Count de Bournon on Bardlgllone, 379 



joined to the hardness of the carbonate of lime itself, must im- 

 part 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 rege- 

 nerated 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 Epighie^ on the authority of the Abbe Haiiy, 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 Epigenc^ he considers the part of the specimen, which is in the 

 state of compact gypsum, as having been originally lamellar bardig- 

 lione similar to the other part ; and as having undergone this change 

 in consequence of the intervention of ivater^ which has introduced 

 itself into the interior of the substance^ 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 bardig- 

 lione only by the interposition of a certain quantity of water ; or, if 

 the expression " introduced into its interior^'' implies the combi- 

 nation 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 interpo- 

 sition of water in its substance, the difference of figure between the 



3 B 2 



