Paper, hay, excelsior, sawdust, cork, wool, feathers, and many other materials are poor conductors of heat. If any hot substance is surrounded by any of these poor conducting materials, the heat of that substance is retained for some time. Also, if any cold substance is surrounded by a poor conductor, the substance remains cold. In throwing a piece of carpet or newspaper over an ice cream freezer, to prevent the ice from melting, one makes use of the latter principle.
[Illustration: FIGURE 26.—FIRELESS COOKER HAVING EXCELSIOR PACKING.]
The walls of a well-built refrigerator consist of a number of layers of non-conducting materials (see Figure 25).
To understand the principle involved in "cooking without fire," try the following:
EXPERIMENT 12: RETENTION OF HEAT.—Fill 2 tin measuring cups half full of boiling water. Immediately inclose one cup of water in a paper bag or wrap paper about it so there will be considerable air space between the cup and paper. After 15 minutes, insert a thermometer into the water in each of the cups. Which is hotter? What has "kept in" the heat of the hotter water?
The fireless cooker is a device containing cooking kettles which are surrounded by some poor conductor. When food is heated thoroughly, the heat can be retained for a number of hours by placing the hot food in the fireless cooker.
[Illustration: FIGURE 27—FIRELESS COOKER WITH STONE DISKS Note the kettles of various shapes]
In the ordinary fireless cooker it is possible to cook all foods that can be cooked in water at a temperature below the boiling point of water, i. e. simmering temperature. Another type of fireless cooker has a metallic or an enamel lining and is provided with movable stone disks. Both the stones and food are heated on a range and then introduced into the cooker in such a way that the stones are under and over the kettle of food. By this arrangement, foods can be cooked at a higher temperature than in the ordinary fireless cooker (see Figures 26 and 27).
There are also electric fireless cookers (see Figure 28). Such cookers are equipped with a heating element which is placed in the bottom of the insulated box. With these it is not necessary to heat the food before placing it in the cooker. The uncooked food is put into the cooker and the current turned on. By means of a clock arrangement the current may be cut off when the desired length of time of heating has passed.
[Illustration: Courtesy of the Standard Electric Stove Co FIGURE 28.—ELECTRIC FIRELESS COOKER. Has a heating element in the bottom of the cooker.]