he radiation from
a carbonic oxide flame. Here the product of combustion is carbonic
acid; and on the radiation from this flame even the ordinary carbonic
acid of the atmosphere exerts a powerful effect. A quantity of the
gas, only one-thirtieth of an atmosphere in density, contained in a
polished brass tube four feet long, intercepts 50 per cent. of the
radiation from the carbonic oxide flame. For the heat emitted by
lampblack, olefiant gas is a far more powerful absorber than carbonic
acid; in fact, for such heat, with one exception, carbonic acid is the
most feeble absorber to be found among the compound gases. Moreover,
for the radiation from a hydrogen flame olefiant gas possesses twice
the absorbent power of carbonic acid, while for the radiation from the
carbonic oxide flame, at a common pressure of one inch of mercury, the
absorption by carbonic acid is more than twice that of olefiant gas.
Thus we establish the coincidence of period between carbonic acid at a
temperature of 20 deg.C. and carbonic acid at a temperature of over
3000 deg.C, the periods of oscillation of both the incandescent and the
cold gas belonging to the ultra-red portion of the spectrum.

It will be seen from the foregoing remarks and experiments how
impossible it is to determine the effect of temperature pure and
simple on the transmission of radiant heat if different sources of
heat be employed. Throughout such an examination the same oscillating
atoms ought to be retained. This is done by beating a platinum spiral
by an electric current, the temperature meanwhile varying between the
widest possible limits. Their comparative opacity to the ultra-red
rays shows the general accord of the oscillating periods of the
vapours referred to at the commencement of this lecture with those of
the ultra-red undulations. Hence, by gradually heating a platinum wire
from darkness up to whiteness, we ought gradually to augment the
discord between it and these vapours, and thus augment the
transmission. Experiment entirely confirms this conclusion. Formic
nether, for example, absorbs 45 per cent. of the radiation from a
platinum spiral heated to barely visible redness; 32 per cent. of the
radiation from the same spiral at a red heat; 26 per cent. of the
radiation from a white-hot spiral, and only 21 per cent. when the
spiral is brought near its point of fusion. Remarkable cases of
inversion as to transparency also occur. For barely visible redness
formic ae