What makes smoke?
What makes fire burn?
Why does air keep us alive?
Why does an apple turn brown after you peel it?
If oxygen should suddenly lose its power of combining with other
things to form compounds, every fire in the world would go out at
once. You could go on breathing at first, but your breathing would be
useless. You would shiver, begin to struggle, and death would come,
all within a minute or two. Plants and trees would die, but they would
remain standing until blown down by the wind. Even the fish in the
water would all die in a few minutes,--more quickly than they usually
do when we take them out of the water. In a very short time everything
in the world would be dead.
Then suppose that this condition lasted for hundreds and hundreds of
years, the oxygen remaining unable to combine with other elements.
During all that time nothing would decay. The trees would stay as they
fell. The corpses of people would dry and shrivel, but they would lie
where they dropped as perfectly preserved as the best of mummies. The
dead fish would float about in the oceans and lakes.
This is all because life is kept up by burning. And burning is simply
the combining of different things with oxygen. If oxygen ceased to
combine with the wood or gas or whatever fuel you use, that fuel could
not burn; how could it when "burning" _means_ combining with oxygen?
The heat in your body and the energy with which you move come entirely
from the burning (oxidation) of materials in your body; and that is
why you have to breathe; you need to get more and more oxygen into
your body all the time to combine with the carbon and hydrogen in the
cells of which your body is made. Plants breathe, too. They do not
need so much oxygen, since they do not keep warm and do not move
around; but each plant cell needs oxygen to live; there is burning
(oxidation) going on in every living cell. Fishes breathe oxygen
through their gills, absorbing the oxygen that is dissolved in the
water. They do not take the water apart to get some of the combined
oxygen from it; there is always some free oxygen dissolved in any
water that is open to the air. It is clear that fires would all go out
and everything would die if burning (combining with oxygen) stopped.
The reason things would not decay is that decay usually is a slow
kind of oxidation (burning). When it is not this, it is the action
of bacteria. But bacteria themselves could not live if they had _no_
oxygen; so they could not make things decay.
Not only would the dead plants and animals remain in good condition,
but the clothes people were wearing when they dropped dead would stay
unfaded and bright colored through all the storms and sunshine. And
the iron poles and car tracks and window bars would remain unrusted.
For bleaching and rusting are slow kinds of oxidation or burning
(combining with oxygen).
Here are two experiments which show that you cannot make things burn
unless you have oxygen to combine with them:
EXPERIMENT 94. Light a candle not more than 4 inches long and
stand it on the plate of the air pump. Cover it with the bell
jar and pump the air out. What happens to the flame?
EXPERIMENT 95. Fasten a piece of candle 3 or 4 inches long to
the bottom of a pan. Pour water into the pan until it is about
an inch deep. Light the candle. Turn an empty milk bottle
upside down over the candle. Watch the flame. Leave the bottle
over the candle until the bottle cools. Watch the water around
the bottom of the bottle. Lift the bottle partly out of the
water, keeping the mouth under water.
The bubbles that came out for a few seconds at the beginning of the
experiment were caused by the air in the bottle being heated and
expanded by the flame. Soon, however, the oxygen in the air was used
so fast that it made up for this expansion, and the bubbles stopped
going out. When practically all the oxygen was used, the flame went
out.
The candle is made mostly of a combination of hydrogen and carbon. The
hydrogen combines with part of the oxygen in the air that is in the
bottle to form a little water. The carbon combines with the rest of
the oxygen to make carbon dioxid, much of which dissolves in the water
below. So there is practically empty space in the bottle where the
oxygen was, and the air outside forces the water up into this space.
The rest of the bottle is filled with the nitrogen that was in the air
and that has remained unchanged.
About how much of the air was oxygen is indicated by the space that
the water filled after the oxygen was combined with the candle.
[Illustration: FIG. 167. The water rises in the bottle after the
burning candle uses up the oxygen.]
CARBON AND HYDROGEN THE CHIEF ELEMENTS IN FUEL. Carbon and hydrogen
make up the larger part of almost every substance that is used for
fuel, including gas, gasoline, wood, and soft coal; alcohol, crude
oil, kerosene, paper, peat, and the acetylene used in automobile and
bicycle lamps. Hard coal, coke, and charcoal are, however, chiefly
plain carbon. Since burning is simply the combining of things with
oxygen, it is plain that when the carbon of fuel joins oxygen we shall
get carbon dioxid (CO_2). When the hydrogen in the fuel joins oxygen,
what must we get?
When things do not burn up completely, the carbon may be left behind
as charcoal. That is what happens when food "burns" on the stove.
But if anything burns up entirely, the carbon or charcoal burns too,
passing off as the invisible gas, carbon dioxid, just as the hydrogen
burns to form steam or water.
It is because almost every fuel forms water when it burns, that we
find drops of water gathering on the outside of a cold kettle or cold
flatiron if either is put directly over a flame. The hydrogen in the
fuel combines with the oxygen of the air to form steam. As the steam
strikes the cold kettle or iron, it condenses and forms drops of
water.
NOTHING EVER DESTROYED. One important result of the discovery that
burning is only a combining of oxygen with the fuel was that people
began to see that nothing is ever destroyed. There is exactly as much
carbon in the carbon dioxid that floats off from a fire as there was
in the wood that was burned up; and there is exactly as much hydrogen
in the water vapor that floats off from the fire as there was in the
wood. Chemists have caught all the carbon dioxid and the water vapor
and weighed them and added their weight to the weight of the ashes;
and they have found them to weigh even more than the original piece of
wood, because of the presence of the oxygen that combined with them in
the burning.
If everything in the world were to burn up, using the oxygen that is
already here, the world would not weigh one ounce more or less than it
does now. All the elements that were here before would still be here;
but they would be combined in different compounds. Instead of wood
and coal and oxygen we should have water and carbon dioxid; instead of
diamonds, we should have just carbon dioxid; and so on with everything
that can burn.
WHY WATER PUTS OUT A FIRE. Water puts out a fire because it will not
let enough free oxygen get to the wood, or whatever is burning, to
combine with it. The oxygen that is locked up in a compound, like
water, you remember, has lost its ability to combine with other
things. Sand puts out a fire in the same way that water does. Most
fire extinguishers make a foam of carbon dioxid (CO_2) which covers
the burning material and keeps the free oxygen in the air from coming
near enough to combine with it.
Water will not put out burning oil, however, as the oil floats up on
top of the water and still combines with the oxygen in the air.
WHY ELECTRIC LAMPS ARE USUALLY VACUUMS. Electric lamps usually have
vacuums inside because the filament gets so hot that it would burn up
if there were any oxygen to combine with it. But in a globe containing
no oxygen the filament may be made ever so hot and it cannot possibly
burn.
High-power electric lamps are not made with vacuums but are
"gas-filled." The gas that is oftenest put into lamps is
nitrogen,--the same gas that is mixed with the oxygen in air. By
taking all the oxygen out of a quantity of air, the lamp manufacturers
can use in perfect safety the nitrogen that is left. It will not
combine with the glowing filament. There is no oxygen to combine with
the filament; so the lamp does not burn out.
WHAT FLAMES ARE. When you look at a flame, it seems as if fire were
a real thing and not merely a process of combining something with
oxygen. The flame is a real thing. It is made up of hot gases,
rising from the hot fuel, and it is usually filled with tiny glowing
particles of carbon. When you burn a piece of wood, the heat partly
separates its elements, just as heating sugar separates the carbon
from the water. Some of the hydrogen gas in the wood and some of the
carbon too are separated from the wood by the heat. These are pushed
up by the cooler air around and combine with the oxygen as they rise.
The hydrogen combines more easily than the carbon; part of the carbon
may remain behind as charcoal if your wood does not all burn up,
and many of the smaller carbon particles only glow in the burning
hydrogen, instead of burning. That is what makes the flame yellow. If
you hold anything white over a yellow flame, it will soon be covered
with black soot, which is carbon.
WHAT SMOKE IS. Smoke consists mostly of little specks of unburned
carbon. That is why it is gray or black. When you have black smoke,
you may always be sure that some of the carbon particles are not
combining properly with oxygen.
Yellow flames are usually smoky; that is, they usually are full of
unburned bits of carbon that float off above the flame. But by letting
enough air in with the flame, it is possible to make all the little
pieces of carbon burn (combine with the oxygen of the air) before they
leave the heat of the burning hydrogen. That is why kerosene lamps do
not smoke when the chimney is on. The chimney keeps all the hot gases
together, and this causes a draft of fresh air to blow up the chimney
to push the hot gases on up. The fresh air blowing up past the flame
gives plenty of oxygen to combine with the carbon. The drum part of
an oil heater acts in the same way; when the drum is open, the heater
smokes badly; when it is closed up, enough air goes past the flame
to burn up all the carbon. But if you turn either lamp or heater
too high, it will smoke anyway; you cannot get enough air through to
combine with all the carbon.
The hottest flames are the blue flames. That is because in a blue
flame all the carbon is burning up along with the hydrogen of the
fuel--both are combining with the oxygen of the air as rapidly as
possible. A gas or gasoline stove is so arranged that air is fed
into the burner with the gas. You will see this in the following
experiment:
EXPERIMENT 96. Light the Bunsen burner in the laboratory. Open
wide the little valve in the bottom. Now put your finger and
thumb over the hole in the bottom of the burner. What happens
to the flame? Turn the valve so that it will close the hole
in the same way. Now hold a white saucer over the flame, being
careful not to get it hot enough to break. What is the black
stuff on the bottom of the saucer?
Examine the gas plate (small gas stove) in the laboratory.
Light it, and see if you can find the place where the air is
fed in with the gas. Close this place and see what happens.
Open it wider and see what happens. If the air opening is too
large, the flame "blows"; there is too much cold air coming in
with the gas, and your flame is not as hot as it would be if
it did not "blow." Also, the stove is liable to "back-fire"
(catch fire at the air opening) when the air opening is too
wide.
[Illustration: FIG. 168. The Bunsen burner smokes when the air holes are
closed.]
_APPLICATION 69._ An oil lamp tipped over and the burning oil
spread over the floor. Near by were a pail of water, a pan of
ashes, a rug, and a seltzer siphon. Which of these might have
been used to advantage in putting out the fire?
_APPLICATION 70._ My finger was burned. I wanted the flesh
around it to heal and new skin cells to live and grow rapidly
around the burn.
"Put a rubber finger cot on the finger and keep all air out,"
one friend advised me. "Air causes decay and will therefore be
bad for the burn."
"He's wrong; you should bandage it with clean cloth; you want
air to reach the finger, I've heard," said another friend.
"Oh, no, you don't; air makes things burn, and the burn will
therefore get worse," still another one said. What should I
have done?
_APPLICATION 71._ Two students were discussing how coal was
formed.
"The trees must have fallen into water and been completely
covered by it, or they would have decayed," said one.
"Water makes things decay more quickly; there must have been
a drought and the trees must have fallen on dry ground," said
the second.
Which was right?
[Illustration: FIG. 169. Regulating the air opening in a gas stove.]
_APPLICATION 72._ A gas stove had a yellow flame. In front,
by the handles, was a metal disk with holes so arranged that
turning it to the left allowed air to mix with the gas on the
way to the flame, and turning it to the right shut the air off
(see Fig. 170).
One member of the family said, "Turn the disk to the left and
let more air mix with the gas."
But another objected. "It has too much air already; that's why
the flame is yellow. Turn it to the right and shut off the air
from below."
"You're both wrong. Why do you want to change it?" said a
third member of the family. "The yellow flame is the hottest,
anyway. Can't you see that the yellow flame gives more light?
And don't you know that light is just a kind of radiant heat?
Of course the yellow flame is the hottest. Leave the stove
alone."
Who was right?
[Illustration: FIG. 170. The air openings in the front of a gas stove.]
INFERENCE EXERCISE
Explain the following:
441. Iron tracks are welded together with an electric arc.
442. The cool mirror in a bathroom becomes covered with
moisture when you take a hot bath.
443. This prevents you from seeing yourself in the mirror.
444. Carbon dioxid has oxygen in it, yet a burning match
dropped into a bottle of it will go out.
445. A ship that sinks to the bottom of the ocean does not
decay.
446. When women put their hair in curlers, they usually
moisten the hair slightly.
447. To dry a pan after washing it, a person often sets it on
the hot stove for a few minutes.
448. When you put a kettle of cold water over a gas flame,
drops of water appear on the lower part of the sides of the
kettle.
449. Electric power plants are often situated where running
water will turn the dynamo. Explain the necessity of turning
the dynamo.
450. We make carbon dioxid by burning carbon, but you cannot
put different things together to make carbon.