Fire and Heat 105 



its rays fall. Is the glass as warm as the objects in the direct 

 sunlight? If glass conducts the sun's energy, it must be as 

 warm or even warmer than the objects. But you find that 

 glass is only slightly warmed by the transmitted rays. With 

 the aid of a reading glass start a fire in some paper or burn a 

 hole through a piece of cloth. When the substance is smoking 

 hot, feel the glass. Is it as hot as the burning substance? Is 

 it much hotter than the air? Plainly, glass is a good medium 

 for the transmission of sunshine, absorbing and converting 

 into heat only a very small amount. Observe its use in hot- 

 house or sun parlor. Suppose the windows were made of thin 

 sheets of copper instead of glass. Would the room be warmed 

 by the sun when shining directly upon such " windows"? 

 Do you think it would be possible to keep the room warm with 

 heat from fuel on a cold day? Does it seem that the sun's 

 energy can be transmitted by conduction through the air any 

 more than through glass? 



Exercise : Third way of heat transference. The principal points 

 may be brought out clearly by observations on an incandescent lamp. 

 Observe that the hot filament of a tungsten or carbon lamp throws 

 out some heat. This cannot be transmitted entirely by conduction, 

 for the vacuum or inert gas within the bulb and the glass are poor 

 conductors. Clasp the bulb with the hand and turn on the electric 

 current. Is the heat instantly sensible? Turn off the current. 

 Does the heat diminish gradually as it does in the case of cooling iron 

 or water? Repeat this until the facts are clearly grasped, and you 

 have proved whether or not convection or conduction accounts for 

 the heat transfer. 



An iron ball, three to five inches in diameter, with a hook or knob 

 to which a wire may be attached, is heated until it is practically red 

 hot ; then hung by the wire in about the center of the room and half 

 way from the ceiling to the floor. Now find by trials in what direc- 

 tions and at what distances you can feel with your hand the heat 

 radiating from the ball. Is the heat thrown out in all directions, 

 downward as well as upward? Do convection currents move down- 

 ward? Do they move upward? Interpose between your hand and 

 the hot ball successively a sheet of paper, a piece of glass, and a sheet 

 of asbestos paper. What happens? With a small mirror see if you 



