[Illustration: FIG. 39.--The decomposition of water.]
If we send an electric current through water (acidulated to make it a
good conductor), as shown in Figure 39, we see bubbles of gas rising
from the end of the wire by which the current enters the water, and
other bubbles of gas rising from the end of the wire by which the
current leaves the water. These gases have evidently come from the
water and are the substances of which it is composed, because the
water begins to disappear as the gases are formed. If we place over
each end of the wire an inverted jar filled with water, the gases are
easily collected. The first thing we notice is that there is always
twice as much of one gas as of the other; that is, water is composed
of two substances, one of which is always present in twice as large
quantities as the other.
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