Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/385

 in la Mancha, and elsewhere in Spain, at Northwich in Cheshire, in the salt-works of Alfamonte in Calabria, &c. may not much of the accompanying gypsum, as it has been hitherto considered, be rather in the state of bardiglione? It appears besides, that under certain circumstances bardiglione 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 magnesia carbonate of lime has proved, since Mr. Tennant turned his attention towards it, and showed, that it exists in deposits of considerable extent, where its presence had not been before suspected.

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;