oiler may have been employed for the
purpose.

A pound of coal or any other fuel has a definite heat producing
capacity, and is capable of evaporating a definite quantity of water
under given conditions. That is the limit beyond which even perfection
cannot go, and yet I have known, and doubtless you have heard of, cases
where inventors have claimed, and so-called engineers have certified to,
much higher results.

The first step in generating steam is in burning the fuel to the best
advantage. A pound of carbon will generate 14,500 British thermal units,
during combustion into carbonic dioxide, and this will be the same,
whatever the temperature or the rapidity at which the combustion may
take place. If possible, we might oxidize it at as slow a rate as that
with which iron rusts or wood rots in the open air, or we might burn it
with the rapidity of gunpowder, a ton in a second, yet the total heat
generated would be precisely the same. Again, we may keep the
temperature down to the lowest point at which combustion can take place,
by bringing large bodies of air in contact with it, or otherwise, or we
may supply it with just the right quantity of pure oxygen, and burn it
at a temperature approaching that of dissociation, and still the heat
units given off will be neither more nor less. It follows, therefore,
that great latitude in the manner or rapidity of combustion may be taken
without affecting the quantity of heat generated.

But in practice it is found that other considerations limit this
latitude, and that there are certain conditions necessary in order to
get the most available heat from a pound of coal. There are three ways,
and only three, in which the heat developed by the combustion of coal in
a steam boiler furnace may be expended.

1st, and principally. It should be conveyed to the water in the boiler,
and be utilized in the production of steam. To be perfect, a boiler
should so utilize all the heat of combustion, but there are no perfect
boilers.

2nd. A portion of the heat of combustion is conveyed up the chimney in
the waste gases. This is in proportion to the weight of the gases, and
the difference between their temperature and that of the air and coal
before they entered the fire.

3rd. Another portion is dissipated by radiation from the sides of the
furnace. In a stove the heat is all used in these latter two ways,
either it goes off through the chimney or is radiated into the
surrounding space