HEAT 



/nsv/at/or). 



thrust into the oven and set afire. The logs in burning heated 

 the oven, and when they had heated it sufficiently, they were 

 quickly removed. The bread to be baked was then placed in 

 the hot oven, the opening was closed, and the oven left to 

 take care of itself. Bricks are poor conductors of heat, and 

 the heat left by the burning logs did not escape, but remained 

 to bake the bread. 



The fireless cooker and the refrigerator. The invention 

 of the " fireless cooker " depended in part upon the principle 

 of non-conduction. Two vessels, one inside the other, are 



separated by sawdust, 

 asbestos, or other poor 

 conducting material 

 (Fig. 1 8). Foods are 

 heated in the usual way 

 to the boiling point or 

 to a high temperature, 

 and are then placed in 

 the inner vessel. The 

 heat of the food cannot 

 escape through the non- 

 conducting material which surrounds it, and hence remains 

 in the food and slowly cooks it. 



A very interesting experiment for testing the power of non- 

 conductors to retain heat can be easily performed at home. 

 Put some hot water, at about 75 F., in a vessel covered with 

 wool or felt, and an equal quantity at the same temperature 

 in an uncovered vessel of the same kind. By means of ther- 

 mometers notice the rapidity with which the water cools in 

 the uncovered vessel, and the slowness with which it cools 

 in the covered vessel. Evidently the non-conductor serves 

 to retain the heat within the vessel and to prevent its escape 

 into the air. 



FIG. 18. A fireless cooker. 



