TRANSFERENCE OF HEAT 



55 



that in this method of transferring, no material substance 



is necessary, as the hot filament is surrounded by an almost 



perfect vacuum. Now astrono- 

 mers tell us that there is no 



material medium between our 



atmosphere and the sun and that 



the heat of the sun travels to 



us with the tremendous speed of 



light, 186,000 miles per second, 



and does not warm the inter- 

 vening space. The convection 



and conduction processes are, 



when compared to this, very 



slow. Radiation is the name 



given to this method of heat 

 transference. If 

 heat did not 

 travel in this way 

 the earth would 

 be uninhabitable. 

 If a body, 



heated to ordinary temperatures, is sur- 

 rounded by substances which do not readily 

 permit of conductional or convectional 

 heat transference, the heat is retained 

 within the body. Application of this is 

 made in the fireless cooker and the thermos 

 bottle (Fig. 30). In one, the hot substance 

 is surrounded by felt, wood fiber, asbestos 

 or similar nonconducting substances, and 

 in the other by glass and a space from 

 which the air has been nearly exhausted. 

 Both of these arrangements prevent the transference of 



heat from the hot body. The cooking therefore continues 



HOT WATER FURNACE. 

 The hot water rises from 

 the top, passes through the 

 radiator and returns as 

 colder w#ter to the bottom. 



Fig. 30. 



