and is said
to be very efficient.

In a close stove and in many furnaces the second condition is violated;
there is an insufficient supply of air; fresh coal is put on, and the
feeding doors are shut. Gas is distilled off, but where is it to get any
air from? How on earth can it be expected to burn? Whether it be expected
or not, it certainly does not burn, and such a stove is nothing else than a
gas works, making crude gas, and wasting it--it is a soot and smoke
factory.

Most slow combustion stoves are apt to err in this way; you make the
combustion slow by cutting off air, and you run the risk of stopping the
combustion altogether. When you wish a stove to burn better, it is
customary to open a trap door below the fuel; this makes the red hot mass
glow more vigorously, but the oxygen will soon become CO_{2}, and be unable
to burn the gas.

The right way to check the ardor of a stove is not to shut off the air
supply and make it distill its gases unconsumed, but to admit so much air
above the fire that the draught is checked by the chimney ceasing to draw
so fiercely. You at the same time secure better ventilation; and if the
fire becomes visible to the room so much the better and more cheerful. But
if you open up the top of a stove like this, it becomes, to all intents and
purposes, an open fire. Quite so, and in many respects, therefore, an open
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 thro