A. Class Experiments. How Heat passes from One Place to Another.
1. Radiation of heat.
a. Stand in front of a hot stove or fire and notice the heat. Then hold a screen between your face and the fire. Do you feel the heat on your face as before? Yet the air that surrounds you is still warm. Heat that passes in straight lines directly from one object to another at a distance is called radiant heat. The heat is said to pass by radiation.
b. Determine whether a bright, clean surface or a dull, rough one radiates heat more easily. Take two cheap tin cups, one that is bright and new and polished as highly as possible, the other that has been held in a flame until it is rough and dull and sooty. Have both cups at room temperature and fill each with equal amounts of boiling water. Test with thermometers to see which cools first. Is it economical of heat to keep the sides of a saucepan smooth and clean ?
2. Conduction of heat.
a. Hold one end of a long piece of wire, or an iron poker, in a flame, while you hold the other end in your hand. Feel how the heat is led along or conducted from one part of the metal to another.
b. Are all substances equally good conductors of heat? Repeat (a) with a glass rod or a long splinter of wood, instead of the wire. Does the other end grow as hot? Hold a test tube two-thirds full of water in a flame, but at an angle so that the water at the top is directly heated. Can you get the water at the top hot while the water at the bottom is still cool ? Is water a good conductor of heat ? Is air ?
c. Test the relative conductivity of two saucepans by taking two of different materials (as for example, one of aluminum and one of granite). Pour into them equal amounts of cold water, and place them over two flames which are equally hot, or heat first one and then the other. Determine which boils in the shorter time.
B. Scramble an Egg.
Serve on a slice of toast. Decide how the heat has been transmitted to each in the cooking.
Beat an egg slightly, add a tablespoon of milk, and season with pepper and salt. Pour into a pan in which a teaspoon of butter has been melted, and cook, scraping the mixture from the pan as it sets, until all is creamy.
There is an old story of a man who was held to be a wizard because he could blow both hot and cold; he blew on his hands to warm them and on his soup to cool it. But fireless cookers can do just as seemingly contrary things, since they can keep hot things hot and cool things cool. All that is necessary is that they be constructed in such a way that little heat can pass in or out of them.
The simplest form of fireless cooker is the hay box; literally a box stuffed with hay which, with the air spaces between, makes a non-conductor of heat. Usually, the hay is kept from scattering by covering it with stout cloth. Whatever is to be cooked is placed in water in a saucepan or pail, heated to boiling, tightly covered, and placed in the center of the hay. The difficulty with this type of cooker is that some steam with the odor of the food escapes from the cooking vessel and is absorbed by the hay, which gradually becomes musty and must be replaced. More efficient cookers are lined with non-absorbing material, such as enameled metal, which can be washed if desired. The metal, being a fairly good conductor of heat, even when enameled, must be made double with an air space between. Further insulation can be put between these two layers. This type is commonly furnished with soapstones, which can be heated as hot as desired and placed inside to increase the heat and make even baking possible.
Of course, none of these cookers is so constructed that no heat can escape, and, gradually, the material inside becomes cold. The efficiency of different cookers is measured by the heat that is retained after some hours of standing. To test, equal amounts of boiling water are put in different cookers, and the temperature of water is taken after a given number of hours.
There is now on the market a combination of a gas stove and a fireless cooker which is convenient. The range can always be used as an ordinary gas stove, but over some of the top burners are hung cylinders which may be lowered at will to cover the kettle which has been heating over the burner. At the same time, the flame is extinguished by the automatic turning off of the gas. The oven, too, is exceptionally well insulated and may be used as a fireless cooker also. While the first cost of these stoves is more than that of the ordinary types of ranges, they are undoubtedly great savers of gas.
A thermos bottle uses the same principle as a fireless cooker. Since heat cannot pass in any more readily than it can pass out, both may be used to keep cold drinks cold by protecting them from the heat of the air. Thermos bottles are made with a vacuum between the inner and outer layers, and are more resistent to the passage of heat than the ordinary cookers.
Ice-boxes and refrigerators, too, are efficient in the measure in which they are non-conductors of heat, this depending on the kind and number of layers of material used for "packing." Some people recommend the wrapping of the ice in the ice-box in paper of some heavy material to keep it from melting so fast by protecting it from the heat of the air inside the box. This must not be done if it checks the melting too much, for it is the melting of the ice which causes the low temperature in the box, and the lowering of the temperature is in proportion to the melting, the heat of the air being rendered latent as the ice changes to water.
Any good school textbook in Physics on heat. U. S. Dept. of Agriculture. Farmers' Bulletin No. 298, "The Fire-less Cooker."
U. S. Dept. of Commerce. Bureau of Standards, Circular No. 55, sections on Radiation, Refrigeration, and Ice.
1. Is the iron used for the top of stoves a good or bad conductor of heat? Does this matter in cooking?
2. Why can you hold your hand for a few moments in an oven whose temperature is above that of boiling water?
3. How are steam and hot water radiators usually finished? Why?
4. Which boils more quickly, a new tin kettle or one which is dull on the bottom?
5. Describe different types of fireless cookers.
6. Why are the metal bails on cooking vessels often made with wooden handles?
7. What was the old-fashioned tea cosey and how did it work?
8. What are the materials commonly used in making ice-boxes?
9. Why is the ice compartment at the top instead of the bottom of an ice-chest?
10. How may the weight of a piece of ice be determined by its measurements in inches?