fiY COMPRESSION. 383 



]^a^ ma(sof Renwnur's porcelain*. It now appeared, that by 

 placing this fubftance immediately behind the borax, the 

 penetrating quality of this laft might be efFedlually reftrained ; 

 lor, Reaumur's porcelain has the double advantage of being 

 refractory, and of not cracking by change of temperature. I 

 found, however, that in the ad of confolidation, the pounded 

 bottle-glafs flirunk, fo as to leave an opening between its mat's 

 and the tube, through which the borax, and, along with it, the 

 carbonic acid, was found to efcape. But the objed in view was Improvement 

 obtained by means of a mixture of pounded bottle-glafs, and °" '*^'* "^"^'''** 

 pounded flint, in equal parts. This compound ftill agglutinates, 

 not indeed into a mafs fo hard as Reaumur's porcelain^ but luf- 

 ficiently fo for the purpofe; and this being done without any 

 fenfible conlradion, an efie6lual barrier was oppofed to the bo- 

 rax; (this arrangement is fhewnin Fig. II.); and thus the me- 

 thod of clofing the tubes was rendered fo complete, as feldom 

 to fail in practice +. A ftill further refinemeot upon this me- 

 thod was found to be of advantage. A fecond feries of powders, 

 like that already delcribed, was introduced iowards the muzzle, 

 (as fliewn in Fig. 12). During the firft period of the experi- 

 ment, this lafi-mentioned feries was cxpoled to heat, with all 

 the outward half of the tube fa b) ; and by this means, a folid 

 mafs was produced, which remained cold and firm during the 

 fubfequent a<5lion of heal upon the carbonate. 



I foon found, that noiwithftanding all the above-mentioned Remedy for po- 



, .... , . - , , . lofity in the 



precautions, the carbonic acid made its elcape, and that it earthen tubes, 

 pervaded the fubflance of the Wedgwood tubes, where no flaw 

 could be traced. It occurred to me, that this dei\it\ might be 

 remedied, were borax, in its thin and penetrating fiate of fu- 

 fion, applied to the inlideof the tube; and that i-ie pores of the 

 porcelain might thus be clofed, as thofe of leather are clofed 

 by oil, in an air-pump. In this view, I rammed the carbonate 

 into a fmall tube, and furrounded it with pounded glafs of boi. 

 rax, which, as foon as the heat was applied, fpread on the in*- 



f In the fame temperature, a mafs of the glafs of equal bulk 

 would undergo the fame change ; but it would occupy an hour. 



■j- A fubftance tqually efficacious in reftraining the penetrating 



quality cfbornx, was difcovered by another accident. It confifts 



of a mixture of borax and common fand, by which a fubftance is 



formed, which, in heat, aflumesthe ftate of a very tough palle, and 



■ becomes hard and compact on cooling, ■ ' . . ; 



fide 



