Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/390

 gypsum from which it was produced. It is known too, that the moment the water is absorbed by the plaster, there is always a disengagement of heat, though much less than that produced in the absorption of the same liquid by quicklime.

As the integrand molecules of gypsum, in their regeneration from plaster, can only crystallize, as has been said, in a very irregular and confused manner, the space occupied by the whole mass, after this reproduction, must be in proportion to this irregularity. Accordingly it is found that plaster acquires a very evident increase of bulk, a circumstance that is often very detrimental in the purposes for which it is employed; and this increase is greater, in proportion to the purity of the gypsum from which it was made, The swelling allows part of the water, superfluous to the regeneration of the gypsum, to remain interposed in the plaster, till the caloric of the ambient air has in its turn absorbed it, during the process of drying. In this state, the plaster must be considered as perforated with little vacuities interspersed throughout its substance: and in consequence of this texture, when it afterward comes again into contact with water, or even with air loaded with humidity, these vacuities exert a capillary attraction, and imbibe the water afresh; and plaster of this sort can be used only in places that are dry, and sheltered from all accidental dampness.

Gypsum mixed with carbonate of lime affords a better plaster than such as contains none; that is to say, it acquires in time greater solidity and hardness, because, while by calcination it loses only its water of composition, the carbonate of lime loses its carbonic acid, and passes to the state of quicklime, which, in course of time regaining its carbonic acid, returns to its former state by a process of real crystallization. This reproduction of the carbonate of lime, occasioning it to occupy a more considerable space, compresses the particles of the plaster, which it has been shown had a very loose texture; and this