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to see that, as every wave arrives just in time to repeat the
action of its predeCessor, the molecules must finally be
caused to swing through wider spaces than if the arrivals
Were not so timed. In fact, it is not difficult to see that an
assemblage of molecules, operated upon by contending
waves, might remain practically quiescent, and this is act-
ually the case when the waves of the visible spectrum pass
through a transparent gas or vapor. There is here no sen-
sible transference of motion from the ether to the molecules;
in other words, there is no sensible absorption of heat.

One striking example of the influence of period may here
be recorded. Carbonic-acid gas is one of the feeblest of
absorbers of the radiant heat emitted by solid sources. It
is, for example, to a great extent transparent to the rays
emitted by the heated copper-plate. already referred to.
There are, however, certain rays, comparatively few in num-
ber, emitted by the copper, to which the carbonic acid is
impervious ; and could we obtain a source of heat emitting
such rays only, We should find carbonic acid more opaque to
the radiation from that source than any other gas. Such a
source is actually found in the flame of carbonic oxide,
where hot- carbonic acid constitutes the main radiating body.
Of the rays emitted by our heated plate of copper, olefiant
gas absorbs ten times the quantity absorbed by carbonic
acid. Of the rays emitted by a carbonic-oxide flame, car-
bonic acid absorbstwice as much as olefiant gas. This won-
derful change in the power of the former as an absorber is
simply due to the fact that the periods of the hot and cold
carbonic acid are identical, and that the waves from the
flame freely transfer their motion to the molecules which
synchronize with them. Thus it is that the tenth of an at-
mosphere of carbonic acid, enclosed in a tube four feet long,
absorbs 60 per cent. of the radiation from a carbonic-oxide
flame, while one-thirtieth of an atmOSphere absorbs 48 per
cent. of the heat from the same origin.

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