What makes a gun shoot?
What makes an automobile go?
Usually we think of explosions as harmful, and they often are, of
course. Yet without them we could no longer run automobiles; gasoline
launches would stop at once; motorcycles would no longer run; gasoline
engines for pumping water or running machinery would not be of any
use; and all aviation would immediately cease. Tunneling through
mountains, building roads in rocky places, taking up tree stumps,
and preparing hard ground for crops would all be made very much more
difficult. War would have to be carried on much as it was during
the Middle Ages; soldiers would use spears and bows and arrows;
battleships would be almost useless in attacking; modern forts would
be of little value; cannon, guns, rifles, howitzers, mortars, and
revolvers would all be so much junk.
[Illustration: FIG. 179. The explosion of 75 pounds of dynamite. A
"still" from a motion-picture film.]
[Illustration: FIG. 180. Diagram of the cylinder of an engine. The
piston is driven forward by the explosion of the gasoline in the
cylinder.]
WHAT MAKES AN AUTOMOBILE GO. In all the above cases the explosions
are caused by chemical action. When gasoline mixed with air is sprayed
into the cylinder of an automobile, an electric spark makes the
gasoline combine with the oxygen of the air; the gasoline suddenly
burns and changes to steam and carbon dioxid. As you already know,
when a liquid like gasoline turns to gases such as steam and carbon
dioxid, the gases take much more room. But that is not all that
happens. Much heat is released by the burning of the gasoline spray,
and heat causes expansion. So the gases formed by the burning gasoline
are still further expanded by the heat released by the burning.
Therefore they need a great deal more room; but they are shut up in a
small place in the top of a cylinder. The only thing to hold them up
in this small space, however, is a piston (Fig. 180), and the suddenly
expanding gases shove this piston down and escape. The piston is
attached to the drive wheel of the automobile, and when the piston is
pushed down it gives the automobile a push forward. If it were not for
the expansion of a gas in the cylinder, this gas being confined to a
small space, the piston would not be pushed down.
An explosion is simply the sudden pushing of a confined gas expanding
on its way to freedom. The gasoline vapor and air were the confined
gas. Their chemical combining made them expand; by pushing the piston
out of its way the newly formed gas suddenly freed itself. This was
an explosion, and it gave the automobile one forward push. But the
automobile engine is so arranged that the piston goes up into the
cylinder again, and is pulled down again, drawing a spray of gasoline
and air into the cylinder after it. Then it goes up a second time,
an electric spark explodes the gasoline, the piston is forced down
violently once more, and so it goes on. There are several cylinders,
of course, and the explosions take place within them one after the
other so as to keep the automobile going steadily.
HOW A GUN SHOOTS. Pulling a trigger makes a gun shoot by causing an
explosion. There is a spring on the hammer of a gun. This drives
the hammer down suddenly when you release the spring by pulling the
trigger. The hammer jars the chemicals in the cap and causes them to
explode. The heat and flame then cause the oxygen in the gunpowder to
combine with some of the other elements in the powder to make a gas.
The gas requires more room than the powder and is further expanded
by the heat released by the chemical change. The expanding gas frees
itself by pushing the bullet out of its way. The bullet gets such a
push through the exploding of the gunpowder that it may fly to a mark
and pierce it.
[Illustration: FIG. 181. The most powerful explosions on earth occur
in connection with volcanic activity. The photograph shows Mt. Lassen,
California, the only active volcano in the United States.]
There is a slight explosion even when you shoot an air gun. First you
compress some air in the upper part of the barrel of the air gun; then
you suddenly release it. The only thing in the way of the expanding
air is the bullet; so the air pushes this out in front 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 firecracker, the flame travels down the
wick until it reaches the gunpowder, and then the firecracker
bursts with a bang.
493. If you light a small pile of gunpowder in the open, as
you do when you make a squib by breaking the firecracker in
two, you merely have a blaze.
494. Air-filled tires make bicycles ride much more evenly than
solid tires would.
495. When clay has once been baked into brick, you can never
change it back to clay.
496. A photographic negative turns black all over if it is
exposed to the light before it is "fixed."
497. The outside of a window shade fades.
498. A vacuum electric lamp globe feels hot instantly when
turned on, but if turned off again at once, it immediately
feels cold.
499. Coal gas is made by heating coal very hot in an air-tight
chamber.
500. White straw turns yellow when it is long exposed to the
sunlight.
CHAPTER ELEVEN
SOLUTION AND CHEMICAL ACTION