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In fact, the presence of the minutest quantity of car-
bonic 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 efi'ected. 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, has made this determination.
The absorption produced by the breath freed from its moist-
ure, 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 analyzed chemi-
cally by Dr. Frankland, and physically by Mr. Barrett, gave
the followingaresults :

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 ethereal motion
which it is competent to take up, we have a practical meas-
ure of the carbonic acid of the breath, and hence of the
combustion going on in the human lungs.

Still this question of period, though of the utmost im-
portance, is not competent to account for the whole of the
observed facts. The ether, as far as we know, accepts
vibrations of all periods Withthe same readiness. To it
the oscillations of an atom of oxygen are just as acceptable
as those of a molecule of olefiant gas; that the vibrating
oxygen then stands so far below the olefiant gas in radiant

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