io8 EFFECTS of HEAT 



lation between the various forces of expansion, affinity, tena- 

 city, &c), much more ftrain has been given to the veifels than 

 was requilite. In fuch cafes, barrels have been deftroyed, 

 which, as the refults have proved, had a&ed with fufficient 

 ftrength during the fir ft flages 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 conflrain 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 conftru&ion. 



I had 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 

 fatisfactory, and fuch as to convince me, that the mode now 

 adopted was the belt 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 flood 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 fpar ', 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 1 it was very much fhrunk, ftill re- 

 taining a cylindrical form, though bent by partial adhefions. Its 

 furface bore fcarcely any remains of the impreilion taken by 



the 



