Count de Bournon on BardigUo?te. S77 



Hence we can readily explain the cause why plaster, when 

 In contact with water, absorbs this liquid, and acquires solidity; 

 while bardiglione, in similar circumstances, undergoes no change. 

 From what has been said of the effect of calcination on the integrant 

 molecules of gypsum, it appears that the moment when plaster is 

 brought into contact with water, the molecules of the fluid are 

 attracted into the vacuities of similar shape with themselves by the 

 compound attraction of the bare surfaces of the constituent mole- 

 cules of lime and sulphuric acid, and are fixed there anew. The 

 plaster then returns in reality to the state of gypsum ; and this 

 change takes place more perfectly, when the same water that has 

 completed anew the imperfect integrant molecules of gypsum, 

 affords by its temporary superabundance a vehicle, by the inter- 

 vention of which, the new formed molecules are enabled to approach 

 each other afresh, and crystallize. The gypsum however neither 

 recovers the form, the hardness, nor the transparency which previously 

 belonged to it: the crystallization just mentioned cannot be other- 

 wise than greatly confused, on account of the considerable motion 

 that must exist at the moment in which the process takes place, on 

 the one hand, from the absorption of the water in the transition of 

 the integrant molecules from the state of plaster to that of gypsum ; 

 and on the other, from the evaporation of that liquid, which is occa- 

 sioned by the disengagement of caloric, expelled by the return of the 

 molecules of water of which it had occupied the place, added to that 

 set free by the water of combination, at the moment of its passage 

 from the liquid to the solid state. The superabundance of the water, 

 beyond what is necessary for the regeneration of the molecules of the 

 gypsum, is indicated by the volume of that absorbed by the plaster. 

 It is well known, that the volume of this is at least equal to that of 

 the plaster, which is itself of more considerable bulk than the 



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