202 FRAGMENTS OF SCIENCE. 



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 ; some of these 

 bodies are so impervious that in lengths of a few feet only 

 they intercept every ray as effectually as a layer of pitch 

 would do. The substances, however, which show themselves 

 thus opaque to radiant heat are perfectly transparent to 

 light. Now the T&ys of light differ from those of invisible 

 heat only 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 vapors 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 cer- 

 tain definite rates of oscillation, and those waves of ether 

 are most copiously absorbed whose periods of recurrence 

 synchronize with the periods of the molecules among which 

 they pass. Thus, when we find the invisible rays absorbed 

 and the visible ones transmitted by a layer of gas, we con- 

 clude that the oscillating periods of 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 

 strokes of the waves, the vibration of the molecules aug- 

 ments, as a heavy pendulum is set in motion by well-timed 

 puffs of breath. Millions of millions of shocks are received 

 every second from the calorific waves, and it is not difficult 



