Radiant Heat and Light 



127 



FIG. 61. How a thermos bottle is made. Notice the double layer of glass in 

 the broken one. 



radiation or conduction. And that is why thermos bottles 

 will keep things very hot or ice-cold for such a long time. 



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 for- 

 ever 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 day- 

 time radiates away at night. And since it keeps on 



