Iron. 
to sustain in burning at different iron works* F ieids of iron-; 
Stone are commonly impressed with a general distinguish? 
ing characteristic feature : some are of the calcareous ge- 
nus ; some of the argillaceous ; and others again have a 
more intimate alliance with silex than the former two. 
Hence we find, at those works where the chief supply is 
drawn from the argillaceous iron-stones, that the loss in tor- 
refaction on the great scale is computed from 30 to 33 
per cent. Where calcareous iron-stones form the chief 
supply, the loss is estimated at from 35 to 37 1-2 per 
cent. Few or no iron- works are obliged to have recourse 
to iron-stones abounding with sand for their chief con- 
sumption : a general estimation of the loss sustained by 
this stone in the large way is therefore difficult to be made, 
though, I have heard, that at some works 25 per cent, 
was all that was allowed to be expelled during burning. 
Nature has been extremely kind in the formation of our 
secondary ores of iron ; as the bulk of them, that contain 
iron sufficient to entitle them to be smelted, are combined 
with superior proportions of clay and lime. 
As the burning or torrefying of iron-stones is of 
great importance to the manufacturer, and as it may be 
in some respects gratifying to the man of science, I shall 
particularly enter into the various phenomena attending 
the operation; dividing torrefaction into two classes* 
that which deprives iron-stones of certain substances ca- 
pable of becoming aeriform by the combination of caloric 
in contact with atmospheric air ; and that which deprives 
ores of their oxygen— hepce called de-oxygenation— by 
heating them ip contact with charcoal, in closed vessels., 
or in cavities impervious tp the external air. 
1. The consequences of heating iron-stone exposed to 
air is a loss of water, sulphur, and carbonic acid*. A 
* I have seen some iron-stones in torrefying deposit a beautiful 
oxyd upon their surface, of a lake colour, and light as down. The 
