RADIATION. 65 



periments 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 extra- 

 ordinary way. 



15. Influence of Vibrating Period and Molecluat 

 Form. Physical Analysis of the Human Breath. 



In the foregoing experiments with gases and vapours 

 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, how- 

 ever, 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 depend- 

 ence ? 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 invisible rays absorbed and the visible ones trans- 

 mitted 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. 



