ears to
boil. If a splinter, having a spark on the end, is now inserted in the
tube, it is kindled into a flame. This shows the presence of _free_
oxygen, the heat having caused the potassium chlorate to decompose. The
difference between free and combined oxygen may also be shown by
decomposing other compounds of oxygen, such as water and mercuric oxide.

*Preparation and Properties of Oxygen.*--Intimately mix 3 grams (1/2
teaspoonful) of potassium chlorate with half its bulk of manganese
dioxide, and place the mixture in a large test tube. Close the test tube
with a tight-fitting stopper which bears a glass tube of sufficient length
and of the right shape to convey the escaping gas to a small trough or pan
partly filled with water, on the table. Fill four large-mouthed bottles
with water and, by covering with cardboard, invert each in the trough of
water. Arrange the test tube conveniently for heating, letting the end of
the glass tube terminate under the mouth of one of the bottles (Fig. 58).
Using an alcohol lamp or a Bunsen burner, heat over the greater portion of
the tube at first, but gradually concentrate the flame upon the mixture.
Do not heat too strongly, and when the gas is coming off rapidly, remove
the flame entirely, putting it back as the action slows down. After all
the bottles have been filled, remove the end of the glass tube from the
water, but leave the bottles of oxygen inverted in the trough until they
are to be used. On removing the bottles from the trough, keep the tops
covered with wet cardboard.

[Fig. 58]

Fig. 58--*Apparatus* for generating oxygen.

1. Examine a bottle of oxygen, noting its lack of color. Insert a small
burning splinter in the upper part of the bottle and observe the change in
the rate of burning. The air contains free oxygen, but it is diluted with
nitrogen. Compare this with the undiluted oxygen in the bottle as to
effect in causing the splinter to burn.

2. In a second bottle of oxygen insert a splinter without the flame, but
having a small spark on the end. As soon as the oxygen kindles the spark
into a flame, withdraw from the bottle and blow out the flame, but again
insert the spark. Repeat the experiment as long as the spark is kindled by
the oxygen into a flame. This experiment is usually performed as a test
for undiluted oxygen.

3. Make a hollow cavity in the end of a short piece of crayon. Fasten a
wire to t