28 PRACTICAL LESSONS IN SCIENCE. 



from the source. The first law needs no illustration, and our 

 experience leads us to believe that the other may be true also, 

 but its truth can be better illustrated in the discussion on the 

 subject of light, which varies according to the same law. 



Radiant heat or energy moves in straight lines, as can be 

 shown by arranging screens about the source of heat. A body 

 with a rough surface will radiate heat more readily than a 

 similar body with a smooth surface. The teapot is given a 

 polished surface because such surfaces retain heat better than 

 rough ones do. 



Suppose one end of an iron rod is held in the flame of a lamp, 

 the molecules in contact with the flame are made to vibrate 

 more rapidly. They swing against their neighbors and put them 

 also in more rapid motion ; they in turn give motion to the next, 

 and so on until those at the end of the rod are in more rapid 

 motion, and can put in more rapid motion molecules of other 

 bodies in contact with them, thereby increasing their tempera- 

 ture. This method of transferring heat from molecule to molecule, 

 through a body, is called conduction. The metals are, in general, 

 the best conductors of heat. Liquids and gases are poor con- 

 ductors of heat. If we take a test tube and put a piece of ice in 

 the bottom, weighted by a piece of iron or lead, and then fill the 

 tube with water, the water at the top of the test tube may be 

 boiled without melting the ice, showing that water does not 

 conduct heat as rapidly as some solids do. 



The heat applied to a body is divided by it into two parts, 

 one part is used to increase the temperature of the body, an- 

 other part is used to expand the body. The first can be appre- 

 ciated by the touch, and is called sensible heat; the second 

 can not be appreciated by the touch or by the thermometer, 

 and is called latent heat. The latent heat has not been lost, 

 but it has simply been used up in doing the work of expansion. 

 Different substances do not divide heat in the same proportion. 

 Let equal parts of water and mercury be placed over the same 

 source of heat. Each divides the heat it receives into two parts 

 as above; but the heat devoted to temperature is more in the 



