RADIATION. 65 



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 Doth be- 

 ing 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 va- 

 pours 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 transparent to light. Now the rays 

 of light differ from those of invisible heat merely in 

 point of period, the former failing to affect the retina 

 because their periods of recurrence are too slow. Hence, 

 in some way or other, the transparency of our gases and 

 vapours depends upon the periods of the waves which 

 impinge upon them. What is the nature of this de- 

 pendence? 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 periods of 

 recurrence synchronise with those of the atomic groups 

 amongst which they pass. Thus, when we find the in- 

 visible rays absorbed and the visible ones transmitted 

 by a layer of gas, we conclude that the oscillating 

 periods of the atoms constituting the gaseous molecules 

 coincide with those of the invisible, and not with those 

 of the visible spectrum. 



