HEAT: DISTRIBUTION AND MEASUREMENT 75 



If the hand touches an object having a temperature lower 

 than the hand itself, the object feels cold because it conducts 

 heat /row the hand. If the object has a higher temperature 

 than the hand, it feels warm because it conducts heat to the 

 hand. Bodies which have been for a long time in the same 

 room have the same temperature, but the good conductors 

 feel cold, the poor ones less cold. 



Metals are the best conductors of heat ; stone, hard wood, 

 silk, and linen are very good ones; asbestos, wool, paper, 

 and air are poor conductors. Because of the layers of air 

 between articles of our clothing, several thin garments are 

 warmer than one thick layer of the same weight as all the 

 thin ones. Clothing does not make us warm but keeps us 

 warm if heat is not conducted away from the body. 



77. Convection. Heat is distributed through liquids 

 and gases by moving currents. This method is called con- 

 vection. It may be illustrated by studying a kettle of water 

 over a fire. The bottom of the kettle becomes heated, and 

 the layer of water in contact with it is heated by conduction. 

 This rise in temperature causes expansion in the bottom layer 

 and therefore a smaller density. One gram of cold water 

 occupies one cubic centimeter of space; when the water is 

 warmed, a gram occupies more than one cubic centimeter. 

 One cubic centimeter of warm water weighs less than one 

 gram. The lighter, warmer water in the kettle is pushed 

 away by the colder, denser water and rises. This process 

 is continued until the liquid at the top as well as at the 

 bottom has reached the boiling point. 



In convection, there is a downward movement of the 

 colder liquid or air and an upward movement of the warmer. 

 The air of a room is always warmer near the ceiling than 

 near the floor. 



78. Radiation. Besides conduction of heat through 

 matter in contact with a source of heat, and convection in 

 currents of liquids and gases, there is another method by 



