90 EFFECTS of HEAT 
I rnus expected to confine the carbonic acid; but the at- 
tempt was attended with confiderable difficulty, and has led 
to the employment of various devices, which I fhall now fhort- 
ly enumerate, as they occurred in the courfe of practice. The 
fimple application of the principle was found infufficient, from 
two caufes: Firft, The carbonic acid being driven from the 
breech of the tube, towards the muzzle, among the pores of 
the pounded filex, efcaped from the compreffing force, by 
lodging itfelf in cavities which were comparatively cold: 
Secondly, The glafs of borax, on cooling, was always found to 
crack very much, fo that its tightnefs could not be depended 
on. 
To obviate both thefe inconveniences at once, it occurred to 
me, in addition to the firft arrangement, to place fome borax 
(fig. 10. C) fo near the breech of the tube, as to undergo heat 
along with the carbonate (A); but interpofing between this 
borax and the carbonate, a ftratum of filex (B), in order to 
prevent contamination. I trufted that the borax in a liquid 
or vifcid ftate, being thruft outwards by the expanfion of the 
carbonic acid, would prefs againft the filex beyond it (D), 
and totally prevent the elaftic fubftances from efcaping out of 
the tube, or even from wandering into its cold parts. 
In fome refpects, this plan anfwered to expectation. The 
glafs of borax, which can never be obtained when cold, with- 
out innumerable cracks, unites into one continued vifcid mafs 
in the loweft red-heat ; and as the ftrefs in thefe experiments, 
begins only with rednefs, the borax being heated at the fame 
time with the carbonate, becomes united and impervious, as 
foon as its action is neceflary. Many good refults were accord- 
ingly obtained in this way. But I found, in practice, that as the 
heat rofe, the borax began to enter into. too thin fufion, and 
was often loft among the pores of the filex, the fpace in which 
it had lain being found empty on breaking the tube. It was 
therefore. 
