MODIFIED by COMPRESSION. 127 



weight, and the compound retaining its fufibility in low 



heats *. 



In the early part of 1804, fome experiments were made 

 with barrels, which I wifhed to try, with a view to another 

 feries of experiments. The remits were too interefting to be 

 palTed over ; for, though the carbonic acid in them was far 

 from being completely conftrained, they afforded fome of the 

 fineft examples I had obtained, of the fufion of the carbonate, 

 and of its union with filex. 



On the 13th of February, an experiment was made with 

 pounded oyfter-fhell, in a heat of 33 °, without any water be- 

 ing introduced to ailift compreffion. The lofs was apparently 

 of 1 2 per cent. The fubftance of the fhell had evidently been 

 in vifcid fufion : it was porous, femitranfparent, mining in fur- 

 face and fracture ; in moil parts with the glofs of fufion, in 

 many others with facettes of cryflallization. The little tube 

 had been fet with its muzzle upwards ; over it, as ufual, lay a 

 fragment of porcelain, and on that a round mafs of chalk. 

 At the contact of the porcelain and the chalk, they had run 

 together, and the chalk had been evidently in a very foft flate ; 

 for, refting with its weight on the porcelain, this laft had been 

 prefTed into the fubftance of the chalk, deeper than its own 

 breadth, a rim of chalk being vifible without the furface of 

 the porcelain j juft as when the round end of a knife is prefTed 



upon 



* The retentive power here afcribed to the procelain tubes, feems not to accord 

 ■with what was formerly mentioned, of the carbonic acid having been driven 

 through the fubftance of the tube. But the lofs by this means has probably been 

 fo fmall, that the native properties of the carbonate have not been fenfibly 

 changed. Or, perhaps, this penetrability may not be fo univerfal as" I have 

 been induced to think, by having met with it in all the cafes which I tried. 

 In this doubt, I ftrenuoufly recommend a further examination of this fubject to 

 gentlemen who have eafy accefs to fuch procelains as that of Drefden or of 

 Seve. 



