GYPSUM DEPOSITS OP NEW YORK ^"J 



solved in the water which became saturated with respect to it. 

 The heat of the chemical reaction causes an evaporation of some 

 of the water and a consequent crystallization of the saturated solu- 

 tion, the first crystal developed determining and hastening the 

 crystallization of the whole mass. Le Chatelier later showed, how- 

 ever, that plaster would set in a vacuum so that evaporation was 

 not the means of causing the crystallization. 



Le Chatelier^ in taking up the question utilized the observation 

 of Marignac^ that calcium sulfate in contact with water gives a 

 supersaturated solution which allows the deposition of crystals of 

 the hydrous calcium sulfate. With plaster cooked at 140° a solu- 

 tion is obtained containing 9 grams of CaSO^ per liter, i. e. four 

 times more than can normally exist in solution. Le Chatelier goes on 

 to say that such supersaturated solutions, capable of uniting directly 

 with water to form their hydrates are common, for example 

 Na^ SO4 Nao CO3 etc., all o:^ which salts set when mixed with water. 

 Finally he believes that the set is the result of two simultaneous 

 phenomena : *' On the one hand the masses of the plaster mixed 

 with water dissolve themselves on hydrating and produce a super- 

 saturated solution. On the other hand, this solution allows at the 

 same time a deposition of crystals of hydrous calcium sulfate. They 

 are added to little by little and bind themselves together." 



G. P. Grimsley," although agreeing with Le Chatelier and others 

 that the set of gypsum is due to a formation O'f a network of 

 crystals of gypsum crystallized from a saturated solution of the 

 half hydrate, to account for the cause of the beginning of the crys- 

 tallization advances this theory : " The effect of heat on gypsum 

 in the burning of plaster as we have shown, 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 ultramicroscopic 

 in size. If the heat is not carried too far, certain particles through 

 the mass may still possess their crystalline form and are true 

 crystals though small. These minute crystals in tlie saturated so- 

 lution would start the process of crystallization. . . If the plas- 

 ter is underburned the gypsum is not reduced to the proper fineness 

 and uniformity, and so would not permit the crystallization to go 

 on in the way it would in a properly burned plaster. But of more 

 importance, the hydrate represented by plaster of paris would not 

 be formed. If the plaster is overburned, it will be so completely 



comminuted that no minute crystals will be left to start the crvstal- 



\ — . . . . ' 



1 Acad, de Sci. Compt. Rend. 96, 714. 1883. 



' Ann. de Chimie de Physique Tome I, 279. 1874. 



3 Kansas Univ. Geol. Sur. 1899. 5:95. 



