152 THE PRODUCTION AND USE OF HEAT 



Reopen the damper slowly, noting just the amount of air necessary 

 to produce a non-luminous flame. 



The most intense, i.e., the hottest, flame is secured when 

 the damper is so set that there is just a sufficient amount of 

 air to produce a non-luminous flame. An excess of air, not 

 only reduces the intensity of the heat, but it also tends to cause 

 the flame to " strike back," i.e., to burn down in the burner 

 instead of above the burner as it should. 



FIRELESS COOKERS 



178. Cooking Temperatures. The cooking of foods is ac- 

 complished by raising them to a certain temperature and then 

 maintaining that temperature for a certain length of time. 

 Both the temperature required and the time required vary, 

 first, with the nature of the food to be cooked, and second, 

 whether it is to be cooked wet, i.e., STEWED or BOILED, or cooked 

 relatively dry i.e., BAKED. "Stewing" and "boiling" usu- 

 ally require a temperature near the boiling point of water, 

 or from 180 to 212F. Baking requires a much higher tem- 

 perature. Bread is commonly baked at about 375F. 



179. Conductors and Non-conductors. If it were possible 

 to discover a device which would entirely prevent the loss of 

 heat, it is evident that it would be necessary only to bring the 

 food once to the proper temperature; if no heat were lost, 

 the food would then remain at that, temperature indefinitely 

 or until cooked. Unfortunately we know of no frieans of 

 preventing heat from escaping through the walls of any 

 vessel we can construct. Heat passes through every known 

 material. However, it passes through some materials much 

 more readily than through other materials. Materials 

 through which heat passes readily are said to be GOOD CON- 

 DUCTORS OF HEAT; materials through which heat passes less 

 readily are said to be POOR CONDUCTORS OF HEAT. All metals 

 are good conductors of heat; air, asbestos, and paper are poor 

 conductors. 



