fire is an improvement on a close stove. An open fire has faults, and it
certainly wastes heat up the chimney. A close stove may have more
faults--it wastes less _heat_, but it is liable to waste _gas_ up the
chimney--not necessarily visible or smoky gas; it may waste it from coke or
anthracite, as CO.

You now easily perceive the principles on which so-called smoke consumers
are based. They are all special arrangements or appendages to a furnace for
permitting complete combustion by satisfying the two conditions which had
been violated in its original construction. But there is this difficulty
about the air supply to a furnace: the needful amount is variable if the
stoking be intermittent, and if you let in more than the needful amount,
you are unnecessarily wasting heat and cooling the boiler, or whatever it
is, by a draught of cold air.

Every time a fresh shovelful is thrown on, a great production of gas
occurs, and if it is to flame it must have a correspondingly great supply
of air. After a time, when the mass has become red hot, it can get nearly
enough air through the bars. But at first the evolution of gas actually
checks the draught. But remember that although no smoke is visible from a
glowing mass, it by no means follows that its combustion is perfect. On an
open fire it probably is perfect, but not necessarily in a close stove or
furnace. If you diminish the supply of air much (as by clogging your
furnace bars and keeping the doors shut), you will be merely distilling
carbonic oxide up the chimney--a poisonous gas, of which probably a
considerable quantity is frequently given off from close stoves.

Now let us look at some smoke consumers. The diagrams show those of Chubb,
Growthorpe, Ireland and Lowndes, and of Gregory. You see that they all
admit air at the "bridge" or back of the fire, and that this air is warmed