Reading references and experiments illustrating the principles upon which fireless cookery is based.

1. A Test Of The Insulating Powers Of Different Materials

Apparatus

One or more boxes and fittings, described on pages 9 to II.

One or more pails of the same size, shape and material, preferably of from two to four quarts' capacity, with close fitting covers.

Cooking thermometer

Wool

Mineral wool

Cotton batting or waste

Excelsior

Hay

Sawdust Newspapers Ground cork Southern moss Pencil Notebook

Pack the box successively with as many of the different packing materials given above as are to be tested, following the directions given on page 15; or have several exactly similar boxes packed at the same time. For all tests fill the cooker-pail with water, bring it to the boiling point, let it boil one minute, to permit all parts of the utensil and its contents to reach the same temperature; then put it at once into the cooker-box and leave it for an equal length of time, not less than one hour. Record the temperature of the contents of the pail at the expiration of this period. In order to get a full record and a fair comparison it would be well to repeat this experiment with varying periods of time, taking the temperature, for instance, at the end of one, three, six, nine, and twelve hours. In taking temperatures do not wholly remove the cushion and cover of the pail, but slip them to one side, enough to insert the thermometer. This is, of course, a crude method of taking temperatures, but answers for purposes of comparison. If it is desired to make more accurate records this can be done by boring the cover of the box, the cushion and the pail cover, and inserting a thermometer through corks which are used to close the bored holes. The temperature can then be read while the apparatus is closed. However, the first method, if carefully done, will give probably within one degree of the correct temperature. Record the results in tabular form.

Which material do you find gives the best insulation?

Winkelman,* Duff,+ and other writers on physics give tables of the conductivity of felt, asbestos paper, paper, cotton, flannel, and other materials; but as different figures are shown, from different sources, for the same material, it is likely that the insulating power of any material used for packing a cooker will depend as much or more upon the way it is packed as upon the material used.

Experiment: Conductivity Of Different Materials

Take a piece of copper wire about six inches long in one hand, and a piece of steel wire of the same length and thickness in the other. Put one end of each piece in a flame, holding the wire by the extreme end. Notice which first becomes too hot to hold at the end farthest from the flame. This illustrates the different conductivity of the two materials, steel and copper. There is not a great deal of difference in the conductivity of different materials, but metals are relatively good conductors, and air is a very poor conductor.

2. Heat Is Carried From The Pail Partly By Convection

2. Heat Is Carried From The Pail Partly By Convection, except where solid insulating material, such as wood or indurated fibre, is used; and

* "Handbuch der Physik." + "Textbook of Physics." that manner of packing which best entangles the air and prevents air currents will, therefore, most increase the effectiveness of the insulation.

Experiment: Convection

Into a glass flask of cold water drop a few crystals of potassium permanganate, being careful not to agitate the flask. Apply a flame to the bottom of the flask. As the water becomes heated its density is reduced and it rises, forming convection currents which are coloured by the permanganate and may be distinctly seen.

Convection currents may be formed in any liquid or gas; for instance, air. By means of them heat will be carried from one part of the liquid or gas to another. Thus air heated by contact with a kettle of food will, if allowed to flow freely, carry the heat away from the food.

3. Heat Is Also Lost By Radiation

This takes place less rapidly from a bright, highly polished surface, and for this reason "Thermos" and similar bottles are encased in polished nickle. A cooker-pail with polished outside surface retains heat better than one with a dull finish. In those cookers made with a metal outside retainer, the surface should not be painted or roughened or dulled by any means.

Experiment: Radiation

Take two empty tin cans of the same size and shape. Wash off the paper labels. Keep one of them bright and shining, but move the other through a candle flame until the entire outer surface is smoked. Into each pour exactly the same quantity of water at the same temperature. Note carefully the temperature and the time. At the end of any given period, say one hour, again take the temperature of each. Which has lost the most heat, that in the bright can or that in the dull can?