I must leave it to others to describe the progress of that splendid
series of spectroscopic discoveries by which the chemistry of the
heavenly bodies has been brought within the range of human inquiry. I
wish rather to direct your attention to the fact that, not only has
every molecule of terrestrial hydrogen the same system of periods of
free vibration, but that the spectroscopic examination of the light of
the sun and stars shews that, in regions the distance of which we can
only feebly imagine, there are molecules vibrating in as exact unison
with the molecules of terrestrial hydrogen as two tuning-forks tuned
to concert pitch, or two watches regulated to solar time.
Now this absolute equality in the magnitude of quantities, occurring
in all parts of the universe, is worth our consideration.
The dimensions of individual natural bodies are either quite
indeterminate, as in the case of planets, stones, trees, &c., or they
vary within moderate limits, as in the case of seeds, eggs, &c.; but
even in these cases small quantitative differences are met with which
do not interfere with the essential properties of the body.
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