Fire and Heat 



99 



or heater, here represented by the lower bottle, 

 lower bottle. Explain results. 



Apply heat to the 



Meaning of convection. The process of transfer of heat 

 in this manner by gases, such as air, and by liquids, such as 

 water, is known as convection. The heated portions of a given 

 volume of a substance are moved from place to place through 

 differences in density. Eventually the whole of a given mass, 

 as the air of a room or the 

 water in a boiler, becomes hot 

 by the actual circulation of 

 the heated portions or parti- 

 cles. Very ancient peoples 

 made use of convection, as is 

 evident from their practice of 

 putting hot stones into water 

 contained in vessels of bark or 

 skin. To-day the fact is uti- 



lized in hot-water systems, in 

 steam and hot- water heating, 

 as well as in hot-air heating. 



Exercise. In the diagram of v7"ve 

 Fig. 32, identify the cold water 

 supply pipe, the course of water 

 to and from the range, and the FIG. 32. Water heater and hot-water 



hot-water delivery pipe leading tank - Trace the course of the ho * and 

 , , , ., , , , ,, cold water through the tank and the 



to laundry, kitchen, and bath. water _back. 

 Explain how the water is heated ; 



how circulation is brought about and the water in the tank heated. 

 Apply the facts that have been brought out experimentally. 



Exercise. The use of hot water for heating buildings is becoming 

 very common. The accompanying diagram (Fig. 33), presents the 

 chief features of such a system. Trace the course of cold and hot 

 water in the system. Explain why the water circulates in the pipes 

 and boiler. What is the use of the "expansion tank"? Of the 

 overflow pipe? Why are these above the rest of the system? Men- 

 tion other familiar uses of water as a medium for the transfer of heat, 

 in the household and in manufacturing. 



