92 EFFECTS of HEAT 
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 laft-mentioned feries was expofed to heat, with all 
the outward half of the tube (¢4); 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 notwithftanding all the above-mention- 
ed precautions, the carbonic acid made its efcape, and that it 
pervaded the fubftance of the Wedgwood tubes, where no flaw 
could be traced. It occurred to me, that this defeé& might be 
remedied, were borax, in its thin and penetrating ftate 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: furrounded it with pounded glafs of borax, 
which, as foon as the heat was applied, {pread on the infide of 
the large tube, and effectually clofed its pores.. In this man- 
ner, many good experiments were made with barrels lying ho~ 
rizontally in common muffles, (the arrangement juft deferibed: 
being reprefented in fig. 13.) 
I was thus enabled to carry on experiments with this. 
porcelain, to the utmoft 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 {pent 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 Corni/b-/tone, 
which I believe to be a granite in a ftate of decompofition. 
Thefe tubes were feyen or eight inches long, with a bore. 
tapering 
