by a fall of temperature, but he did not know that that fall was due
to the conversion of heat into work. We should state this clause more
correctly by saying that "the cooling of the agent must be caused by
the external work it performs." In accordance with these propositions,
it is immaterial what the heated gases or vapors in the furnace of a
boiler may be, provided that they cool by doing external work and, in
passing over the boiler surfaces, impart their heat energy to the
water. The temperature of the furnace, it follows, must be kept as
high as possible. The process of combustion is usually complex. First,
in the case of coal, close to the fire-bars complete combustion of the
red hot carbon takes place, and the heat so developed distills the
volatile hydrocarbons and moisture in the upper layers of the fuel.
The inflammable gases ignite on or near the surface of the fuel, if
there be a sufficient supply of air, and burn with a bright flame for
a considerable distance around the boiler. If the layer of fuel be
thin, the carbonic acid formed in the first instance passes through
the fuel and mixes with the other gases. If, however, the layer of
fuel be thick, and the supply of air through the bars insufficient,
the carbonic acid is decomposed by the red hot coke, and twice the
volume of carbonic oxide is produced, and this, making its way through
the fuel, burns with a pale blue flame on the surface, the result, as
far as evolution of heat is concerned, being the same as if the
intermediate decomposition of carbonic acid had not taken place. This
property of coal has been taken advantage of by the late Sir W.
Siemens in his gas producer, where the supply of air is purposely
limited, in order that neither the hydrocarbons separated by
distillation, nor the carbonic oxide formed in the thick layer of
fuel, may be consumed in the producer, but remain in the form of crude
gas, to be utilized in his regenerative furnaces.