Iron . 
69 
small proportion of oxygen may at times be unfixed, when 
the fuel may chance to come into contact with the heated 
iron-stone under a partial exclusion from air. But the as- 
sociation of circumstances necessary to effect this can so 
seldom be the effect of chance, that it is never to be look- 
ed for with certainty* When the operation is properly 
performed, the last particle of acid and water may be ex- 
pelled* But this point is difficult to be ascertained with 
any degree of exactitude: for, in proportion as these 
gaseous substances are carried off, the metal becomes 
more and more revived, of course more and more liable to 
attract, and fix oxygen by the decomposition of the igni- 
ted gas ; and, as the last portions of the acid and water are 
distilling, the stone is apt to gain weight by the calcina- 
tion (oxygenation) of its iron* This will positively be 
the case when the heat is carried beyond the necessary de- 
gree, and is indicated by the iron-stone swelling in bulk, 
becoming specifically lighter and porous on the surface, 
but gaining weight in a great degree internally* As 
oxygenation goes on, the magnetic virtue decreases, until 
at last it becomes entirely annihilated* 
The first stage in torrefaction is indicated by the first 
general change of colour in the iron-stone* This is corn- 
same substance, I have observed, effloresced upon the surface of 
the fracture of highly oxygenated crude iron, which had been bro- 
ken immediately after the metal had lost its fluidity. From this 
coincidence of effect, I am inclined to suppose, that the oxyd de- 
posited on the iron-stone in burning, is the consequence of the de- 
composition of the sulphuric acid ; a portion of which had been 
mineralised with the stone, holding iron in solution : and that in 
the latter it was occasioned by a superabundance of oxygen in the 
blast-furnace, probably from the introduction of raw iron-stone, 
which had escaped the effects of the fire : that the sulphur, as for- 
merly stated, had become oxygenated, dissolving a portion of the 
metal $ which was again deposited in the state of a calcined suF 
phat, when the acid was suffered to escape, by being freely expos- 
ed 5$ a high degree of heat to open air. 
