass slide and then ex- 
amining it under a high-power microscope. 
Crystallization is aided by the small size of the grains or 
particles in the plaster, and the finer grained plasters set more 
rapidly than the coarser ones, as one may observe in the fine 
dental plasters as compared with ordinary plaster of Paris. 
My own experiments agree then with those given by Lavoisier, 
Payen, Landrin, and Chatelier, in that the set of plaster is due 
to the formation of a crystalline network. The cause of the 
formation of this network of crystals, or the factor which starts 
the crystallization is the troublesome part to explain, and this 
has attracted less attention among investigators along these 
lines. | 
When gypsum is burned it forms, as Landrin showed and as 
analyses prove, the hydrate (CaSO,),, H,O. Marignac called 
attention to the fact that if water is added in excess, this 
hydrate in part is dissolved, forming first a clear liquid which 
then becomes turbid, and crystals of CaSO,, 2H,O, or gypsum, 
are thrown down. Now an examination of these formule shows 
that three parts of water have been taken up by the hydrate, 
(CASIO) a, Il,Oq-slel O==2 (CarsO,,, Ziel) - 
