ave
direct current in the laboratory, attach the copper wires to
the two poles of a battery instead.

[Footnote 8: If the copper wire is drawn through a piece of
1/4-inch soft glass tubing so that only the platinum wire
projects from the end of the tube, and the tube is then sealed
around the platinum by holding it in a Bunsen burner a few
minutes, your electrodes will be more permanent and more
satisfactory. The pieces of glass tubing should be about 6
inches long (see Fig. 160).]

Bend the platinum electrodes up so that they will stick up
into the test tubes from below. Bubbles should immediately
begin to gather on the platinum wire and to rise in the test
tubes. As the test tubes fill with gas, the water is forced
out; so you can tell how much gas has collected at any time by
seeing how much water is left in each tube.

One tube should fill with gas twice as fast as the other. The
gas in this tube is hydrogen; there is twice as much hydrogen
as there is oxygen in water. The tube that fills more slowly
contains oxygen.

When the faster-filling tube is full of hydrogen--that is,
when all of the water has been forced out of it--take the
electrode out and let it hang loose in the glass. Put a piece
of cardboard about 1 inch square over the mouth of the test
tube; take the test tube out of the water and turn it right
side up, keeping it covered with the cardboard. Light a match,
remove the cardboard cover, and hold the match over the open
test tube. Does the hydrogen in it burn?

When the tube containing the oxygen is full, take it out,
covered, just as you did the hydrogen test tube. But in this
case make the end of a stick of charcoal glow, remove the
cardboard from the tube, and then plunge the glowing charcoal
into the test tube full of oxygen.

[Illustration: FIG. 160. The electrodes are made of loops of platinum
wire sealed in glass tubes.]

Only oxygen will make charcoal burst into flame like this.

When people found that they could take water apart in this way and
turn it into hydrogen and oxygen, and when they found that whenever
they combined hydrogen with oxygen they got water, they knew, of
course, that water was not an element. Maybe some day they will find
that some of the eighty-five or so substances that we now consider
elements can really be divided into two or more e