148 Heat emitted by Condtnfaticn of Fluids. [Book II. 



conveyed into a clofe bottle filled with this air, with a 

 fmall piece of tinder or any combiiftible matter upon 

 it, to which fire has been communicated, the wire will 

 be obferved to burn, after the other combuftible mat- 

 ter is confumed, with a clear and bright flame, and if 

 there is a large quantity of pure air, the whole of the 

 iron will be converted into a calx. 



It has been amply demonftrated, that the conden- 

 fation, not only of pure air, but of every fluid, is at- 

 tended with the emiffion of heat or elementary fire ; 

 and even the partial condenfation of a fluid, or the 

 reduction of it from a rarer to a denfer ftate, will 

 produce the fame effe<5b. Thus air and vapour are 

 rarer fluids than water, and their condenfation into 

 water always produces fcnfible heatj thus fixable air 

 is a denfer fluid than atmofpherical or pure airj. 

 and when a quantity of the latter is by any procefs 

 converted into the former, a quantity of fuperfluous 

 caloric is confequently emitted. This is the cafe in 

 all fermenting bodies, which abforb a large quantity of 

 the pure ai-r of the atmofphere, and emit that denfe 

 acid fluid, which always hangs over their furface like a 

 vapour, and is univerfally known by the name of fix- 

 able air: and this procefs is always attended with heat, 

 that is, with the feparation of a quantity of elementary 

 fire. The accenfion or ignition of the mafs depends, 

 however, on the fpeedy emiiTion of the matter of fire, 

 that is, upon the violence of attraction between the 

 two fubftances which occafipns the condenfation. 

 When fulphur, iron -filings, and water, are mixed to- 

 gether snd kneaded into a pafte, the air is rapidly at- 

 tra&ed, and the mafs becomes fo hot as to take 

 fire. Play-ricks and other fermenting mafTes are fre- 

 quently fired by this kind of fpontaneous procefs, 



If 



