owever this radiation may occur, since it would be a
volume phenomenon rather than a surface phenomenon it would be
considered somewhat differently from ordinary radiation. It might apply
as increasing the conductivity of the gas which, however independent of
radiation, is known to increase with the temperature. It is, therefore,
to be expected that at high temperatures the rate of transfer will be
greater than at low temperatures. The experimental determinations of
transfer rates at high temperatures are lacking.

Although comparatively nothing is known concerning the heat radiation
from gases at high temperatures, there is no question but what a large
proportion of the heat absorbed by a boiler is received direct as
radiation from the furnace. Experiments show that the lower row of tubes
of a Babcock & Wilcox boiler absorb heat at an average rate per square
foot of surface between the first baffle and the front headers
equivalent to the evaporation of from 50 to 75 pounds of water from and
at 212 degrees Fahrenheit per hour. Inasmuch as in these experiments no
separation could be made between the heat absorbed by the bottom of the
tube and that absorbed by the top, the average includes both maximum and
minimum rates for those particular tubes and it is fair to assume that
the portion of the tubes actually exposed to the furnace radiations
absorb heat at a higher rate. Part of this heat was, of course absorbed
by actual contact between the hot gases and the boiler heating surface.
A large portion of it, however, must have been due to radiation. Whether
this radiant heat came from the fire surface and the brickwork and
passed through the gases in the furnace with little or no absorption, or
whether, on the other hand, the radiation were absorbed by the furnace
gases and the heat received by the boiler was a secondary radiation from
the gases themselves and at a rate corresponding to the actual gas
temperature, is a question. If the radiations are direct, then the term
"furnace temperature", as usually used has no scientific meaning, for
obviously the temperature of the gas in the furnace would be entirely
different from the radiation temperature, even were it possible to
attach any significance to the term "radiation temperature", and it is
not possible to do this unless the radiations are what are known as
"full radiations" from a so-called "black body". If furnace radiation
takes place in this manner, the indications o