196 Effects of Heat modijied hj Compression. 



by this means a solid mass was proclucecl, which remained 

 cold and firm during the subsequent action of heat upon the 

 carbonate. 



I soon found that, notwithstanding all the above-men- 

 tioned precautions, ih.c carbonic acid made its escape, and 

 that it pervadud the substance of the Wedgewood tubes, 

 where no flaw could be traced. It occurred to me, that this 

 defect might be remedied, were borax, in its thin and pene- 

 trating state of fusion, applied to the inside of the tube; and 

 that the pores of the porcelain might thus be closed, as those 

 of leather are closed by oil in an air-pump. In this view 

 T rammed the carbonate into a small tube, and surrounded 

 it with pounded glass of borax, which, as soon as the heat 

 was applied, spread on the inside of the large tube, and ef- 

 fectuallv closed its pores. In this manner, many good ex- 

 periments were made with barrels lying horizontally in com- 

 mon muffles (the arrangement just described being repre- 

 sented in fig. 13). 



I was thus enabled to carry on experiments with this 

 porcelain, to the utmost that its strength would bear. Bt»t 

 I was not satisfied with the force so exerted ; and, hoping 

 to obtain tubes of a superior quality, I spent much time in 

 experiments with various porcelain compositions. In this 

 I so far succeeded as to produce tubes, by which the car- 

 bonic acid was, in a great measure, retained without any in- 

 ternal glaze. The best material I found for this purpose was 

 the pure porcelain clay of Cornwall, or a composition in 

 the proportion of two of this clay to one of what the potters 

 call Cornish stone, which I believe to be a granite in a state 

 of decomoosition. These tubes were seven or eight inches 

 lonfT, with a bore tapering from 1 inch to 0*6 Their thick- 

 ness was about 0"3 at the breech, and tapered towards the 

 muzzle to the thinness of a wafer. 



I now adopted a new mode of operation, placing the tivbe 

 verticallv, and not horizontally as before. By observing 

 the thin state of borax whilst in fusion, I was convinced that 

 it ought to he treated as a complete liquid, which being 

 supported in t^ic coarse of the experiment from belnvv, wouW 



secure 



