A. Prepare Scalloped Potatoes.
Wash, pare, and cut a potato into very thin slices. Put in layers in a baking dish. Season each layer with salt and butter, and pepper if desired. Cover with milk, and bake in a slow oven until the potato is soft.
B. Class Experiments. 1. Fuels.
Take a narrow test tube and fill it two-thirds full of wood - the stems of matches will do. Heat, holding it cautiously in a flame. As smoke escapes, put a lighted match in the smoke and see if it can be set on fire. Notice the black residue left in the tube. This is charcoal.
2. Is air necessary to combustion?
Lower a candle or a burning splinter of wood into a bottle of air and cover as closely as possible. Does it continue to burn ?
3. What is formed when fuel burns ?
a. Hold a cold glass tumbler for a moment over a burning candle. Observe whether moisture forms on the inside of the glass.
b. Burn a candle or a piece of wood in a covered bottle till flame is extinguished.
Remove the candle or wood quickly and pour in a little lime water, and shake it around. Does it become milky? Try lime water in a clean bottle of air. Carbon dioxide is the gas which turns lime water milky. c. It is commonly said that food acts as fuel in the body. See if the " products of combustion", water and carbon dioxide, can be detected in the air breathed out. Test as follows:
(1) Breathe on a cold pane of glass. Does moisture collect?
(2) Breathe through a glass tube or a lemonade straw into lime water. Do we breathe out carbon dioxide?
When the word combustion is used, it ordinarily means burning, that is, the union of a substance with the oxygen of the air with such rapidity that both heat and light are produced. But in order to have this combustion take place, it is necessary to have something more than a combustible substance and oxygen. Wood is a combustible substance, but it does not burn unless it is sufficiently heated to "take fire." Not all materials have to be heated to the same degree to make them burn, and the point to which each must be heated is called the kindling temperature of the substance. Phosphorus has a low kindling temperature and can easily be set on fire by the heat of friction; that is why it is used on the heads of matches.
Most combustible substances contain both carbon and hydrogen as well as a little oxygen. When they are burned in the air the carbon unites with the oxygen to make carbon dioxide, and the hydrogen with oxygen to form water, oxygen from the air being used in the process. The common fuels are inexpensive substances which are largely composed of these three elements. Foods, too, contain the same elements in large amounts. When food is burned in the body the process is a much slower one than ordinary burning and no light at all is produced, but the heat maintains the body temperature. Some fuels, like some foods, have nitrogen in them, but this does not help in the production of heat. The elements in fuels and foods are put together so differently, however, that they are entirely unlike in their nature, and the body could not burn coal or wood instead of food.
The fuels that are most widely used in this country are wood, coal, kerosene, and gas. Wood is becoming so expensive and requires so much space for storage, that, in cities, it is used only in starting a coal fire. In country districts where wood is cheap, wood stoves are still in common use. Wood must be set on fire by piling it on top of burning paper, straw, or shavings. Such kindling is not sufficiently hot to set fire to coal, so, in laying the coal fire, both paper and wood are used. Wood is roughly divided into two classes, hard and soft; the first is desirable when long-continued, steady heat is necessary; the other for quick, hot fires. For kindling, soft wood must, of course, be chosen. The usual way of selling wood is by the cord, which consists of one hundred and twenty-eight cubic feet.
Coal is of two general kinds, anthracite or hard coal, containing about ninety per cent of carbon and very little gas, and bituminous or soft coal which contains gas and burns with considerable flame. The latter variety is dirty to handle and gives off much soot. It costs less by the ton, however, than hard coal, especially in some parts of the country, so it is often commonly used. A fire made with it requires more frequent attention than one made with hard coal, and when this and the cost of the cleaning and laundering which it necessitates, as well as the wear that this extra laundering means for fabrics, are all taken into account, it is doubtful whether the use of soft coal is really cheaper. Coal is sold by the ton; a long ton is 2240 pounds, a short ton only 2000 pounds.
Kerosene is also a much-used fuel, and in the blue-flame stoves a very satisfactory one. It is one of the oils present in petroleum, a mixture of natural oils found in the ground in large quantities in some parts of the country. In order to be sure that the more inflammable oils are not left in the kerosene, in most states the quality is regulated by a requirement that the flashing point shall not be below a certain temperature. The flashing point is the temperature at which the vapor from the kerosene will catch fire or flash. The kerosene itself does not burn, and the vapor only for an instant. The temperature required below which the vapor must not flash varies from 110° F. to 200° F., the latter meaning a very high-grade oil. Probably 149° F. is sufficiently safe, but as all kerosene is explosive, care must be taken in its use. Stoves and lamps should be filled only by daylight and never when they are lighted or hot. Kerosene is sometimes poured on a fire of coal or wood to act as kindling, and there have been many accidents from such use. Safety requires that it should never be used in kindling. The danger lies in pouring it on after the fire is started, or when there are hot ashes in the bed of the fire.
Gas is a much cleaner fuel to use than any of those already mentioned. There are many varieties. Natural gas, like kerosene, is found in the ground in certain parts of the country. Its cost is much below that of artificial gas. The latter gas is made by two different methods, one giving us coal gas, the other water gas. Coal gas is obtained by heating coal, usually semi-bituminous, in retorts so as to drive off the gas which it contains. Water gas is made by passing steam over heated coal; then this is enriched by the addition of other gases in order to make it more efficient. Any gas is dangerous, because, when it is mixed with a certain amount of air, it becomes explosive, and because some of the gases present, if they escape un-burned, are highly poisonous. Water gas is particularly poisonous. Leaks in gaspipes should not be neglected. A light should never be taken into a room where there is a strong smell of gas, windows and doors should be opened, and, if necessary, the gas should be turned off from the whole house by the main stopcock near the meter. The surest way to detect small leaks is to paint over the suspected places with strong soapsuds, and notice where bubbles are blown.
Acetylene is another sort of gas used for fuel in special stoves. It is manufactured, usually on a small scale, by the automatic dropping of calcium carbide into water. The gas requires special burners, but gives a brilliant light. It seems still to be a matter of dispute whether it is highly poisonous as well as explosive.
Gasoline gas, sometimes called air gas, is made by pumping air through gasoline. The law requires the gasoline tank to be outside and at a certain distance from the house, although the mixing chamber where more air is added may be nearer. The gas makes an excellent fuel and does not contain carbon monoxide, the compound in coal gas which is most poisonous.
There are two kinds of alcohol which are common. One, wood alcohol, ought not to be used, because its fumes are poisonous. It is much cheaper than the other variety called grain, or ordinary alcohol, because that is so highly taxed. In recent years a way out of this difficulty has been found in the use of denatured alcohol. This is merely grain alcohol to which some substance has been added that makes it impossible as a beverage and thus it escapes having to pay the heavy tax otherwise imposed. The sub-stance added in no way impairs its use as a fuel.
The use of electricity for cooking has certain advantages which are furnished by none of the fuels. In its use there are none of the products of combustion to get rid of, there is no flame to set fire to the unwary, no matches to be looked after, and its control is simple. It is, however, generally too expensive for common use. Electricity is measured by the kilowatt hour, the cost varying from about ten to fifteen cents. The dials on an electric meter are not unlike those on a gas meter and can be read easily.
White. "Fuels of the Household."
U. S. Dept. of Commerce. Bureau of Standards Circular No. 55, sections on Coal, Wood, and Heating Value of Fuels.
1. Name the different substances which may be used as fuels and arrange them in the order of their kindling temperatures.
2. Name the different kinds of coal used in the house, and, briefly, characterize each.
3. Determine the comparative costs of the different fuels used in your locality. Which is most commonly used and why?
4. In an ordinary wood or coal stove, what becomes of the products of combustion? Where do they go in a gas range? Account for the difference in arrangement.
5. How is illuminating gas manufactured?
6. What source of heat, sometimes used for cooking, is not the direct result of combustion?