144 GENERAL SCIENCE 



through to the upper side. If it is ignited above the 

 gauze, the flame will burn as shown in Figure 125 (a), but 

 it will not pass through to the lower side. 



The metal gauze conducts the heat away from the flame 

 so rapidly that the gas on the other side is not heated to the 

 ignition point. It is on 

 this principle that the 

 Davy Safety Lamp, much 

 used in mines, depends. 

 The distinctive feature of 

 the lamp is that the flame 

 is completely inclosed by 



a Wire gauze Chimney, SO FIG. 125. Effect of Wire Gauze on a 



that if the mine is full of 



inflammable gases, they are not ignited by the lamp 



burning inside the gauze. 



Convection in Liquids. Although liquids are poor 



conductors of heat, they may be readily heated by con- 

 vection. In conduction there was no move- 

 ment of the molecules from one end of the 

 metal rod to the other, but only vibrations 

 of the molecules in a limited space. In 

 convection, however, the molecules move 

 from place to place. If the heat had been 

 applied to the bottom of the test tube in 

 Figure 121, the ice would have melted 

 quickly, and the whole tube of water would 

 FIG. 126. The have heated evenly. This shows that heat 



water is^eveniy is transferred much more rapidly from the 



heated because bottom of the tube toward the top than 



of the convection 



currents. from the top toward the bottom. 



Experiment 46. Fill a Florence flask two thirds full of water 

 and drop into it a crystal of potassium permanganate. Heat the 



