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NEW YORK STATE MUSEUM 



Le Chatelier's theory. Le Chatelier shoAved that Landrin's 

 third priniciple as mentioned above was not necessarily true, 

 inasmuch as plaster would set in a yacuum flask. 



His own theory was that plaster of paris partially dissolves 

 in the water, thus diminishing solubility, the solution becomes 

 supersaturated and gypsum crystallizes out. 



Grimsley's theory. Grimsleyi noted that, " 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 distin- 

 guished. Open spaces are left in the mass apparently filled with 

 water, and finally these are closed, and a firm solid mass results." 



He also noted that crystallization was more rapid in the finer- 

 grained plasters than in the coarser ones. He agrees with the 

 investigators mentioned above that the setting of plaster is due 

 to the formation of a crystalline net-work, but his explanation of 

 the cause of the formation of this net-work is somewhat dif- 

 ferent, and a summary of the same is given below. 



1 When water is added to plaster of paris. the plaster be- 

 comes partially dissolved. 



2 When the solution becomes hydrated and supersaturated 

 crystallization is started, and the liquid becomes turbid. 



In explanation of this theory, Grimsley says : 



The solution of the hydrate in these experiments is certainly 

 saturated, and all that is needed is something to start the crys- 

 tallization. From a study of saturated solutions in the labora- 

 tory, it is well known that if crystals are introduced into such 

 solutions, crystallization will result and go on until the salt has 

 crystallized out. 



The effect of heat on gypsum in the burning of plaster is to 

 remove a certain percentage of water, and to break up the small 

 masses of the rock into finer and finer particles, microscopic and 

 even ultra-microscopic in size. If the heat has not been carried 

 too far, certain particles through the mass may still possess 

 their crystalline form, and so they are true crystals, though 

 Tery small. These minute crystals in the saturated solution 



^Grimsley, G. P. & Bailey, E. H. S. Special Report on Gypsum and 

 Gypsum Cement Plasters. Univ. Geol. Sur. of Kan. Topeka 1899. p. 92-96. 



