On the Component Tarts of Iron-Jlones. 205 



Thefe, as in metallic fubftances, are the effects of the pre- 

 tence of air at certain degrees of heat*. As the progreffive 

 diffipation of the volatile mixtures takes effect, the colour 

 deepens ; and, according to the nature of the iron-ftone, 

 becomes fixed, of a brown, dark brown, or deep claret 

 colour. Thefe indicate the almoft entire expulfion of the 

 water, acid, &c. In this- ftate all iron-ftones are poffeffed 

 of magnetic attraction, and exhibit the various phenomena 

 already defcribed, as being peculiar to their refpeftive na- 

 tures. 



The methods commonly in ufe for torrefying iron-ftone, 

 in the large way, are of two different kinds : that of burn- 

 ing in kilns or conical furnaces; and that of expofing iron- 

 ftone in open air, ftratified with coals, to combaftion. The 

 former is ufed in fome places ill Wales ; the latter is almoft 

 univerfally adopted in England, and totally fo in Scotland. 



In the operation with furnaces, they become filled entirely 

 with iron-ftone, except a ftratum of coals irt the bottom^ 

 which is afterwards inflamed. The combaftion is then 

 carried on by means of a current of air pafling through the 

 furnace, and forcing the heat along with it. When the 

 iron-ftone is deemed fufficiently burnt, the regifter is fliut 

 up ; and the combuftion, no longer maintained by means 

 of external air, foon dies away, and leaves the furnace to cool. 



The moft common method, however, of barning iron- 

 ftones, confiits in levelling a piece of ground, and covering 

 it with a layer of fmall pit-coals. This is of various thick- 

 neffes, 4, 6 or 8 inches, according to the height the pile is 

 to be built, and the nature of the iron-ftone. Upon this 

 ftratum of coals the pieces of iron-ftone are imbedded, as 

 near to the fame fizc as poflible, in order that all may be 

 equally adted apon. Thefe are reared to various heights, 



* From fmooth-furfaced iron-ftones and ores, when expofed to heat 

 partially flickered by charcoal duft from the a&ion of air, 1 have obtained 

 all the (hades of colour peculiar to polifhed iron and ftce! ; but with lefs 

 •f the metallic luftre, owing to the furface being more porous.. 



7 18, 



