[Illustration: Courtesy of McCray Refrigerator Co FIGURE 25. --INSULATED WALL OF A REFRIGERATOR.]

THE PRINCIPLE OF FIRELESS COOKERY.--In Experiment 2 it was found that wood did not transmit heat rapidly, while tin did. Another familiar illustration will show the difference between wood and metal in transmitting heat. A metal door knob feels very cold on a winter day, because the metal conducts the heat away from the hand rapidly, while a wooden knob is comfortable to touch. Wood is termed a poor conductor of heat. Metals are good conductors of heat.

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.]

The principle of the fireless cooker is used on some of the modern gas and electric ranges. The walls of the ovens of these ranges are surrounded by insulating materials. When an oven is heated and has reached the desired temperature, the gas or electricity is cut off, but the baking temperature is retained for some time. The top burners of some gas ranges have a fireless cooker attachment in the form of an insulated hood. The food is first heated over the burner, then the hood is lowered over the food, and the gas is cut off. The food continues to cook, however, by the retained heat (see Figure 29).

SUGGESTIONS FOR USING A FIRELESS COOKER.--One should keep the following in mind in using the ordinary fireless cooker:

[Illustration: Courtesy of the Chambers Manufacturing Co. FIGURE 29.--GAS RANGE HAVING FIRELESS COOKER ATTACHMENT, INSULATED OVEN AND HOODS.]

1. Have the food heated thoroughly before placing in the fireless cooker. (This direction does not apply to an electrical fireless cooker such as shown in Figure 28.) If the foods are small, as cereals, 5 minutes' boiling is usually sufficient cooking on the range; if large in size, as a piece of beef, 30 minutes is required to heat it through.

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