GRIMSLEY. | ~ Technology of Gypsum. 91 
remains to-day a valuable and suggestive memoir. The set of 
plaster was divided by him into four periods : 
‘‘1. The calcined plaster, on contact with water, unites with this liquid and 
takes a crystalline form. 
‘>. The plaster dissolves partially in the water, which becomes Seared 
with this salt. 
“3, A part of the liquid evaporates, due to the heat set free in the chemical 
combination. A crystal is formed and determines the crystallization of the entire 
mass; a phenomenon which is analogous to that which takes place when a piece 
of sulphate of soda is placed in a saturated solution of this salt. 
‘¢4, The maximum hardness is reached when the plaster loses enough water 
to correspond exactly to the formula SO3CaO, 2H2O; this maximum being to 
the remainder in proportion to the quantity of water added to the plaster to 
transform it into mortar.”’ 
In order to test the third and fourth principles, Landrin tried 
the following experiment. He mixed 23.358 grammes of plaster 
with ten grammes of water, and the parelig nis found at different 
intervals were : 
lim, TO) soaMONKS, 5 coco seu boc good ue 33.100 grammes, or loss of water, 0.258 grammes. 
In one hour and 10 minutes ... .32.623 EG os ee 0.735 OC 
J Esa (3) CWI AS) see ely ea eA ree ee 29.218 B6 OG OY 4.140 6 
lin. WO) Cee. a5 aaa pera a maer 27.290 Be oe ae 6.068 66 
Jim, 1s} (CENYSARS Aas ee eae ae 27.283 es ue é 6.075 OG 
After this time, no change. 
‘¢The plaster lost in calcining 5.715 grammes, equal to the combined water. 
In 27.283 grammes of plaster, by formula SO,CaO, 2H,0O, there would be 5.710 
grammes of water, so that drying ceased when plaster reached its original com- 
position.”’ 
The maximum hardness then was reached when the plaster 
contained about 20 per cent. of water. The loss of water was 
regarded as due to the evaporation, according to Landrin’s 
third principle. 
- CHATELIER S THEORY. 
Chatelier showed that plaster would set in a vacuum flask, 
so that evaporation was not a necessary step in the set of plas- 
ter, as Landrin maintained in his third principle above. 
According to Le Chatelier, the plaster of Paris compound 
(CaSO,),, H,O dissolves in part in the added water, which 
diminishes the solubility, and the solution becomes therefore 
food 
w= 
