154 



OUR PHYSICAL WORLD 



air. These, of course, must be air-tight. Any hot substance 

 in the bottle cannot lose heat to the surrounding air, while if 

 the bottle contains a cold substance the heat of the surrounding 

 air cannot get to it to warm it. We put a storm sash on our 

 windows to inclose a layer of air between the window and the 



storm sash. These numerous 

 substances in the path of the 

 radiating heat tend to reflect it 

 and prevent its escape, for heat 

 is reflected just as is light. Put 

 a thermometer bulb at the point 

 at which the light from a lamp 

 is brought to a focus by a con- 

 cave mirror and the mercury 

 rises rapidly. 



Now, too, to avoid dust and 

 dirt in our homes the heating 

 plant for the house is often put 

 in the basement. Hot-air pipes 

 from the furnace conduct the 

 heated air to the rooms above 

 on the same principle that the 

 chimney carries the hot air up. 

 In the same way the hot- water 

 pipes conduct the hot water up 

 to the radiators and as it cools 

 off in them, it flows back to the 

 heater, so forcing up the hot 

 water. In the hot-water sys- 



FIG. 60. Diagram of a hot-water plant 



tern an expansion tank must be used, because when the cold 

 water is heated it expands and unless there were a chance for an 

 overflow it would burst the pipes and radiators (see diagram, 

 Fig. 60). 



In the steam-heating plant, the water is boiled in the base- 

 ment, the steam goes up through pipes to the radiators to warm 



