390 



ACTION OF HEAT MODIFIED 



by which fhelaft could be counlera6led ; bnt I faw no remedy 

 for the firfl. I began, Iherefure, to defpair of ultimate fuccefs 

 with tubes of porcelain *. 

 Thefe laft could Another circumfiance confirmed me in this opinion. I 

 not bearekvated ^^^^^ j^ impraclicable to apply a heat above 2T^ to thele tubes, 

 when charged as above with carbonate, without dellroying 

 them, either by explofion, by the formation of a minute 

 rent, or by the afiual fvvelling of the tube. Sometimes this 

 fwelling took place to tiie amount of doubling the internal 

 diameter, and yet the porcelain held tight, the carbonate fuf- 

 taining but a very fraall lofs. This dudility of the porcelairv 

 in a low heat is a curious fa£t, and fiaews what a range of 

 temperature is embraced by tiie gradual tranfition of fome 

 fubllances from a (olid to a liquid ftale : For the fame porce- 

 lain, which is thus fufceptible of being Wretched out without 

 breaking in a heat of 27**, ftands the heat of 152^, without 

 injury, when expofed to no violence, the angles of its frad^ure 

 remaining (liarp and entire. 



Ixpfrlmcnts 

 with gun bar. 

 rels refumed, 



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 Adiion on 

 Porcelain. — Additional Apparatus required in Confequence of 

 that A&ion. — Good Refidts; in particular ^ four Experiments^ 

 illufirating the Theory of Internal Calcination, andfhewing 

 the Efficacy of the Carbonic Acid as a Flux. 



SINCE I found that, with porcelain tubes, I could neither 

 confine the carbonic acid entirely, nor expofe the carbonate 

 in them to firong 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 ac- 

 cidental circumfiances. 



* T am neverthelefs of opinion, that, in fome fituations, experi- 

 ments with compretfion may be carried on with great eafe and ad- 

 vantnge in fuch tubes. I allude to the fituation of the geologifh 

 of France and Germany, who may eafdy procure, from their own 

 rnanufaftories, tubes of a quality far fupcrior to any thing made 

 for fale in this ccuntry. 



On 



