108 EFFECTS of HEAT 
lation between the various forces of expanfion, affinity, tena- 
city, &c.), much more ftrain has been given to the veflels than 
was requifite. In fuch cafes, barrels have been deftroyed, 
which, as the refults have proved, had acted with fufficient 
ftrength during the firft ftages of the experiments, though they 
had been unable to refift the fubfequent overftrain. Thus, my 
fuccefs with gun-barrels, depended on the good fortune of ha- 
ving ufed a force no more than fufficient, to conftrain the car- 
bonic acid, and enable it. to act as a flux on the lime. I there- 
fore determined to have recourfe to iron barrels of much 
greater ftrength, and tried various modes of conftruction. 
I nap fome barrels executed by wrapping a thick plate of iron 
round a mandrel, as is practifed in the formation of gun-bar- 
rels ; and likewife by bringing the two flat fides together, fo as 
to unite them by welding. Thefe attempts, however, failed. 
I next thought of procuring bars of iron, and of having a cavi- 
ty bored out of the folid, fo as to form a barrel. In this man- 
ner I fucceeded well. The firft barrel I tried in this way was 
of fmall bore, only half an inch: Its performance was highly 
fatisfatory, and fuch as to convince me, that the mode now 
adopted was the beft of any that I had tried. Owing to the 
fmallnefs of the bore, a pyrometer could not be ufed internal- 
ly, but was placed upon the breech of the barrel, as it ftood in 
the vertical muffle. In this pofition, it was evidently expofed 
to a much lefs heat than the fiducial part of the apparatus, 
which was always placed, as nearly as could be guefled, at the 
point of greateft heat. 
On the 4th of April, an experiment was made in this way with 
fome {par ; the pyrometer on the breech giving 33°. The fpar 
came out clean, and free from any contamination, adhering to 
the infide of the porcelain tube: it was very much fhrunk, ftill re- 
taining a cylindrical form, though bent by partial adhefions. Its 
furface bore {carcely any remains of the impreilion taken by 
the 
