Some Applications of Heat 



115 



heat to pass rapidly to or from the inside bottle. A hot liquid 

 poured into it will therefore remain hot, and a cold liquid 

 will remain cold for a long time if the covering of the bottle is 

 tight. 



Any device, such as a fireless cooker or a vacuum bottle, which 

 keeps food hot for a considerable period of time, must do so by 

 holding the heat that is placed in it with the food, as it has no 

 means of forming heat. We have learned that heat may be 

 transferred from place to place by any one or more of three 

 methods ; conduction, convection and radiation. . The efficiency of 

 the vacuum bottle depends upon eliminating to a large extent 

 these methods of heat transfer. 



The vacuum bottle is made of glass, which 

 in itself is a poor conductor of heat. There are 

 two walls of glass, and the space between them 

 is a vacuum. A vacuum will not conduct heat. 

 Thus loss by conduction is almost entirely 

 eliminated. 



A vacuum also eliminates the loss by con- 

 vection because convection depends on the cir- 

 culation of a gas or a liquid and neither exists 

 in a vacuum. 



Radiation may be reflected from a mirror. 

 Both sides of the glass next to the vacuum in a 

 vacuum bottle are silvered as mirrors are. Radi- 

 ation from within the bottle is thus turned back. 

 The silver coating on the inside of the outer 

 wall reflects outward all radiation coming from Landers. Frary & ciark. 

 the outside which might change the temperature A VACUUM BOTTLE 



f , .. , A bottle within a bot- 



Of the Contents Of the bottle. tie, separated by a 



. , i i 11 vacuum. Hot liquids 



About the only heat loss in the vacuum bottle m the inner bottle 



, A . ,..,..,, will keep hot, and 



is through the cover. Air or liquid inside the cold liquids win keep 



^ . cold. 



bottle may carry heat by convection and thus 

 carry heat to the cork. It will then be conducted slowly through 

 the cork and escape. When the bottle is filled with a cold liquid 

 the conduction through the cork is reversed. In either case this 

 transfer of heat is very slow. 



