in order that it shall be so intensely hot when the air enters that
the air shall instantly be heated to the same degree of tenuity as the
hot gases themselves, and the two will then unite like a flash--and
that is heat. And here is the solution of the Wye Williams mystery of
failure when cold air was introduced upon the top of a fire to aid
combustion. The proof of the necessity for heat to aid the chemical
assimilation of the volatilized coal elements is seen in starting a
fire in a common stove. At first there is only a blue flame, in which
the hand may be held; but wait until the lining becomes white hot, and
then throw on a little coal, and you will find a totally different
result. It is also seen in the Siemens gas furnace, with which you are
doubtless familiar. There is the introduction of gas with its
necessary complement of air. Until the furnace and retorts become
heated, the air and gas flutter through only partially united, and do
little good; but as soon as the retorts and furnace become thoroughly
hot, the same gas and air will melt a fire-brick.

These are common phenomena, which are familiar, but apt to be
unnoticed; but they logically point to the truth that no furnaces
should present a cooling medium in contact with fuel which is
undergoing this process of digestion, so to speak. It will be very
evident, I think, from these facts that water-legs in direct contact
with a fire are a mistake. They tend to check a fire as far as their
influence extends, as a thin sheet of ice upon the stomach after
dinner would check digestion, and for the same reason, namely, the
abstraction of heat from a chemical process. If fire-brick could be
laid around a locomotive furnace, and the grate, of course, kept of
the same area as before, it is my belief that a very important
advantage would be at once apparent. An old-fashioned cast iron heater
always produced a treacherous fire. It would grow dead around the
outside next to the cold iron; but put a fire-clay lining into it, and
it was as good as any other stove.

If I have now made clear what I mean by making heat, we will next
consider the steam boiler. What is a steam boiler? It is a thing to
absorb heat. The bottom line of this science is the bottom of a pot
over a fire, which is the best boiler surface in the world; there is
water upon one side of a piece of iron and heat against the other. One
square foot of the iron will transmit through it a given number of
uni