at emitting such rays
only, we should find carbonic acid more opaque to the radiation from
that source, than any other gas. Such a source is actually found in
the flame of carbonic oxide, where hot carbonic acid constitutes the
main radiating body. Of the rays emitted by our heated plate of
copper, olefiant gas absorbs ten times the quantity absorbed by
carbonic acid. Of the rays emitted by a carbonic oxide flame,
carbonic acid absorbs twice as much as olefiant gas. This wonderful
change in the power of the former, as an absorber, is simply due to
the fact, that the periods of the hot and cold carbonic acid are
identical, and that the waves from the flame freely transfer their
motion to the molecules which synchronise with them. Thus it is that
the tenth an atmosphere of carbonic acid, enclosed in a tube four feet
long, absorbs 60 per cent. of the radiation from carbonic oxide
flame, while one-thirtieth of an atmosphere absorbs 48 per cent. of
the heat from the same source.

In fact, the presence of the minutest quantity of carbonic acid may be
detected by its action on the rays from the carbonic oxide flame.
Carrying, for example, the dried human breath into a tube four feet
long, the absorption there effected by the carbonic acid of the breath
amounts to 50 per cent. of the entire radiation. Radiant heat may
indeed be employed as a means of determining practically the amount of
carbonic acid expired from the lungs. My late assistant, Mr. Barrett,
while under my direction, made this determination. The absorption
produced by the breath freed from its moisture, but retaining its
carbonic acid, was first determined. Carbonic acid, artificially
prepared, was then mixed with dry air in such proportions that the
action of the mixture upon the rays of heat was the same as that of
the dried breath. The percentage of the former being known,
immediately gave that of the latter. The same breath, analysed
chemically by Dr. Frankland, and physically by Mr. Barrett, gave the
following results:

Percentage of Carbonic Acid in the Human Breath.

Chemical analysis Physical analysis

4.66 4.56

5.33 5.22

It is thus proved that in the quantity of aethereal motion which it is
competent to take up, we have a practical measure of the carbonic acid
of the breath, and hence of the combustion going on in the human
lungs.

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