378 CountdeBournono;/ Bardiglmie, 



gypsum from which It was produced. It is known too, that the 

 moment the water is absorbed by the plaster, there Is always a disen- 

 gagement of heat, though much less than that produced in the 

 absorption of the same liquid by quicklime. 



As the integrant molecules of gypsum, in their regeneration 

 from plaster, can only crystallize, as has been said, in a very irre- 

 gular 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 inter- 

 spersed 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 soli- 

 dity 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 car- 

 bonic acid, returns to Its former state by a process of real crystalli- 

 zation. 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 shewn had a very loose texture ; and this 



