This section is from the book "Elements Of The Theory And Practice Of Cookery", by Mary E. Williams. Also available from Amazon: Elements Of The Theory And Practice Of Cookery; A Textbook Of Domestic Science For Use In Schools.
Food is cooked chiefly by means of heat. Heat is commonly obtained by burning something. Let us learn what we can about fire.
Experiments. A. Into a clean bottle pour a little clear lime-water; cork the bottle, and shake it so that the lime-water may come in contact with the air in the bottle. Do you see any change in the appearance of the lime-water?
B. Insert a splinter in a cork, light the splinter, and fit the cork into the neck of a bottle. What happens? Pour a little clear lime-water into the bottle and shake. Note the effect on the lime-water.
Air in which a splinter has been burned turns lime-water cloudy. It must therefore differ in some way from ordinary air.
Air is a mixture of gases, chiefly. oxygen and nitrogen. Wood contains carbon. When the splinter burns, the oxygen in the air and the carbon in the wood unite, forming a new substance, the gas carbon dioxide.
Nothing will burn in carbon dioxide. So when all the oxygen in the bottle is used, and carbon dioxide has taken its place, the fire goes out.1
It is carbon dioxide that turns lime-water cloudy. As only carbon dioxide has this effect, a simple way to show its presence in air is to bring the air in contact with lime-water, as in Experiment B.
Experiments with a candle. C. Set a two-inch piece of candle on the table and light it. How does it burn? Notice the appearance of the flame. (Fig. 1,0.)
D. Set over the candle a lamp-chimney supported on two pencils or blocks of wood. (Fig. 1, b.) Notice how the flame has changed. Hold your hand for a moment about two inches above the chimney, and notice the heat felt. Hold a bit of tissue paper just above the chimney. Is it drawn upward or downward? Hold it near the space at the base of the chimney. Is it drawn outward or inward?
1 The blackened part of the splinter which is left is unburned carbon (charcoal).
E. Remove the supports, letting the chimney rest upon the table. (Fig. 1, c.) Test for heat with your hand, then hold" the bit of paper as before. Do you feel any heat? Does the paper move? What happens to the candle? Can you, by recalling the experiment with the splinter and the bottle, explain this?
F. Relight the candle, replace the chimney upon the supports, and cover the top with a piece of thick cardboard. (Fig. 1, d.) What hap-pens? Explain. Removing the cardboard, quickly thrust a lighted splinter inside of the chimney. What gas do you think may be present?
G. Through a tiny hole in the cardboard pass a fine wire bent into a small loop at one end. Arrange candle and chimney as in Exp. D. Dip the wire loop into clear lime-water, which should form a film across the loop. Cover the chimney with the cardboard, letting the wire hang inside. (Fig. 1, e.) About two minutes after the candle goes out examine the film. What gas has been formed? Is the candle as large as it was before it was lighted? What has become of the part that has disappeared
When a candle is lighted, the wax, by the heat of the burning match, is first melted, and then, being soaked up by the wick, is changed to gas. The oxygen of the air, always eager for something to unite with, seizes upon this gas; in other words, the gas burns. Whenever oxygen unites with another substance so rapidly that light and heat are given off, we have burning, or combustion. The light and heat we call fire. Flame is burning gas.
In still air a candle-flame streams straight upward. This is because hot air is lighter than cold air (p. 27). As the air near the flame becomes heated, it rises, and air from below flows toward the candle to take its place. This starts a draft. And, while the burning of the candle keeps up the draft, this draft supplies the candle with oxygen. When we place a lamp-chimney over the candle, leaving a space at the bottom, we make the draft stronger by shutting off side-drafts. The flame flickers, and the candle burns faster. But if either the opening at the top of the chimney or the space between chimney and table be closed, all draft is stopped; and as soon as the oxygen then inside the chimney is used up, the candle goes out. To keep up combustion, then, we must have a draft. For a draft through an enclosed space two openings are necessary, one to let air in, the other to let it out.
As the candle burns, it grows shorter. The wax is changed into carbon-dioxide, water-vapor, and other gases, which stream off unseen. The water is formed by the union of oxygen with hydrogen from the candle. Not all the carbon unites with oxygen. Some floats off in tiny particles. When there is enough unburned carbon to be visible, we call it smoke. When it is deposited, we call it soot. There are always enough carbon particles in a candle-flame to deposit soot on any cold object, such as a saucer, held in the flame. Anything that will not burn is said to be incombustible. A candle contains nothing incombustible, and so leaves no ashes.
Wherever combustion takes place, whether in fireplace, stove, or lamp; whether a single match burns, or a whole building, something unites with oxygen, giving off heat (and usually light), and forming products of combustion.
In order to manage a kitchen fire successfully, we must understand the construction and purpose of every part of the range. Much fuel is wasted, food spoiled, and time lost because women do not take the trouble to do this.