difftrevl Earths' fur Carbon. 197 



jectcd, for containing " bad iron." This prejudice, without 

 stopping to inquire into the correctness of the deduction, 

 has tended to lock up from general examination numerous 

 strata of iron-stones which unfortunately were impressed 

 with the usual external characteristics of the siliceous genus 

 of stones. Many of these equally rich in iron, with some 

 of those used in the blast-furnace, are buried in the rubbish 

 of quarries, or contumeliously branded as a mischievous va- 

 riety of sand-stone. 



The general theory founded on practice, and which is 

 commonly admitted at iron -works, is, that that iron-stone 

 if! best which tluxes itself, or that, in other words, contains 

 a considerable proportion of hme in the state of crystal, 

 spar, or otherwise. The assertion is, and I believe it to be 

 just, that such iron-stones tend more to make ^' sulphury 

 iron," L e. iron richly carbonated, than any other variety : 

 hence these are always in great request. Again, those va- 

 rieties of iron-stones that present smooth fractures of a 

 dull blackish or grayish black, or gray or gravish white co- 

 lour, and uniform throughout, arc held next in estimation 

 at iron-works. These, with the same justness of remark, 

 make " good iron;" but their tendency to form " sulphury 

 iron" is inferior. The fracture of pig iron made from such 

 iron-stones is generallv less brilliant Than from the former 

 seldom presents a carburated surface, but by many is pre- 

 ferred for excellent melting iron. These varieties generally 

 arrange themselves under the argillaceous class of ores. In 

 these the manufacturer adds another, which is merely a 

 modification of the same class, but united to an evidently 

 large proportion of sand. The theory which is here applied 

 is, that such iron-stones make a coarser quality of iron than 

 the former, and that, when the quantity on the furnace is 

 increased beyond a certain proportion, the quality of the 

 metal becomes hard or less carbonated. 



These facts, which seem to result from general practice 

 may be thus shortly arranged, every circumstance beino- 

 alike to all • ° 



1st, Calcareous iron-stone has a direct tendency to ma- 

 nifest a larger proportion of carburet than any other class 

 in the blasl-furnace, and of course enhances at the same 

 time the absolute value of the metal. 



2d, Argillaceous iron-stones form iron of an equal qua- 

 lity in the estimation of some, but in manufacturing exhi- 

 bit always a less apparent existence of carbon and an in- 

 I'crior tendency to carburate. 



ZiS.) Siliceous iron-stones have uniformly a tendency to 

 N 3 destroy 



