on the spot. And what I want to insist upon is this, that all
well-regulated furnaces are gas retorts and combustion chambers combined.
You may talk of burning coal, but you can't do it; you must distill it
first, and you may either waste the gas so formed or you may burn it
properly. The thing is to let in not too much air, but just air enough.
Look, for instance, at Minton's oven for firing pottery. Round the central
chamber are the coal hoppers, and from each of these gas is distilled,
passes into the central chamber, where the ware is stacked, and meeting
with an adjusted supply of air as it rises, it burns in a large flame,
which extends through the whole space and swathes the material to be
heated. It makes its exit by a central hole in the floor, and thence rises
by flues to a common opening above. When these ovens are in thorough
action, nothing visible escapes. The smoke from ordinary potters' ovens is
in Staffordshire a familiar nuisance. In the Siemens gas producer and
furnace, of which Mr. Frederick Siemens has been good enough to lend me
this diagram, the gas is not made so closely on the spot, the gas retort
and furnace being separated by a hundred yards or so in order to give the
required propelling force. But the principle is the same; the coal is first
distilled, then burnt. But to get high temperature, the air supply to the
furnace must be heated, and there must be no excess. If this is carried on
by means of otherwise waste heat we have the regenerative principle, so
admirably applied by the Brothers Siemens, where the waste heat of the
products of combustion is used to heat the incoming air and gas supply. The
reversing arrangement by which the temperature of such a furnace can be
gradually worked up from ordinary flame temperature to something near the
dissociation point of gases, far above the melting point of steel, is well
known, and has already been described in this place. Mr. Siemens has lent
me this beautiful model of the most recent form of his furnace, showing its
application to steel making and to glass working.