VII. 



HEAT KAYS. 



But how does the heat pass to your hand from a hot body when 

 you are not touching it? This is perhaps not so easy to understand. 

 In some way the heat passes off from the body and crosses the space 

 between the body and your hand, just as the sun's rays come from the 

 sun to the earth. For this reason the process is called radiation, and 

 the body is said to radiate heat. Some surfaces radiate much better 

 than others. Here is a simple experiment to illustrate it: Blacken 

 one side of a tin can with shoe-blacking, or by holding it in a very 

 smoky flame, then fill the can with boiling water and hold one hand 

 near the bright surface and the other equally near the black surface 

 and find which feels warmer. You will thus arrive at the conclusion 

 that black surfaces radiate heat better than bright surfaces. 



While we axe speaking of giving heat out by radiation, it will be 

 well to say something about taking in heat that other bodies radiate 

 out. This taking in of heat is called absorption. Let us first see 

 which takes in heat better, a bright surface or black surface. You can 

 test this by getting two similar tin can covers and blacking the outside 

 of one with shoe blacking. Now, support them just above the middle 

 of a hot stove on little pieces of glass, all of the same thickness, which 

 rest on the stove. Put a couple of matches in each and observe which 

 cover first becomes hot enough to light the matches. If you suspect 

 that one was over a hotter part of the stove than the other, exchange 

 their positions and try again. Instead of putting the matches in, you 

 may pour equal quantities of water into the two covers by using a 

 small cup for measuring, and then see in which the water will boil 

 first. You will thus learn that black surfaces absorb heat more 

 quickly than light surfaces. Now, think of these things and try to 

 decide which part of a kettle should be brightly polished — the bottom 

 or the outside. Try also to think of other cases of such differences. 

 Consider, for instance, house paints, colors of summer 'and winter 

 clothes, etc. 



If you are studying this subject in the winter time you may try the 

 effect of putting different colored cloths of about the saAe thickness 

 on the snow on a bright, sunny day and finding under which the 

 snow is most melted by heat taken in from the sun. These are only 

 some of the ways in which you can make experiments on the subject. 

 You may be able to think of other ways yourself. 



