How many slices of bread are required to make 2 cupfuls of crumbs?
How many slices in one loaf of bread?
LESSON III
FUELS AND COMBUSTION—SAUTED AND BAKED SQUASH
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?