Radiation.
When we stand near the fire, why do we feel warm?
Because the fire gives off heat which makes us feel warm.
How does the fire give off heat?
It gives off heat in straight lines, and in every direction.
What is this giving off of heat called?
It is called Radiation of heat.
Does the sun radiate heat?
The sun does radiate or give off both heat and light.
How do heat and light from the sun come to us?
They come together in the sunbeam.
Does heat travel as rapidly as light?
Yes; heat and light travel together at the rate of 200,000 miles in a second of time.
Do all substances radiate heat equally well?
No; some substances are much better radiators than others.
What kind of objects are the best radiators?
Those with rough and dark surfaces.
What kind of objects are the poorest radiators?
Those with smooth and bright surfaces.
Why does water keep hot a long time in a bright tin vessel?
Because the tin is a poor radiator, and prevents the heat of the water from passing off into the air.
Why does water soon cool in a dull earthen vessel?
Because the earthen vessel is a good radiator, and gives off the heat of the water rapidly.
If the bright tin vessel be painted, will the water keep hot so long?
No; the tin becomes a good radiator when painted, and the water is soon cooled.
Why does a stove cool so quickly?
Because the iron is a good radiator, and gives off its heat rapidly.
Do the rays from the sun give off much heat while passing through the air?
The rays from the sun give off about one-fourth of their heat while passing through the air.
What becomes of the remaining heat from the sun?
It is absorbed or reflected at the earth’s surface.
What does the earth do with the heat?
The earth radiates the heat, and thus warms the air.