Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/391

 joined to the hardness of the carbonate of lime itself, must impart to the plaster that additional solidity, which, in such cases, it obtains by age.

When, on the contrary, the gypsum includes either quartz, sand, or clay, as these substances undergo no change by calcination, they produce an additional want of contiguity between the molecules regenerated from the plaster; and in so far diminish their mutual cohesion, and consequently the compactness of the mass.

I have deferred to this part of my memoir some further observations respecting the bardiglione, placed among the varieties of this substance under the name of Epigène, on the authority of the Abbé Haüy, who has established that variety from a specimen in his collection, one part of which is in the state of lamellar bardiglione, while the other is in that of compact gypsum; and, from the sense in which he uses the word Epigène, he considers the part of the specimen, which is in the state of compact gypsum, as having been originally lamellar bardiglione similar to the other part; and as having undergone this change in consequence if the intervention of water, which has introduced itself into the interior of the susbtance, and which, in his opinion, has rendered its texture more loose, and diminished its hardness.

From this explanation of the transition of lamellar bardiglione to compact gypsum by the mere absorption of water, it would seem as if this learned mineralogist supposed gypsum to differ from bardiglione only by the interposition of a certain quantity of water; or, if the expression “ introduced into its interior,” implies the combination of this fluid, it would necessarily follow, that bardiglione, like plaster, must pass to the state of gypsum on the addition of water, which we have seen is by no means the case. As to the opinion of the transition of bardiglione into gypsum by the mere interposition of water in its substance, the difference of figure between the