Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/389



Hence we em 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 integrand 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 molecules 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 integrand molecules of gypsum, affords by its temporary superabundance a vehicle, by the intervention 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 otherwise 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 integrand molecules from the state of plaster to that of gypsum; and on the other, from the evaporation of that liquid, which is occasioned 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 tree 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