the gas distilled off is for a longtime much too cold for ignition, and
when it does catch fire it is too mixed with carbonic acid to burn
completely or steadily. In order to satisfy the first condition better, and
keep the gases at a higher temperature, Dr. Pridgin Teale arranges a
sloping fire-clay slab above his fire. On this the gases play, and its
temperature helps them to ignite. It also acts as a radiator, 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