300 FRAGMENTS 
No more striking or instructive illustration of the 
influence of coincidence could be adduced than that fur- 
nished by the radiation from a carbonic oxide flarne. 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, defiant 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 car- 
bonic acid at a temperature of 20 degrees C. and carbonic 
acid at a temperature of over 3,000 degrees 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 experi- 
ments how impossible it is to determine the effect of tem- 
perature 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 heating 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 vapors referred to at the com- 
mencement of this lecture with those of the ultra-red un- 
dulations. Hence, by gradually heating a platinum wire 
from darkness up to whiteness, we ought gradually to 
augment the discord between it and these vapors, and thus 
augment the transmission. Experiment entirely confirms 
this conclusion. Formic ether, 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 
