Iron . 
131 
diate contact with the separating metal, but had, by its 
uncommon degree of density, resisted decomposition in 
the ignited passage, and had been decomposed upon the 
coaks at a greater elevation in the furnace. As a proof 
of this, we frequently see a tube formed throughout the 
breadth of the furnace, quite black and apparently cold, 
formed of the fused materials : when this is removed, a 
considerable descent momently takes place of coaks heated 
visibly beyond the common pitch : these inflame rapidly, 
but are soon again cooled to blackness by the incessant 
discharge of air upon them. The descending mi lure 
of iron and lava are in like manner cooled around the line 
of blast ; the tube is again formed, and, if not removed, 
will remain for days together, while the furnace will be 
otherways working in the best manner. 
When by accident or design the compression and velo- 
city of the blast are diminished, the tube begins to burn, 
and throws off a great many red fiery- coloured sparks, the 
sides and roof fail, and are carried before the blast in all di~ 
! rections. ^Sometimes considerable cleats of imperfect iron 
are recoiled with such violence as to escape the vortex of 
blast, and issue from the tuyere-hole with such velocity as 
!j to inflame in the air, and fall down in the state of oxych In 
| the end the tuyere will appear to flame and all the passage 
inwards shews an astonishing degree of whiteness. The 
decomposition of the air is instantaneously effected upon 
!| its entering the ignited passage ; the iron by this means 
is exposed to the oxygen that is disengaged ; and the 
vast quantity of caloric set free, in consequence of its union 
! with the iron and carbon, produces the astonishing heat 
now visible, but which formerly took place at a more pro- 
per height in the furnace. 
From this it will appear, that although a greater apparent 
degree oi heat is visibly produced by the sudden decom- 
position of the air, and a more rapid descent of materials 
■ 
