o
off unless you heat it by the sudden blow of the gun hammer which you
release when you pull the trigger; that you have to concentrate the
sun's rays with a magnifying glass to make it set a piece of paper on
fire; and that to change raw food into food that tastes pleasant you
have to heat it. If heat did not start chemical change, you could
never cook food,--partly because the fire would not burn, and
partly because the food would not change its taste even if heated by
electricity or concentrated sunlight.

Here is an experiment to show that gas will not burn unless it gets
hot enough:

EXPERIMENT 97. Hold a wire screen 2 or 3 inches above the
mouth of a Bunsen burner. Turn on the gas and light a match,
holding the lighted match _above_ the screen. Why, do you
suppose, does the gas below the screen not burn? Hold a
lighted match to the gas below the screen. Does it burn now?

The reason the screen kept the gas below it from catching fire
although the gas above it was burning was this: The heat from the
flame above was conducted out to the sides by the wire screen as soon
as it reached the screen; so very little heat could get through the
screen to the gas below. Therefore the gas below the screen never got
hot enough for the chemical change of oxidation, or burning, to take
place. So the gas below it did not catch fire.

Another simple experiment with the Bunsen burner, that shows the same
thing in a different way, is this:

[Illustration: FIG. 171. Why doesn't the flame above the wire gauze
set fire to the gas below?]

EXPERIMENT 98. Light the Bunsen burner. Open the air valve at
the bottom all the way. Hold the wood end of a match (not the
head) in the center of the inner greenish cone of flame, about
half an inch above the mouth of the burner. Does the part of
the match in the center of the flame catch fire? Does the part
on the edge? What do you suppose is the reason for this? Where
are the cold gas and air rushing in? Can they get hot all at
once, or will they have to travel out or up a way before they
have time to get hot enough to combine?

[Illustration: Fig. 172. The part of the match in the middle of the
flame does not burn.]

_APPLICATION 73._ Explain why boiled milk has a different
taste from fresh milk; why blowing on a match will put it out;
why food gets black if it is left on the stove too long.

INFERENCE EXERCISE