FUEL.—In order to cook foods, heat in some form must be applied. This heat is obtained usually by burning some substance. Thus the first requisite for obtaining heat is something to burn, i.e. a fuel. The fuels commonly used in households are,—wood, coal, kerosene, and gas. Although electricity is not a fuel, its use in cooking is so well established that it should be mentioned as a source of heat. HEAT; KINDLING TEMPERATURE.—There are fuel substances everywhere,—paper, cloth, wood, etc. These materials do not burn unless heated; even gas does not burn by simply turning on the stopcock. But if a piece of paper is placed in contact with glowing iron, the paper burns. It burns because it is heated. If the blazing paper is placed in contact with kindling wood and coal, the kindling wood soon begins to burn because it is heated by the burning paper. The coal burns when it is heated by the burning wood. All fuels must be heated before they will burn. When one thinks of the ease with which paper "catches fire" and of the difficulty of making hard coal burn, it becomes evident that some substances require only a small amount of heat before they will burn, while others require much heat. Different materials, then, require different degrees of heat to burn. The phosphorus and other substances on the tip of a match ignite readily. The heat that is developed by rubbing the tip over some surface is sufficient to make the phosphorus burn. The burning phosphorus and other substances heat the match stick to the temperature at which it begins to burn; the burning match stick applied to paper heats the latter to the temperature at which it burns. The temperature to which a substance must be heated in order to burn and continue to burn is called the kindling temperature of that substance. DRAFT; OXYGEN.—EXPERIMENT 3: LACK OF DRAFT.—(a) Place a short candle on a pan. Light the candle and put a tall slender lamp chimney over it. Does the candle continue to burn? Why? (b) Again light the candle and replace the chimney, but this time support it on two sticks of wood or on the handles of a knife and fork so that it will not rest directly on the pan. Place a saucer or a piece of cardboard over the top of the chimney. Does the candle continue to burn? Why? EXPERIMENT 4: PRESENCE OF DRAFT.—Remove the cover from the top of the chimney, and again light the candle. Does it continue to burn? What substance necessary for combustion is present in the chimney? Explain why the candle soon went out in Experiment 3, but continued to burn in this experiment. If a blanket is thrown upon a burning stick of wood, the wood soon ceases to burn. The wood stops burning because the oxygen of the air is excluded from it. The act of burning, i.e. combustion, is the union of any substance with oxygen, with the result that heat and light are produced. We have learned that a fuel cannot unite with oxygen until heated to a certain temperature. And, no matter how hot it is, the fuel will not burn unless it unites with oxygen. Oxygen, then, is the third requisite for combustion. The necessity for a draft, i.e. a continuous supply of fresh air which furnishes oxygen, is shown by Experiments 3 and 4. SAUTED [Footnote 4: To saute is to brown in a small quantity of fat.] Wash summer squash. Cut it in slices 3/4 inch thick. (Do not remove the skin or the seeds.) Dip each slice in flour. In a frying pan put some fat and heat it. Add the squash and cook each slice on both sides until golden brown in color. Sprinkle with salt and pepper. Then place a cover over the frying pan and continue to cook the squash until it is tender. Serve at once. BAKED WINTER SQUASH [Footnote 6: See "Note to Teacher," Footnote 3] Wash a squash and cut or split it into pieces of suitable size for serving. Remove the seeds from each piece and make several gashes (at right angles to one another) cutting through the pulp down to the shell. Place the pieces (shell down) on the grating in the oven and bake (at moderate temperature) until the pulp is tender. Serve hot, with butter, salt, and pepper. QUESTIONSName the three requisites for combustion. Which has the higher kindling temperature, wood or coal? Explain your answer. What is the price of summer and of winter squash? How much of each kind of squash is required to serve 6 persons? |