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 Radiation, the reciprocity of radiation and 

 absorption has been conclusively proved.* 



In the excellent researches of Leslie, De la Provostaye 

 and Desains, 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 period, 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 trans- 

 mutation of vibrations. 



