202 On the Component Tarts of ' Iron-Jloncs, 



2. Argillaceous iron-ftones, expofed to a fimilar degree of 

 heat, and treated in the fame manner, commonly lofe in 

 weight from 32 to 35 per cent. 



3. Siliceous iron-ftones always give outlcfs weight when 

 expofed to torrefa&ion under fimilar circumftances with 

 other (tones. In common they lofe from 27 to 30 per cent. 

 I have, though rarely, found them to give out 25 per cent. 

 only. 



The extreme lofs of weight in each clafs muft be coufi- 

 dered as the utmoft point to which tor refaction can with 

 fafety be urged, without expofing the iron-itone to an accu- 

 mulation of weight by the combination of oxygen. 



From the varioufly compounded natures of iron-uones 

 arife the various calculations of the lofs which they are faid 

 to fuftain in burning, at different iron-works. Fields of 

 iron-ftone are commonly impreffed with a general diftin- 

 guifhing charafteriftic feature : fome are of the calcareous 

 genus ; fome of the argillaceous ; and others again have a 

 more intimate alliance with filex than the former two. 

 Hence we find, at thofe works where the chief fupply is 

 drawn from the argillaceous iron-ftones, that the lofs in tor- 

 refaftion on the great fcale is computed from 30 to 33 per 

 cent. Where calcareous iron-ftones form the chief fupply, 

 the lofs is eftimated at from 35 to 37^ per cent. Few or 

 110 iron-works are obliged to have recourfe to iron-ftones 

 abounding with fend for their chief confumption : a general 

 efiimation of the lofs fuftained by this ftone in the large 

 way is therefore difficult to be made, though, I have heard, 

 that at fome works 25 per cent, was all that was allowed to 

 be expelled daring burning. Nature has been extremely 

 kind in the formation of our fecondary ores of iron ; as the 

 bulk of them, that contain iron fufficient to entitle them to 

 be fmelted, arc combined with fuperior proportions of clay 

 and lime. 



As the burning or torrefying of iron-ftones is of great im- 

 portance 



