50 FRA GMENTS OF SCIENCE. 
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 synchronize with them. Thus it is 
that the tenth of an atmosphere of carbonic acid, enclosed 
in a tube four feet long, absorbs 60 per cent, of the radia- 
tion from a carbonic oxide flame, while one-thirtieth of an 
atmosphere absorbs 48 per cent, of the heat from the same 
sou rce. 
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 
ffhere effected by the carbonic acid of the breath amounts 
to 50 per cent, of the entire radiation. Radiant heat may 
indeed be employed us 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 car- 
bonic 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 chemically by Dr. 
Frankland, and physically by Mr. Barrett, gave the follow- 
ing results: 
Percentage of Carbonic Acid in the Human Breatli. 
Chemical analysis Physical analysis 
4.66 4.56 
5.33 5.22 
Jt is thus proved that in the quantity of ethereal 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. 
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 with the same readiness. To it 
the oscillations of an atom of free oxygen are just as 
