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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. N ow the rays of light differ from those of invisible
heat only in point of period, theformer 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
pufi's of breath. Millions of millions of shocks are received
every second from the calorific waves, and it is not difficult

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