MODIFIED by COMPRESSION. TOI 
IV. 
Experiments in Gun-Barrels refumed.—The Vertical Apparatus applied 
to them.—Barrels bored in folid Bars.—Old Sable Iron.—Fufion of 
the Carbonate of Lime.—Its action on Porcelain.— Additional appa- 
ratus required in confequence of that action.—Good re/ults ; in par- 
ticular, four experiments, illustrating the theory of Internal Calcina- 
tion, and /hewing the efficacy of the Carbonic Acid us a Flux, 
Since I found that, with porcelain tubes, I could neither 
confine the carbonic acid entirely, nor expofe the carbonate 
in them to ftrong heats; I at laft determined to lay them afide, 
and return to barrels of iron, with which I had formerly ob- 
tained fome good refults, favoured, perhaps, by fome acciden- 
tal circumftances. 
-On the rath of February ty I began a feries of experi- 
ments with gun-barrels, ref{uming my former method of working 
with the fufible metal, and with lead; but altering the pofition of 
the barrel from horizontal to vertical ; the breech being placed 
upwards during the action of heat on the carbonate. This very 
fimple improvement has been productive of advantages no lefs 
remarkable, than in the cafe of the tubes of porcelain. In this 
new pofition, the included air, quitting the air-tube on the fu- 
-fion of the metal, and rifing to the breech, is expofed to the 
greateft heat of the furnace, and muft therefore reat with its 
greateft force; whereas, in the horizontal pofition, that air 
might go.as far back as the fufion of the metal reached, where 
its elafticity would be much feebler. The fame difpofition 
enabled me to keep the muzzle of the barrel plunged, du- 
ring the action of heat, in a veffel filled with water; which 
contributed 
