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.

73. The Composition of Water. On testing the gases into which water
is broken up by an electric current, we find them to be quite
different. One proves to be oxygen, a substance with which we are
already familiar. The other gas, hydrogen, is new to us and is
interesting as being the lightest known substance, being even "lighter
than a feather."

An important fact about hydrogen is that in burning it gives as much
heat as five times its weight of coal. Its flame is blue and almost
invisible by daylight, but intensely hot. If fine platinum wire is
placed in an ordinary gas flame, it does not melt, but if placed in a
flame of burning hydrogen, it melts very quickly.

74. How to prepare Hydrogen. There are many different methods of
preparing hydrogen, but the easiest laboratory method is to pour
sulphuric acid, or hydrochloric acid, on zinc shavings and to collect