Page:On Radiation.djvu/64

 15. Influence of Vibrating Period and Molecular Form. Physical analysis of the human breath.

In the foregoing experiments with gases and vapours 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 rays 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 vapours 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 periods of recurrence synchronize with the periods of the molecules among which they pass. Thus, when we find the invisible rays absorbed