BY COMT'RESSIO;>r. ]Q 



or 80*' without injury. At the same time, the results, even in 

 those high heats, were often inferior, in point of fusion, to those 

 obtained bylow heats in porcelain. The reason of this now 

 plainly appears. In the iron-barrels it has always been con- 

 sidered as necessary to use an air^tube, in consequence of 

 which, some of the carbonic acid has been separated from the 

 earthy basis by internal calcination : what carbonic acid re- 

 mained, has been more forcibly attracted, according to JMr. 

 BerthoUet's principle, and, of course, more easily compressed, 

 than when of quantity sufficient to saturate the lime : but, 

 owing to the diminished quantity of the acid, the compound 

 has become less fusible than in the natural state, and, of 

 course, has undergone a higher heat with less effect. The in- 

 troduction of water, by furnishing a reacting force, has pro- 

 duced a state of things similar to that in the porcelain tubes ; 

 the carbonate sustaining little or no loss of weight, and the 

 compound retaining its fusibility in low heats*. 



In the early part of ISO^, some experiments were made with 

 barrels, which I wished to try, with a view to another series of 

 experiments. The results were too interesting to be passed 

 over ; for, though the carbonic acid in them was far from be- 

 ing completely constrained, they afforded fome of the finest 

 examples I had obtained, of the fusion of the carbonate, and 

 of its union with silex. 



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

 pounded oyster-shell, in a heat of 33°, without any water be- po,'^tioii'of ^cav- 

 ing introduced to assist compression. The loss was apparently bonate (from 

 of 1 2 per cent. The substance of the shell had evidently been tiafljraction 

 in viscid fusion : it was porous, semitransparent, shining in sur- of the carbon- 

 face and fracture; in most parts with the gloss of fusion, in tiie tvibe^* ^"^ " 



* The retentive power here ascribed to the porcelain tubes, 

 seems not to accord with what was formerly mentioned, of the car- 

 bonic acid having been driven through the substance of the tube. 

 But the loss by this means has piobably been so small, that the 

 native properties of the carbonate have not been sensibly chan<i;ed. 

 Or, perliaps, this penetrability may not be so universal as- 1 have 

 been induced to think, by having met with it in all the cases which 

 I tried. In this doubt, I strenuously recommend a further examin- 

 ation of tbis subject; to gentlemen who have easy access to such 

 porcelains as that of Dresden or of Seve. 



D 2 many 



