HEATING BUILDINGS 97 



as good as iron. So glass prevents the escape of heat 

 from a room but allows the radiant heat of the sun to 

 pass through it. The conductivity of water is only about 

 TfW oi that of silver. 



That water is a poor conductor of heat can be proved 

 by taking a tall, narrow vessel, like a test tube, full of 

 water and making the water on top boil while holding 

 the bottom in the hand. The gas flame is applied only 

 to the top of the vessel. 

 When heat is applied to the 

 bottom of a vessel of water, 

 the water on the top is heated 

 by a process different from 

 conduction. Any one who 

 has been in bathing in a 

 river or lake knows that the 

 water is warmer on the top 

 than it is several feet below BOILING WATER 



the surface. Why? I R a test tu be while holding it in 



. J the hand. 



Gases are the poorest con- 



ductors known, only -^ as good as water or -girrijTnr as 

 good as silver. Air is a gas, and because gases are poor 

 conductors of heat, the surface of the earth is not per- 

 mitted to cool rapidly. Gases will transmit heat easily 

 when they are free to move, but not by conduction. 



The difference in the rate of conductivity can be ex- 

 perienced by placing the hand on a piece of iron and 

 then on a piece of dry wood lying near. The iron will 

 feel cooler than the wood in winter because it conducts 

 the heat from the hand. (For this reason never touch 

 your tongue or wet ringer to a piece of iron during zero 

 weather). The iron will feel warmer if both iron and 

 wood are lying in hot sunshine, because the iron will 



