24 INTRODUCTION TO GENERAL SCIENCE 



and iron wires in the flame. Which one conducts the heat to 

 the hand first? Which one last? 



b. Fill the test tube nearly full of water and place it in the 

 test-tube holder. Hold the tube obliquely so that the flame of 

 the burner heats the top of the "vater, but does not touch the 

 glass where there is no water. When the water boils at the top, 

 feel the bottom of the tube. Is water a good conductor of heat ? 



18. CONVECTION 



All bodies expand when heated, whether they are solids, 

 liquids, or gases. It follows that a given weight of material 

 occupies more space, when heated, than when cold. There- 

 fore a given volume of a hot material weighs less than the 

 same volume of the same material when cold. Thus, if there 

 is a mixture of hot air and cold air, or hot water and cold 

 water, the colder, heavier material will push up the warmer, 

 lighter material. We say that hot air rises; it is more cor- 

 rect to say that it is pushed up by colder air. For the same 

 reason a balloon does not rise of itself, but is pushed up by 

 the air, which is heavier than the contents of the balloon. 



In the above-mentioned method of heat distribution, the 

 material heated keeps turning over and over. Thus it is 

 called convection, and the currents, which are produced, are 

 called convective currents. Upon convection are based 

 methods of hot-air heating, and hot-water heating of houses, 

 ventilation, and domestic hot-water boilers. These are de- 

 scribed in the next section. The winds, which are caused by 

 convective currents on an enormous scale, are treated in 

 Section 94, Winds. 

 References : 



1. 1803 : 219-221. Convection of Liquids. 



a. 1801:288-290. Convection in Water. 



