190 J. E. Keeler—Absorption of Radiant Heat 
Art. XXIV.—On the Absorption of Radiant Heat by Carbor 
Dioxide; by J. E. KEELER. 
AMONG the gaseous bodies whose power of absorbing radiant. 
heat has engaged the attention of physicists, one of the greatest 
interest is carbon dioxide (CO,, commonly called carbonic acid 
gas), since as a constituent of our atmosphere, its*action has an 
immediate bearing upon matters whose importance to us is evi- 
dent. It would seem at first sight as if the determination of 
the absorptive power of a gas were a matter of no great diffi- 
culty; yet this is so far from being the case that most conflicting 
results, especially in regard to the absorption of water vapor, have 
been reached by different investigators, the discussion between 
Tyndall and Magnus on this subject being well known. Quite 
recently the question has been revived, ‘‘In which of the gases 
- composing the atmosphere does the power of absorbing the sun’s 
rays lie?” And, indeed, when we consider that, according to meas- 
urements made by different observers at different times and 
places, the action of any one of the principal constituents of the 
atmosphere is sufficient to account for this absorption, it can 
hardly as yet be regarded as settled. That carbon dioxide has 
a remarkable power of absorbing radiant heat has long beea 
known, and within the last few years, methods of analysis based 
upon this property have been invented. : 
e usual course pursued in investigating the subject is to 
allow radiant heat to pass through a stratum of gas of given 
thickness, and measure the amount stopped by it by com-. - 
paring the indication of a radiometric apparatus placed opposite — 
the source of heat before and after the interposition of the gas. 
In this way (considering the more recent experiments), E. Lecher 
and J. Pernter,* using as a radiant source a metal plate heated 
to 100° C. by steam and a gas stratum 31™ thick, found that 
whereas neither dry nor moist air produced any absorption. 
* Wied. Annalen, xii, p. 180. ' + Wied. Annalen, xii, p. 466. 
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