On the Component Tarts oflron-JJones. 203 



portance to the manufacturer, and as it may be in fome re- 

 fpects gratifying to the man of fcience, I (hall particularly 

 enter into the various phenomena attending the operation ; 

 dividing torrefaction into two clafles : that which deprives 

 iron-ftones of certain fubftances capable of becoming aeri- 

 form by the combination of caloric in contact with atmo- 

 fpheric air ; and that which deprives ores of their oxygen — ^ 

 hence called de-oxvgenation — by heating them in contact 

 with charcoal, in clofed veffels, or in cavities impervious to 

 the external air. 



1 . The confequences of heating iron-ftone expofed to air 

 is a lofs of water, fulphur, and carbonic acid*. A fmall 

 portion of oxygen may at times be unfixed, when the fuel 

 may chance to come into contact with the heated iron-ftone 

 under a partial exclufion from air. But the affbciation of 

 circumftances neceffary to effect this can fo feldom be the 

 effect of chance, that "it is never to be looked for with cer- 

 tainty. When the operation is properly performed, the laft 

 particle of acid and water may be expelled. But this point 

 is difficult to be afcertained with any degree of exactitude : 

 for, in proportion as thefe gafeous fubftances are carried off, 

 the metal becomes more and more revived, of courfe more 

 and more liable to attract, and fix oxygen by the decompo- 



* I have feen fome iron-ftones in torrefying depofit a beautiful oxyde 

 upon their furface, of a lake colour, and light as down. The fame fub- 

 ftance, I have obferved, efflorefced upon the furfa:e of the fracture of 

 highiy oxygenated crude iron, which had been broken immediately after 

 the metal had loft its fluidity. From this coincidence of effect, I am in- 

 clined to fuppofe, that the oxyde depofited on the iron-ftone in burning is 

 the confequence of the decompofition of the fulpliuric acid ; a portion of 

 which had beenmineralifed with the ftone, holding iron in folution : and 

 that in the latter it was occafioned by a fuperabundance of oxygen in the 

 bjaft-fumace, probably from the introduction of raw iron-ftone, which 

 li3d efcaped the effects cf the fire : that the fulphur, as formerly ftated, 

 had become oxygenated, diffolving a portion of the metal ; which was 

 again depofited in the ftate of a calcined fulphat, when the acid was fuf- 

 ftrcd to cfcape, by being freely expofed at a high degree of heat to open 

 air, 



fition 



