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.

The most remarkable and, at first sight, astounding thing about this
furnace is, however, that it works solely by radiation. The flames do not
touch the material to be heated; they burn above it, and radiate their heat
down to it. This I regard as one of the most important discoveries in the
whole subject, viz., that to get the highest temperature and greatest
economy out of the combustion of coal, one must work directly by radiant
heat only, all other heat being utilized indirectly to warm the air and gas
supply, and thus to raise the flame to an intensely high temperature.

It is easy to show the effect of supplying a common gas flame with warm air
by holding it over a cylinder packed with wire gauze which has been made
red hot. A common burner held over such a