It was a cold wet day—so cold and wet that neither Dolly nor her
brothers could venture out. They had grown tired of reading books and
drawing pictures, and were indeed feeling very dull. They sat looking at
the bright fire. Uncle George laid down his paper and said:
“Come, let us have a lesson. What shall it be? The rain? The cat? Or
shall it be the cosy fireside?”
“The fire,” said Frank. “Tell us why the fire burns, Uncle George!”
“I will,” said cheery Uncle George. “Just wait until I get some things
from the kitchen. Come along, boys.”
When Uncle George and the boys came back to the room, they brought a lot
of curious articles with them. These were an empty pickle-bottle, a
small saucer, a glass bell-jar, a large dish, a piece of candle, some
tacks, and a taper.
[Illustration: A Baby Cloud]
[Illustration: THE BALLOON]
“Now,” said Uncle George, “we are ready to begin.”
He first lighted the piece of candle and lowered it into the bottle. It
burned for a short time, then it went out.
“Can you tell me why it goes out?” he asked.
“Want of air,” said Tom and Frank at the same time.
“But there is air in the bottle,” said Uncle George.
“Yes, but not the kind of air the candle wants,” said Frank.
[Illustration: Candle burning in Open Jar.]
“That is a queer answer, Frank. The candle burned for a time in the
bottle before it went out.”
“Because it used up that part of the air which makes things burn,” said
Frank.
“That is _very_ good,” said Uncle George.
Uncle George then poured some water into the large dish. He fixed the
candle on a big cork, lighted it and set it floating on the water. Then
he placed the glass bell-jar over it. But first of all he marked the
level of the water on the outside of the bell-jar.
Very soon the flame of the candle became small, and at last went out.
Just then the water inside the bell-jar rose far up above Uncle George’s
mark. He marked this new level, and asked the boys if they could tell
him why the water rose in the jar.
Both Frank and Tom shook their heads sadly.
[Illustration: Candle in Stoppered Jar—burning.]
“Then I must explain,” said Uncle George.
“Some of the air has gone,” said Tom.
“Yes,” said Uncle George. “How much of the air has gone?”
Frank pointed to the space between the two marks.
“That is right,” said Uncle George. “The water has risen up in the
bell-jar to take the place of the air that has been used up by the
burning of the candle. Where has this used-up air gone?”
“It must have gone into the water,” said Tom.
“Why did it not go into the water before the candle burned?”
“Perhaps the burning of the candle has _changed_ this part of the air,”
said Frank.
“Very good, Frank. You are right again. The burning of the candle has
changed a certain part of the air. It has, indeed, so changed it that it
can dissolve in water just as if it were sugar or salt.”
Uncle George now poured water into the outer dish until it was level
with the water inside the bell-jar. Then he took out the stopper and
pushed a lighted taper into the bell-jar. The taper at once went out.
[Illustration: Candle in Stoppered Jar—gone out.]
“This shows us,” he said, “that a part of the air causes things to burn.
The other part of the air does not. It puts burning things out. If we
blow the fire with a bellows or fan, it burns more brightly and quickly.
Why? Just because we are forcing a stream of air upon it, and a part of
that stream of air is changed by the burning.”
Uncle George next put some bright iron tacks in a small dish. He poured
some water out of the large dish, and placed the bell-jar in the dish.
After that he added water until it was just up to his first mark on the
bell-jar.
Then he floated the dish with the tacks on the water. Next he wetted the
tacks with water, and then placed the bell-jar over them and put in the
stopper.
“Now,” he said, “we will leave this just as it is for a few days.”
The boys watched the bell-jar every day, and this is what they saw. The
water rose slowly in the bell-jar. As it rose the bright tacks turned
red with rust. The water rose higher and the tacks turned redder every
day.
At length it rose to Uncle George’s second mark. It rose no farther,
although left for a whole week.
Then Uncle George called the boys and asked them what had taken place in
the bell-jar.
“The tacks have rusted, and some of the air in the jar has been used
up,” said Frank.
“How much air has been used up?” Uncle George asked.
“Just exactly the same as was used up when we burned the candle,” said
Tom, pointing to the top mark.
“Let us see, then,” said Uncle George, “what part of the air has gone.”
He poured water into the large dish until it was level with the water
inside the bell-jar. Then he put a lighted taper into the bell-jar as
before. It went out at once.
“It is the same part of the air as the burning candle used up,” said
Frank.
“Then we have found out,” said Uncle George, “that when a thing burns it
uses up a certain part of the air; and that when iron rusts, exactly the
same part of the air is used up.
“In the first case, the burning of the candle changed part of the air
into a gas which dissolved in the water. In this case, that same part of
the air has joined up to part of the iron tacks to form that red powder
which we call _rust_.”
Questions and Exercises.
1. How is rust formed?
2. Explain why things which are made of iron should be painted.
3. A grate full of coals burns away and only a small quantity of ash
is left in the grate. What has become of the coals?