of it.

In Experiment 36, where you stoppered a test tube containing a little
water and then held the tube over a flame until the cork flew out, you
were causing an explosion. As the water changed to steam, the steam
was an expanding gas. It was at first confined to the test tube by the
cork. Then there was an explosion; the gas freed itself by blowing out
the cork.

Steam boilers have safety valves to prevent explosions. These are
valves so arranged that when the steam expands and presses hard enough
to endanger the boiler, the steam will open the valves and escape
instead of bursting the boiler to get free.

EXPLOSIVES. Dynamite, gunpowder, and most explosives are mixtures of
solids or liquids that will combine easily and will form gases that
expand greatly as a result of the combination. One of the essentials
in explosives is some compound of oxygen (such as the manganese dioxid
or potassium chlorate you used to make oxygen in Experiment 93) which
will easily set its oxygen free. This oxygen combines very swiftly
with something else in the explosive, releasing heat and forming a
gas that takes much more room. In its effort to free itself, this
expanding gas will blast rocks out of the way, shoot cannon balls, or
do any similar work.

But if gunpowder does not have to push anything of much importance out
of its way to expand, there is no explosion. That is why a firecracker
merely fizzes when you break it in two and light the powder. The
cardboard no longer confines the expanding gas; so there is nothing to
burst or to push violently out of the way.

Useful explosions are generally caused by a chemical action which
suddenly releases a great deal of heat and combines solid things into
expanding gases. But the bursting of a steam boiler, or the "blow
out" of an automobile tire, or the bursting of a potato in the oven,
although not caused by chemical action, still are real explosions. An
explosion is the _sudden_ release of a confined gas.

_APPLICATION 77._ Explain how gasoline makes a motorcycle go,
and why it goes "pop, pop, pop." Explain why a paper bag
will burst with a bang, when you blow it up and then clap it
between your hands; why a Fourth-of-July torpedo "goes off"
when you throw it on the pavement.

INFERENCE EXERCISE

Explain the following:

491. The engine of an automobile is cooled by the water that
passes over it from the radiator.

492. When you light a