shoveling on a quantity of coal and shutting it up completely. Every
condition of combustion is thus violated, and the intended furnace is a
mere gas retort.

_Gas Producers_.--Suppose the conditions of combustion are purposely
violated; we at once have a gas producer. That is all gas producers are,
extra bad stoves or furnaces, not always much worse than things which
pretend to serve for combustion. Consider how ordinary gas is made. There
is a red-hot retort or cylinder plunged in a furnace. Into this tube you
shovel a quantity of coal, which flames vigorously as long as the door is
open, but when it is full you shut the door, thus cutting off the supply of
air and extinguishing the flame. Gas is now simply distilled, and passes
along pipes to be purified and stored. You perceive at once that the
difference between a gas retort and an ordinary furnace with closed doors
and half choked fire bars is not very great. Consumption of smoke! It is
not smoke consumers you really want, it is fuel consumers. You distill your
fuel instead of burning it, in fully one-half, might I not say nine-tenths,
of existing furnaces and close stoves. But in an ordinary gas retort the
heat required to distill the gas is furnished by an outside fire; this is
only necessary when you require lighting gas, with no admixture of carbonic
acid and as little carbonic oxide as possible. If you wish for heating gas,
you need no outside fire; a small fire at the bottom of a mass of coal will
serve to distill it, and you will have most of the carbon also converted
into gas. Here, for instance, is Siemens' gas producer. The mass of coal is
burning at the bottom, with a very limited supply of air. The carbonic acid
formed rises over the glowing coke, and takes up another atom of carbon to
form the combustible gas carbonic oxide. This and the hot nitrogen passing
over and through the coal above distill away its volatile constituents, and
the whole mass of gas leaves by the exit pipe. Some art is needed in
adjusting the path of the gases distilled from the fresh coal with
reference to the hot mass below. If they pass too readily, and at too low a
temperature, to the exit pipe, this is apt to get choked with tar and dense
hydrocarbons. If it is carried down near or through the hot fuel below, the
hydrocarbons are decomposed over much, and the quality of the gas becomes
poor. Moreover, it is not possible to make the gases pass freely through a
mass of hot coke;