48 FRAGMENTS OF SCIENCE. 
small quantity of vapor, the warmed air communicates its 
heat by contact to the vapor, the molecules of which con- 
vert into the radiant form the heat imparted to them by 
the atoms of the air. By this process also, which I have 
called Dynamic Eadiation, the reciprocity of radiation and 
absorption has been conclusively proved.* 
In the excellent researches of Leslie, De la Provostaye 
and Desaius, and Balfour Stewart, the same reciprocity, 
as regards solid bodies, has been variously illustrated; 
while the labors, theoretical and experimental, of Kirch- 
hoff have given this subject a wonderful expansion, and 
enriched it by applications of the highest kind. To their 
results are now to be added the foregoing, whereby gases 
and vapors, which have been hitherto thought inaccessible 
to experiments with the thermo-electric pile, are proved by 
it to exhibit the indissoluble duality of radiation and 
absorption, the influence of chemical combination on both 
being exhibited in the most decisive and extraordinary 
way. 
15. Influence of Vibrating Period and Molecular Form. 
Physical Analysis of the Human Breath. 
In the foregoing experiments with gases and vapors we 
have employed throughout invisible rays, and found some 
of these bodies so impervious to radiant heat, that in 
lengths of a few feet they intercept every ray as effectually 
as a layer of pitch. The substances, however, which show 
themselves thus opaque to radiant heat are perfectly trans- 
parent to light. Now the rays of light differ from those 
of invisible heat merely in point of peliod, the former fail- 
ing to affect the retina because their periods of recurrence 
are too slow. Hence, in some way or other, the trans- 
parency of our gases and vapors depends upon the periods 
of the waves which impinge upon them. What is the 
nature of this dependence ? The admirable researches of 
Kirchhoff help us to an answer. The atoms and molecules 
of every gas have certain definite rates of oscillation, and 
those waves of ether are most copiously absorbed whose 
* When heated air imparts its motion to another gas or vapor, the 
transference of heat is accompanied by a change of vibrating period. 
The Dynamic Radiation of vapors is rendered possible by this tra,ns- 
wutation. of vibrations, 
