These atoms are so
small, and, when grouped to molecules, are so tightly clasped together,
that they are capable of tremors equal in rapidity to those of light and
radiant heat. To a mind coming freshly to these subjects, the numbers
with which scientific men here habitually deal must appear utterly
fantastical; and yet, to minds trained in the logic of science, they
express most sober and certain truth. The constituent atoms of molecules
can vibrate to and fro millions of millions of times in a second. The
waves of light and of radiant heat follow each other at similar rates
through the luminiferous ether. Further, the atoms of different
molecules are held together with varying degrees of tightness--they are
tuned, as it were, to notes of different pitch. Suppose, then,
light-waves, or heat-waves, to impinge upon an assemblage of such
molecules, what may be expected to occur? The same as what occurs when a
piano is opened and sung into. The waves of sound select the strings
which respectively respond to them--the strings, that is to say, whose
rates of vibration are the same as their own--and of the general series
of strings these only sound. The vibratory motion of the voice, imparted
first to the air, is here taken up by the strings. It may be regarded as
_absorbed_, each string constituting itself thereby a new center of
motion. Thus also, as regards the tightly locked atoms of molecules on
which waves of light or radiant heat impinge.
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