126 Common Science 



The radiant heat from the incandescent filament in 

 the lamp passed right out through the vacuum of the 

 lamp, and much of it went on through the glass to your 

 hand. You already know what a poor conductor of 

 heat glass is; yet it lets a great deal of radiant heat 

 pass through it, just as it does light. As soon as the 

 lamp stops glowing, the heat stops coming; the glass 

 is not made hot and you no longer feel any heat. In 

 one way the electric filament shining through a vacuum 

 is exactly like the sun shining through empty space: 

 the heat from both comes to us by radiation. 



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 



