For furnace work, where gas is needed in large quantities, it must be made 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.

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.