Count de Bournon on Bard'iglione. 373 



granilla In la Mancha, and elsewhere in Spain, at Northwich in 

 Cheshire, in the sah-works of Altamonte in Calabria, &c. may not 

 much of the accompanying gypsum, as it has been hitherto consi- 

 dered, be rather in the state of bardiglione ? It appears besides, that 

 under certain circumstances bardigHone forms particular deposits, 

 such as that at Vulpino, at a little distance from Bergamo in Italy, 

 which has long been employed by the statuaries of that city, and of 

 Milan : but we are still in want of information respecting its exact 

 position, as well as the nature of the accompanying rocks. 



This substance must still be considered a new object of observation 

 for the geologist. I am strongly persuaded, that it will be to him 

 what the magnesian carbonate of lime has proved, since Mr. Tennant 

 turned his attention towards it, and shewed, that it exists in deposits 

 of considerable extent, where its presence had not been before sus- 

 pected. 



I have postponed to this part of my paper the grounds of my 

 opinion as to the essential difference between bardiglione and gypsum, 

 the detail of which will oblige me to take a view of the calcination 

 of gypsum, and of the different phenomena it exhibits. 



By calcination, gypsum is known to undergo no other alteration 

 than the loss of the water that formed part of its substance. The 

 residue of this process, properly called plaster, contains nothing but 

 lime and sulphuric acid in the same proportions as in bardiglione. 

 These two substances might therefore be ranked together, but we 

 shall presently see that there is a wide difference between them. In 

 this state of calcination, when we would make use of it, we mix it 

 with water, by tempering and beating them together. A certain 

 quantity of the water is absorbed, and after the evaporation of the 

 superabundant part of this liquid, which is promoted in some degree 

 by the caloric evolved in the operation, the mixture acquires solidity ; 



