RADIATION 71 



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 effect- 

 ually 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 for- 

 mer 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 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 mole- 

 cules of every gas have certain definite rates of oscillation, 

 and those waves of ether are most copiously absorbed 

 whose periods of recurrence synchronize with those of the 

 atomic groups among 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. 



It requires some discipline of the imagination to form 

 a clear picture of this process. Such a picture is, however, 

 possible, and ought to be obtained. "When the waves of 

 ether impinge upon molecules- whose periods of vibration 

 coincide with the recurrence of the undulations, the timed 



