If a lamp glows for a long time, however, the glass really does become hot. That is partly because there is not a perfect vacuum within it (there is a little gas inside that carries the heat to the glass by convection), and it is partly because the glass does not let quite all of the radiant heat and light go through it, but absorbs some and changes it to the regular conducted heat.
One practical use that is made of a knowledge of the difference between radiant and conducted heat is in the manufacture of thermos bottles.
Experiment 43. Take a thermos bottle apart. Examine it carefully. If it is the standard thermos bottle, with the name "thermos" on it, you will find that it is made of two layers of glass with a vacuum between them. The vacuum keeps any conducted heat from getting out of the bottle or into it. But, as you know, radiant heat can flash right through a vacuum. So to keep it from doing this the glass is silvered, making a mirror out of it. Just as a mirror sends light back to where it comes from, it sends practically all radiant heat back to where it comes from. Heat, therefore, cannot get into the thermos bottle or out of it either by radiation or conduction. And that is why thermos bottles will keep things very hot or ice-cold for such a long time.
Fig. 61. How a thermos bottle is made. Notice the double layer of glass in the broken one.
Fill the thermos bottle with boiling water, stopper it, and put it aside till the next day. See whether the water is still hot.
If we could make the vacuum perfect, and surround all parts of the bottle, even the mouth, with the perfect vacuum, and if the mirror were perfect, things put into a thermos bottle would stay boiling hot or icy cold forever and ever.
Why it is cool at night and cold in winter. It is the radiation of heat from the earth into space that makes the earth cooler at night and cold in winter. Much of the heat that the earth absorbs from the sun in the daytime radiates away at night. And since it keeps on radiating away until the sun brings us more heat the next day, it is colder just before dawn than at midnight, more heat having radiated into space.
For the same reason it is colder in January and February than in December. It is in December that the days are shortest and the sun shines on us at the greatest slant, so that we get the least heat from it; but we still have left some of the heat that was absorbed in the summer. And we keep losing this heat by radiation faster than we get heat from the sun, until almost spring.
Application 33. Distinguish between radiant and conducted heat in each of the following examples:
(a) The sun warms a room through the window. (b) A room is cooler with the shades down than up, when the sun shines on the window. (c) But even with the shades down a room on the sunny side of the house is warmer than a room on the shady side. (d) When a mirror is facing the sun, the back gets hot. (e) If you put your hand in front of a mirror held in the sun, the mirror reflects heat to your hand. (f) If you put a plate on a steam radiator, the top of the plate gradually becomes hot. (g) If anything very hot or cold touches a gold or amalgam filling of a sensitive tooth, you feel it decidedly. (h) The handle of your soup spoon becomes hot when the bowl of it is in the hot soup. (i) The moon is now very cold, although it probably was once very hot.
Inference Exercise
Explain the following:
181. Trees bend in the wind, then straighten up again. Why do they straighten up?
182. A cloth saturated with kerosene and placed in the bottom of a clock will oil the clockworks above it.
183. In cold weather the doorknob inside the front door is cold.
184. It is cool in the shade.
185. Clothes get hot when you iron them.
186. Potatoes fried in deep fat cook more quickly than those boiled in water.
187. If you hold your hand near a vacuum electric lamp globe that is glowing, some of the heat will go out to your hand at once.
188. Rubbing silver with fine powder polishes it.
189. A mosquito can suck your blood.
190. A hot-water tank becomes hot at the top first, then gradually heats downward. When you light the gas under an ordinary hot-water heater, the hot water circulates to the top of the boiler, while the cold water from the boiler pushes into the bottom part of the heater, as shown in Figure 59. What causes this circulation?
Section 22. Reflection.