93 EFFECT'S of HEJT 



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

 like that already defcribed, was introduced towards the muzzle, 

 (as fhewn in fig. 12.). During the firft period of the experi- 

 ment, this 1 aft-mentioned feries was expofed to heat, with all 

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

 mafs was produced, which remained cold and firm during the 

 fubfequent action of heat upon the carbonate. 



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

 ed precautions, the carbonic acid made its efcape, and that it 

 pervaded the fub fiance of the Wedgwood tubes, where no flaw 

 could be traced. It occurred to me, that this defedt might be 

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

 fion, applied to the infide of the tube ; and that the 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 unrounded it with pounded glafs of borax, 

 which, as foon as the heat was applied, fpread on the infide of 

 the large tube, and erTe&ually clofed its pores. In this man- 

 ner, many good experiments were made with barrels lying ho- 

 rizontally in common muffles, (the arrangement juft defcribed 

 being reprefented in fig. 13.) 



I was thus enabled to carry on experiments with this 

 porcelain, to the utmofl that its ftrength would bear. But I 

 was not fatisfied with the force fo exerted; and, hoping to 

 obtain tubes of a fuperior quality, I fpent much time in expe- 

 riments with various porcelain compofitions. In this, I fo 

 far fucceeded, as to produce tubes by which the carbonic acid, 

 was in a great meafure retained without any internal glaze* 

 The beft material I found for this purpofe, was the pure por- 

 celain-clay of Cornwall, or a compofition in the proportion of 

 two of this clay to one of what the potters call Cornijh-Jlonc, 

 which I believe to be a granite in a ftate of decompofition. 

 Thefe tubes were feven or eight inches long, with a bore 



tapering 



