REPORT OF THE DIRECTOR AND STATE GEOLOGIST 1903 133 



Set of plaster. Layoisier, Landiin, and Le Chatelier all gave 

 theories to show the cause of plaster of paris setting or becom- 

 ing solid. 



Lavoisier's theory, as quoted^ by Grimsley from the original 



account, is as folloAVS : 



I took the calcined plaster, as has been described before, and 

 which hardens readily with water. I threw it into a consider- 

 able amount of water, in a pan or in a large dish. Each mole- 

 cule of plaster, in passing through the liquor, seized its molecule 

 of water of crystallization and fell to the bottom of the dish in 

 the form of small brilliant needles, visible only with a strong 

 lens. These needles, dried in the free air or with the aid of a 

 very moderate heat, are very soft and silky to the touch. If 

 placed on the stage of a microscope, it is perceived that what 

 was taken under the lens for needles are also parallelopipeds, 

 very fine, so they are described as thicker, or many thinner, and 

 many more elongated. The plaster in this state is not capable 

 of uniting with water, but if it is calcined anew, these small crys- 

 tals lose their transparency and their water of crystallization, 

 and become again a true plaster, as perfect as before. One may 

 in this fashion successfully calcine and recrystallize the plaster, 

 even to infinity, and consequently give to it at will the property 

 of seizing water. 



Landrin's- theory, which is the result of an elaborate study of 



plaster, divides the set into four periods: 



1 The calcined plaster, on contact with water, unites with this 

 liquid and takes a crystalline form. 



2 The plaster dissolves partially in the water, which becomes 

 saturated 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 sulfate 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 SOgCaO, 

 2HoO; this maximum i being to the remainder in proportion to 

 the quantity of water added to the plaster to transform it into 

 mortar. 



^Landrin. Annales des chimie. 1874, p. 434, 435. Also Grimsley Gyp- 

 sum Deposits of Kansas, p. 90. 



'Landrin. Annales des chimie. 1874. Also Grimsley Gypsum Deposits 

 of Kansas, p. 91. 



