104 Our Surroundings 



mouth of the bottle with a cork through which passes a narrow 

 glass tube reaching to the bottom of the bottle. Heat the bottle 

 gradually. The colored liquid will rise in the tube. This is 

 caused by the expanding of the air. 



To show the transference of heat in matter by convection, 

 hold a burning joss stick under a good-sized inverted glass jar. 

 The movements of the air will be indicated by the smoke. These 

 movements are convection currents. Water and air expand when 

 warmed, and in each case convection currents are caused by this 

 expansion. Without this expansion convection would be im- 

 possible. 



Conduction. Conduction is the process of transferring heat 

 by contact. In order to understand the difference between con- 

 duction and convection it is necessary to keep in mind that all 

 bodies are made up of molecules. In conduction, each molecule 

 heats the neighboring molecules by striking against them, and 

 thus the heat is passed along from molecule to molecule. In 

 convection, masses of molecules, when heated, start moving and 

 other masses, less heated, move in to take their place. Thus cur- 

 rents are set flowing, and these carry the heat. 



Experiments to Show Conduction. To show the transference 

 of heat by conduction, place one end of an iron rod about a foot 

 in length in the flame of a lamp or burner. Hold the other end 

 in your hand until you feel the heat. Try the same experiment 

 with a glass rod of the same length as the iron rod. Experi- 

 ment with wooden and copper rods of the same length, and 

 with rods of other substances. Is the heat felt on the hand as 

 soon with one substance as with another? If not, what da 

 you conclude? Why is copper a good material for cooking- 

 utensils ? 



Methods of Heating Buildings. There are five important 

 ways of heating buildings: by the fireplace, the stove, the hot 

 air furnace, the steam heater and the hot water heater. All of 

 these require a fire and, therefore, a chimney. 



Importance of the Chimney. In early times houses were 

 heated by building a fire on the floor. A hole was left in the 

 roof through which much of the smoke passed out, though some 



