A. How Heat Passes from One Place to Another.
1. Convection of heat.
Put cocoa shells or sawdust into water and heat - in a glass beaker if you have it. Notice the movement of the particles. Does this indicate movement of the water also?
B. Prepare Potato Salad.
Use one potato. Serve with a crisp slice of bacon. Explain carefully exactly how the heat in cooking passes from the fire to the potato: to the bacon.
Cut a boiled potato into half-inch cubes. Season with salt and pepper, mix with a very little chopped onion and parsley, add two teaspoons of oil, and one teaspoon of vinegar.
Lay in a frying pan a thin slice of bacon with the rind cut off. When one side is brown turn the slice.
In some ways the passing of heat by convection is harder to understand than the two other methods of transfer. In conduction, heat is said to be passed on from one particle to the next. Water is known to be a poor conductor, although nobody knows why this is so. In the experiment in which water with sawdust in it was heated in a beaker, the water would not readily pass the heat from one particle to the next. Instead, the water at the bottom of the beaker became heated by contact, and heat has exactly the same effect on water that it has on air. The water which is heated expands and so becomes lighter. Then it is pushed up by the colder water above, which is pulled down harder by gravity, to become heated in its turn. The whole of the water becomes hot, not by the passage of heat from one particle to the next, but by the movements of the particles themselves, carrying the heat with them. Thus a circulation of the water is started, the hottest water rising to the top. So a water boiler, when it is heating, first becomes warm at the top. This can easily be felt by placing a hand on the outside of the boiler.
Boilers and hot-water pipes lose heat through radiation and contact with air, and, to decrease the loss, are sometimes jacketed with asbestos. Asbestos is a mineral sub-stance, finely shredded and pressed into a sheet. It is not only a poor conductor of heat, but is fire-proof as well. It is fairly expensive.
In choosing utensils for the kitchen many things must be taken into consideration. First of all, the probable number of people to be cooked for will govern the size of many of the utensils. Moreover, the style of living will affect the kinds and number of them. In general, for the sake of storage room and convenience of access, the number of utensils should be kept as small as possible. Utensils which can be used for many purposes should be selected rather than those that fit a single need, unless that need is frequent. In nearly any kitchen are to be found a number of utensils that are good, but are so seldom needed that they are never used, because it is too much trouble to find and wash them for the occasion. Beside these considerations, convenience in handling, ease in cleaning, and durability must all be taken into account. There are saucepans which upset easily; and saucepans with handles with sharp edges or which grow hot too quickly; saucepans and skillets with lips on only one side, and that the wrong one, so that if one tries to pour from them and stir at the same time, the stirring must be done with the left hand. Ease of cleaning demands that the utensils be smooth, with rounded sides and no seams or corners, and that they should be wide enough to permit easy access for cleaning.
A Group or Kitchen Utensils.
Durability depends partly on make and partly on the material used. Saucepans are usually of aluminum, enamel or granite ware, or tin. The so-called tin utensil is steel, or sometimes wrought iron or copper, covered with tin. It is the least expensive of the three types and also the least durable. Cheap grades are easily attacked by the weak acids in fruits and vegetables, and even the better grades are not proof against these acids when hot; but tin vessels are light and good conductors of heat. Since tin is sufficiently soft to scratch easily and to wear off it is better suited for baking pans and bread and cake-boxes than for saucepans.
Granite and enamel ware is made by coating an iron or steel foundation with a glaze which is not unlike glass in its nature. The quality depends upon the character of the foundation, upon the ingredients of the glaze, and the number of coatings, as well as on the success with which every particle of the metal is covered. The durability is greatly affected by the care that is exercised in using it. Sudden heating or cooling, too vigorous scouring, and dropping, all tend to make it crack and chip off, exposing the metal beneath.
Aluminum is light, and also an excellent conductor of heat. It darkens if any alkaline substances are used in cleaning it, and should be scoured inside with fine steel wool, not soaked in washing powders. It is affected slightly by acids, but the experts tell us that the amount dissolved is insufficient to harm us. It warps if subjected to too much heat. However, it makes a durable saucepan and probably justifies its cost.
Clark. "Care of the House."
U. S. Dept. of Commerce. Bureau of Standards, Circular No. 55, section on Water.
1. Examine a coal range and see whether it has a water-back or a water-front. What makes the water pass from this container into the boiler?
2. Make a diagram of the hot-water system in your school or in a house, and explain the circulation through the pipes.
3. Of what material is the boiler made? What other materials may be used? Is it always wise to use the water from the hot-water faucet for cooking or drinking?
4. Has your boiler a faucet connected with the lower part of the boiler? What is its use?
5. What is the water rate in your town? If meters are installed in the houses, learn to read one.