The number of atoms present
remains unchanged. The transparency of the compound is quite equal to
that of the mixture prior to combination. No change is perceptible to
the eye, but the keen vision of experiment soon detects the fact that
the perfectly transparent and highly attenuated ammonia resembles pitch
or lampblack in its behavior to the rays of heat.
There is probably boldness, if not rashness, in the attempt to make
these ultra-sensible actions generally intelligible, and I may have
already transgressed the limits beyond which the writer of a familiar
article cannot profitably go. There may, however, be a remnant of
readers willing to accompany me, and for their sakes I proceed. A
hundred compounds might be named which, like the ammonia, are
transparent to light, but more or less opaque--often, indeed, intensely
opaque--to the rays of heat from obscure sources. Now the difference
between these latter rays and the light rays is purely a difference of
period of vibration. The vibrations in the case of light are more rapid,
and the ether waves which they produce are shorter, than in the case of
obscure heat. Why, then, should the ultra-red waves be intercepted by
bodies like ammonia, while the more rapidly recurrent waves of the whole
visible spectrum are allowed free transmission? The answer I hold to be
that, by the act of chemical combination, the vibrations of the
constituent atoms of the molecules are rendered so sluggish as to
synchronize with the motions of the longer waves.
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