On the Component Paris of Iron-flows. 199 



lion to the quantity of clay and its kind contained in the 

 ftonc, than to its real contents in iron. Iron -{tone may alio 

 be torrefied in fuch a manner as to deprive its internal furface 

 of this property ; as it is only peculiar to the ftone at a cer- 

 tain ftage of torrefaction. 



The influence which the magnet poffeffes over fome ores 

 of iron is no direct proof of the quantity of iron contained ; 

 as fome ores which contain 15 parts in 100 are completely 

 magnetic, while others again that contain 60 to 70 parts of* 

 iron in 100 are not in the fmalleft degree affected with this 

 property. The magnetic teft is more ufed to ascertain the 

 exiftent ftate of the metal, whether mineral ifed with an acid, 

 combined with fulphur or with oxygen, or exifting in a dif- 

 engaged ilate more or lefs metallic. No iron-ftones which 

 in their native ftate contain their iron mineralifed with oxy- 

 gen, or in the ftate of an oxyde, completely difperfed through 

 an intimate combination of clay, lime and filex, containing 

 water, carbonic acid, and fometimes concrete fulphur, are 

 obedient to the magnet till fuch time as torrefadtion has 

 paffed upon them, either expofed to open air, or in contact 

 with charcoal in clofe vends. If this procefs is continued 

 for a fliort time, the whole mafs will become obedient ; but 

 this affection will ftill depend upon the relative quantity of 

 concrete oxygen fixed with the iron. Individually, how- 

 ever, iron-ftones are affected by the proportion of heat con- 

 veyed to them while torrefying. If the quantity communi- 

 cated has been fparing, fo as not to have carried oft" all the 

 water, carbonic acid, &c. the magnetic virtue will alfo be 

 proportionally abfent: if the diffipation of thefe fubftances 

 has been complete, the magnet will poflefs an influence in 

 the exact ratio of the quantity of oxygen which remain-; 

 combined with the metal. Should it happen that a degree 

 of heat capable of exciting fufion is applied, the mafs will 

 then rapidly lofe its magnetic obedience by an extra-fixation 

 of oxygen; if driven fo far as to make it exhibit a femi-vitri- 



O 4 fied 



