The idea of atoms proved an early want on the part of minds in pursuit
of the knowledge of nature. It has never been relinquished, and in our
own day it is growing steadily in power and precision.
The union of bodies in fixed and multiple proportions constitutes the
basis of modern atomic theory. The same compound retains, for ever, the
same elements, in an unalterable ratio. We cannot produce pure water
containing one part, by weight, of hydrogen and nine of oxygen, nor can
we produce it when the ratio is one to ten; but we can produce it from
the ratio of one to eight, and from no other. So also when water is
decomposed by the electric current, the proportion, as regards volumes,
is as fixed as in the case of weights. Two volumes of hydrogen and one
of oxygen invariably go the formation of water. Number and harmony, as
in the Pythagorean system, are everywhere dominant in this under-world.
Following the discovery of fixed proportions we have that of _multiple_
proportions. For the same compound, as above stated, the elementary
factors are constant; but one elementary body often unites with another
so as to form different compounds. Water, for example, is an oxide of
hydrogen; but a peroxide of that substance also exists, containing
exactly double the quantity of oxygen. Nitrogen also unites with oxygen
in various ratios, but not in all.
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