451 
If on. 
follows, that were we to roast a sulphureous ifonstone without ad- 
mitting* the air as Mr. Mushet advises, we should not get rid of the 
sulphur at all, that small portion excepted which might he able 
to make its way through his nearly impenetrable covering of pit 
coal coak dust, and thus instead of improving upon the process 
in common use, we should really substitute one incomparably 
worse : a mode indeed, where the ore is very sulphureous, which 
would alone be fully sufficient to frustrate every future operation — *» 
to ruin the undertakers and cast disgrace upon the undertaking. 
To sum up the whole of what has been stated, I should com- 
prise it in the following rules, viz : 
That when the ore contains neither sulphur nor arsenic it can- 
not contain any other principle which requires the process of tor- 
refaction for its expulsion. Therefore, That when the ore is not 
mineralized and does not require pounding, roasting is unnecessa- 
ry. That when it is not mineralized and does require to be pound- 
ed, it is no matter for that purpose how the heat is applied, whe- 
ther with air or without it. That when the ore is mineralized, 
whether it needs pounding or not, it must be torrefied,— and that 
in this latter case, air is not only not prejudicial but absolutely ne- 
cessary for the purpose. 
There is yet one part of this subject which demands notice and 
that is the degree to which the heat should be raised in the process 
of torrefaction. The importance of this circumstance also de- 
pends in a great measure upon sulphur. When this injurious 
substance is present, the heat should be long continued, and must 
not be so great as to fuse the ore. When the ore contains no sul- 
phur, it is no longer of so much consequence, except that it is a 
waste of combustibles in the pile and of some small, quantity of coal 
In' the furnace. The reasons for this opinion are I conceive as fol- 
lows. 
It is generally found, at least throughout the argillaceous 
species of ores, that sulphur is chemically combined with a small 
portion of the iron, in the state of pyrites, but only mechanically 
combined with the great body of the ironstone. Now the united 
effect of heat and air, is to expel this sulphur in the form of sul- 
phureous acid gas, so long as the ore is not heated to the fusing 
point. But as soon as this is the case, the surface becoming glazed, 
denies a passage inwards to the air, and outwards to the sulphure- 
ous vapour, and the chemical combination which before existed 
only in a small part, becomes extended through the whole mass, 
Vol, III. ' 3 K 
