148. 
Iron „ 
fopn that gas ; the ore thus approaches still nearer to a 
metallic nature, becomes magnetic, and requires less char- 
coal in the furnace to make it into metah But if the 
charcoal dust be in too small a quantity, or the roasting 
continued with access of air too long, or if the red hot 
iron-stone be exposed without coal dust to a current of 
air, the ore does not lose, but gains in weight by attract- 
ing the oxygen of the atmosphere ; it becomes more re- 
fractory and infusible, and requires more fuel to metallize 
it : the difference between a piece of iron ore, and a piece 
of iron, being for the most part, that the first is, an d the 
second is not, combined with oxygen, carbonic acid or 
sulphur. 
Perhaps the roasting is better done in ovens : Jars in 2 
Voy. Mineralogiques planches 1 and 9, has given the 
oven employed for roasting ores of iron in Styria and Ca~ 
rinthia, and the coak oven of England : the only question 
here, is, would the convenience pay the ex pence,, 
4thly, If the washing be not skilfully performed, there 
is great hazard of washing away the ochry or metalliferous 
part of the iron-stone as I think I have seen, 
5thly, That it is worth while to ascertain whether the 
iron ore is accompanied by earths or stones of the clay 
class, or the limestone class, or of the sand- stone or flinty 
class, in order that limestone as a flux, may not be indis- 
criminately used in the same proportion to all kind of ores. 
The less earth of any kind is mixed with the ore, the less 
limestone is needed, 
6thly, Limestone being the flux that brings all other 
earths into fusion, a mixture of limestone ore with clay 
ore, or sand-stone or flint ore, promotes their fusion. 
Hence, limestone is to be added to clay ore and siliceous 
ore, but it would be useless and superfluous to add it to 
a limestone ore. In this case, another kind of ore should 
be brought to and mixed with the limestone ore. In Eng- 
land, the slag of previous fusions is much used® 
