The Composition of Water 1
Water was long thought to be a simple substance, but toward the end of the eighteenth century it was found to consist of two quite different substances, oxygen (O) and hydrogen (H.)
If we send an electric current through water (acidulated to make it a good conductor), as shown in Figure, 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.
FIG. - The decomposition of water.