Evaporation.
Does a wet blanket, when hung in the air, remain wet?
No; it soon becomes dry.
What becomes of the water in the blanket?
The water passes off into the air.
What is this process called?
It is called Evaporation.
What is meant by evaporation?
It is the gradual change of a liquid into a vapor.
When water is left in a shallow vessel, what becomes of it?
The water evaporates or passes off into the air.
Do liquids generally evaporate?
Yes; when exposed to the air, nearly all liquids evaporate.
Does water change into steam when it evaporates?
Water does not change into steam when it evaporates.
How, then, does water evaporate?
It passes off into the air in particles or drops so very small as not to be visible.
Is evaporation always going on around us?
Yes; wherever there is water exposed to the air, evaporation is taking place.
Where does the most evaporation take place?
From the surface of the rivers, the lakes, and the ocean.
What becomes of the water when evaporated?
It is formed into fogs and clouds.
In what kind of air does evaporation go on most rapidly?
In air that is warm and dry.
Does evaporation take place in cool air?
It does; because clothing will dry during the coldest weather of winter.
Why does water evaporate more in warm air than in cold air?
Because the warm air can hold more of the fine particles of water, and, therefore, can absorb more.
Why does water evaporate more slowly in damp weather than in dry weather?
Because the damp air contains so much moisture that but little more can pass off into it.
What example have we of this?
Wet clothes dry much more slowly when the air is damp than when it is dry.
Why do clothes dry more rapidly in windy weather?
Because the damp air in contact with them is blown away, and the dry air is constantly taking its place.
Is heat necessary in evaporation?
Heat is necessary to produce evaporation.
Why do our hands feel cold when wet?
Because the water on our hands evaporates, and thus heat is taken from them.
Why is the air cooler after a rain?
Because evaporation then takes place from every wet object, and this absorbs heat from the air.
Why does watering streets and roads cool the air?
Because the water evaporates and absorbs heat from the air, making it cooler.
Why do we feel cold when our clothes are wet?
Because the moisture in them evaporates and absorbs heat from our bodies.
Why is wet land always cold?
Because so much heat is absorbed from the land in the evaporation which is constantly taking place.
What becomes of the dew that collects on plants and other objects at night?
It is evaporated.
How does the sun hasten the evaporation of the dew?
By supplying the heat needed to change the dew into a vapor, the sun hastens its evaporation.
Why do bread and biscuit become hard when kept a few days?
Because the moisture in them passes off into the air, and thus they become dry and hard.
How can they be kept moist and soft for a longer time?
By keeping them in a covered vessel, so that evaporation will take place more slowly, they may be kept moist for a longer time.