ressure is partially removed. It is
because of this that milk is evaporated in a vacuum for canning; it is
not necessary to make it so hot that it will be greatly changed by
the heat, if the boiling is done in a vacuum. On a high mountain the
slight air pressure lets the water boil at so low a temperature that
it never becomes hot enough to cook food.

_APPLICATION 30._ Two college students were short of money and
had to economize greatly. They got an alcohol lamp to use in
cooking their own breakfasts. They planned to boil their eggs.

"Let's boil the water gently, using a low flame," one said;
"we'll save alcohol."

"It would be better to boil the eggs fast and get them done
quickly, so that we could put the stove out altogether," the
other replied.

Which was right?

_APPLICATION 31._ Two girls were making candy. They put a
little too much water into it.

"Let us boil the candy hard so that it will candy more
quickly," said one.

"Why, you wasteful girl," said the other. "It cannot get any
hotter than the boiling point anyhow, so you can't cook it any
faster. Why waste gas?"

Which girl was right?

INFERENCE EXERCISE

Explain the following:

161. Warm air rises.

162. The lid of a teakettle rattles.

163. Heating water makes a steam engine go.

164. When an automobile with good springs and without shock
absorbers goes over a rut, the passengers do not get a jolt,
but immediately afterward bounce up into the air.

165. Comets swing around close to the sun, then off again into
space; how do they get away from the sun?

166. When you wish to pour canned milk out, you need two holes
in the can to make it flow evenly.

167. Liquid air changes to ordinary air when it becomes even
as warm as a cake of ice.

168. Skid chains tend to keep automobiles from skidding on wet
pavement.

169. A warm iron and a blotter will take candle grease out of
your clothes.

170. Candies like fudge and nougat become hard and dry when
left standing several days open to the air.

SECTION 20. _Conduction of heat and convection._

Why does a feather comforter keep you so warm?

When you heat one end of a nail, how does the heat get through
to the other end?

How does a stove make the whole room warm?

Here is a way to make heat run a race. See whether