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.

SECTION 48. _Chemical change caused by heat._

Why do you have to strike a match to make it burn?

How does pulling the trigger make a gun go off?

What makes cooked foods taste different from raw ones?

Has it struck you as strange that we do not all burn up, since burning
is a combining with oxygen, and we are walking around in oxygen
all the time? The only reason we do not burn up is that it usually
requires heat to start a chemical change. You already know this in a
practical way. You know that you have to rub the head of a match and
get it hot before it will begin to burn; that gunpowder does not g