droplets, and you see
your breath.

Here are two experiments in condensing water vapor by cooling the air
with which it is mixed. Both work best if the weather is warm or the
air damp.

EXPERIMENT 88. Put the bell jar on the plate of the air pump
and begin to pump the air out of it. Watch the air in the jar.
If the day is warm or damp, a slight mist will form.

As part of the air is pumped out, the rest expands and cools, as warm
air does when it rises and is no longer pressed on so hard by the air
above it. And as in the case of the rising warm air, the water vapor
condenses when it cools, and forms the mist that you see. This mist,
like all clouds and fog, consists of thousands of extremely small
droplets.

EXPERIMENT 89. Hold a saucer of ice just below your mouth.
Open your mouth wide and breathe gently over the ice. Can you
see your breath?

Now put the ice into half a glass of water and cover the
glass. Be sure the outside of the glass is thoroughly dry. Set
it aside and look at it again in a few minutes.

What caused the mist when you breathed across the ice?

Where did the water on the outside of the glass of ice water
come from? What made it condense?

[Illustration: FIG. 158. If you blow gently over ice, you can see your
breath.]

_APPLICATION 66._ Explain why clouds are formed high in
the atmosphere; why we have dew at night instead of in the
daytime; why clothes dry more quickly in a breeze than in
still air; why clothes dry more quickly on a sunny day than on
a foggy one.

[Illustration: FIG. 159. The glass does not leak; the moisture on it
comes from the air.]

INFERENCE EXERCISE

Explain the following:

411. A gas-filled electric lamp gets hotter than a vacuum
lamp.

412. You can remove a stamp from an envelope by soaking it in
water.

413. We see our breath on cold days and not on warm days.

414. The electric arc is exceedingly hot.

415. Rock candy is made by hanging a string in a strong syrup
left open to the air.

416. Dishes in which candy has been made should be put to
soak.

417. Moisture gathers on eyeglasses when the wearer comes from
a cold room into a warm one.

418. Sprinkling the street on a hot day makes the air cool.

419. You cannot see things in a dark room.

420. Where air is rising there is likely to be rain.

SECTION