POWDER.
Powder is a very harmless thing of itself. You can take it into your
hand and it will not hurt you; but touch it with fire, and it flashes
and explodes; and if there is much of it, it breaks every thing in
pieces all around it. When a magazine or a powder-mill blows up, there
is great destruction of every thing that is near.
You know that powder is used in blasting rocks. A hole is drilled and
the powder is put in. The blaster lights something which will burn very
slowly down to the powder, so that he may have time to get out of the
way. When the powder explodes, the rock is all broken apart into large
and small pieces.
[Sidenote: Powder produces its effects by changing into gas.]
Now, how is it that the powder does all this? It does it by changing
all at once into a great quantity of gas. That is all. When you look
at some powder, a heap of black grains, there is no gas in it; but the
moment that the fire touches it the powder is all gone. But how? Has
it become nothing? No; it is changed into something else. The black
powder is chiefly gas now. It is not all gas; if it were, you could not
see it. The smoke that you see is gas, with something else from the
burning powder mixed with it. This gas pushes out every way as soon as
it is made, so that it may get room, and it does it so quickly that it
carries every thing before it. It does the same that the air does when
it moves very quickly, only it moves a great deal more quickly, and so
does a great deal more.
[Sidenote: Boy blasting a log.]
This changing of powder into gas is done very quickly--as quick as a
flash, as we say. I knew a boy that once forgot this in using some
powder. He put some powder into a log of wood in order to split it;
but, instead of fixing a slow match, as men do in blasting rocks, he
touched off the powder, intending to get out of the way by running. But
the powder was, of course, too quick for him. It blew him over, burning
him a little, and frightening him a great deal.
[Sidenote: Bursting of a steam-engine.]
Sometimes water is changed into steam so quickly that it is like the
changing of powder into gas in its effects. This is seen in the way
that the boiler of a steam-engine is sometimes burst, as I will explain
to you. By carelessness, there is not a proper supply of water in it.
The fire will, of course, heat the boiler very hot. Now see what must
be the consequence when more water is let into it. The boiler, being so
very hot, changes this fresh supply of water all at once into steam,
and you know it takes but little water to make considerable steam, just
as it takes but little powder to make a great deal of gas. All this
steam so suddenly made acts precisely like the gas made by burning
powder. It must have room, and as there is not room enough for it in
the boiler, it must get out somewhere. The strong boiler can not hold
so much steam in, and it bursts.
But perhaps you will ask, Is it nothing but air or gas that throws the
ball out of the cannon, or the bullet out of the gun, so fast that
you can not see it? Can such a light, thin thing as gas drive a ball
through even thick beams of wood? Yes, the gas that the powder turns
into can do all this.
[Sidenote: How powder sends the ball out of a cannon.]
Now see the reason why the powder and the ball must be put into a
cannon to do this. If the powder should be laid upon the ground, with
the ball lying upon it, and fire should be touched to it, there would
not be much of a sound, and the ball would not be moved much. Why?
Because the gas that the powder turns into has a chance to escape
in every direction; but when the powder and the ball are put into a
cannon, the gas is all shut in, so that it can escape but one way,
instead of every way, as it did when the powder was on the ground. It
goes out of the mouth of the cannon, pushing the ball before it. It
does to the ball just what the air does to you when it blows against
you and pushes you along. It is a very hard blowing of gas that throws
out the ball so fast. The gas is made all at once, as I have before
told you, and it must find room somewhere. There is not room for it in
the cannon, and in going out to find room it throws the ball out.
If you should blow a little ball of paper from your mouth, it would not
go far. This is for the same reason that a ball laid upon a heap of
powder is not moved much when the powder is exploded. But put the paper
ball into a quill, and blow through it, and you can send it across a
room quite swiftly. The reason is, that the air which you blow out can
escape only through the quill, just as it is with the gas in the cannon.
When the gas comes out of the mouth of the cannon, it spreads out in
all directions, because it has room now. It is exactly as it is with
a crowd of people coming through a door; as fast as the crowd gets
through, it spreads out.
[Sidenote: How rocks are blasted.]
Observe, now, how rocks are rent in pieces in blasting. Quite a large
hole is drilled into the rock. It is like the space in the barrel of
a gun when it is done. This is filled with powder. Why, now, when the
powder explodes, does not the gas come out of this in the same way that
it does out of a cannon or a gun? Why, instead of this, does it break
the rock in pieces? It is because the hole is not large enough for so
much gas to come out. If we should put as little powder into it as we
do into a gun, the gas would all come out, as it does out of a gun,
without breaking the rock at all; but it is filled quite full of the
powder, and so a great deal of gas is made. If we should put as much
powder into a gun, it would burst like the rock, because there would
not be room enough for the escape of so much gas unless it went out
slowly, and that it will not do.
[Sidenote: How a rocket goes up in the air.]
[Sidenote: The going up of a rocket compared to jumping and flying.]
Powder is used in various ways. Some kinds of fire-works are made in
such a way that the powder does not burn all at once, as it does in
a gun or cannon. You know that when a rocket goes up, it is not sent
up by one blast of the powder, as a ball is sent out of a gun. The
powder is placed in the tail of the rocket, which is so made that the
powder burns all the time that it is going up, the last of it making
an explosion high up in the air, scattering the sparks which fall in
so beautiful a shower. Now, did you ever think just how it is that the
rocket is made to go up so swiftly? It is the gas of the burning powder
which streams out from its tail all the time that makes it go up. This
pushes down against the air, and it is the resistance of the air to
this that raises the rocket. It is just as the resistance of the air
to the downward stroke of the bird’s wings raises the bird. It is also
just as, in jumping up off the ground, the resistance of the ground to
your feet makes you go up. You press with your feet on the ground, and
so the rocket presses with its gas on the air; and so long as gas keeps
coming out of its tail to press on the air, the rocket keeps going up.
When the gas is exhausted the rocket comes down.
[Illustration]
You have sometimes seen whirling wheels in fire-works. The powder in
the wheel is arranged as you see here; and as it burns, the resistance
of the air to the gas makes the wheel fly around, for the same reason
that it makes the rocket go up into the air.
_Questions._--What is said about powder when no fire touches it? How
is the power of burning powder shown? How are its effects produced?
What is the smoke from powder? What is said about the quickness with
which powder changes into gas? Tell about the boy that split a log of
wood with powder. Give the comparison about steam. How is it that the
gas made by burning powder makes a ball go out of a cannon or gun so
swiftly? Give the comparison of the quill and the ball of paper. Why
does the gas from a cannon spread after it gets out? Tell what is said
about blasting rocks. Explain how a rocket is made to go up in the
air. What is the comparison about flying and about jumping? What is
said about the whirling wheel in fire-works?