not know anything about. What were they? How about the germs of disease, which are almost sure to flourish in the dark and dirty house that has no sunlight? How Is Glass Made? - Do you find it easy to believe that so brilliant and clear a substance as glass (see Figs. 178 and 179) can be made out of sand, limestone, and soda? Yet this is true. The FIG. 178. The glass blower blows a large lump of hot, softened glass into a hollow globe. When this is cut open, and then softened by heat, it forms a flat sheet. FIG. 179. The lump of soft glass attached to the hollow tube is put into the mold. The workman, or a current of compressed air, blows into the glass lump until the glass has the shape of the mold. mixture is melted in fire clay pots about 4 feet high and 4 feet in diameter, until a clear, transparent liquid is formed. In some glass factories a larger amount is melted in a tank. 255. How Have Men Lighted Their Houses at Night? - Few stories are more interesting than the story of the many ways in which men have tried to lengthen the day, for work or pleasure, by making a light in the house to take the place of the light of day. As you know, the pioneers of this country gathered around the open fire for light by which to work or read, as well as for the heat by which to keep warm shows a lamp such as was used by the Greeks and Romans. The Eskimos still use a lamp of this sort, with blubber as the fuel (see § 57). The flame is often two feet high. Can you imagine yourself doing without a furnace, a range, and electric lights, and having only one greasy, bad-smelling lamp instead? Candles. - As the old lamps were easily spilled, and were heavy and inconvenient, the tallow candle was invented. Tallow is a grease which is very hard when cold. To make a candle, men dipped the wick into melted tallow; then took it out until the tallow cooled; and then dipped it again and again until the candle was of the proper size. Do you think this was a rapid or a slow process? A more rapid way is to fasten the wick in a hollow mold, and to pour the melted tallow around the wick. When the tallow hardens, the wick is embedded in it. Watch a candle burn, and see the little bowl of melted grease that supplies the wick with fuel. 256. How Do Kerosene Lamps Work?-Your grandparents will probably remember the first kerosene lamps, which came into use about 1860. Petroleum had Air Wick just been discovered, and when it was distilled it gave kerosene, gasoline, and other combustible liquids. So men turned back from the solid candle to the oil lamp. But kerosene took fire so easily that they had to invent a lamp in which to burn it. This consisted of a bowl and a wick, like the older lamps, but there was a burner to hold the wick, as well as to hold the chimney, and to allow an even current of air to flow constantly past the burning kerosene of the wick. The chimney protected the flame from other drafts, and carried away the gases formed in the burning. What were they, probably? Thus a strong, bright, even flame was produced. Air Air FIG. 181. In the center-draft kerosene lamp, air passes up through the center of the lamp, so that the kerosene on the wick can be supplied with a large amount of air. Some lamps have a center draft (see Fig. 181); this allows more air to enter, and a larger wick to be used. As a result the flame is very bright. Examine a "blue flame " kerosene stove, and find out what kind of burner it has. 257. How Do We Get Light from Gas? Have you ever seen, in the city, the large gas tanks that rise out of the ground like giant toadstools? In them the gas for the city is stored. How do you suppose this gas is made? Some of it is natural gas, which is found in the rocks of the earth in certain places; but most of it men make by heating soft coal, or by passing steam over red-hot coke or coal. Gas is carried to our houses in iron pipes. The joints must be made carefully, because the gas is under pressure; and any leak must be attended to at once. Most gas is poisonous, and a person in a closed room may continue to breathe it until death results. Then, too, there is danger of fire, if a flame is brought near a pipe in which there is a leak. The old way of burning illuminating gas was to pass it through a slit that gave the flame a "fishtail" form. The modern way is to use a gas burner of the form known as a Bunsen burner, which mixes so much air with the gas that it burns with a colorless flame; this flame is very hot. How can a colorless flame be made to give us light? It is allowed to heat a " and this in turn gives mantle," off a brilliant, white light (Fig. 182). 66 FIG. 182. In the mantle light a Bunsen flame heats the film of oxides of certain rare metals. These become incandescent, and give off a brilliant light. Examine a gas stove and its burners. Gas stoves are really Bunsen burners, like those of the mantle light. There are openings in the side of each burner through which air is drawn in by the rushing gas. gas and air burns with a colorless flame, without soot, but very hot. The mixture of 258. How Does a Fuse Box Protect a House? - Is there any danger from the electric light in our houses? Read § 228. There we learned that electric light wires must be insulated carefully, and that builders take special care to keep the wires away from wood and from one another, using porcelain, or rubber, tubes, and metal pipes, or conduits, for the purpose. The danger is that a spark may pass from one wire to another, or that the current intended for several wires may all be loaded upon one of them, wood on fire. Current Enters Switch Fuse Plugs and this may become hot enough to set If the wires were very thick, a great deal of this danger might be avoided; but as copper is expensive, the smallest possible wires are generally used. FIG. 183. A fuse box consists of a set of "fuse plugs" each containing a short piece of easily melted wire. The wire melts, and breaks its circuit, before the circuit is hot enough to set the house on fire. To take care of the danger from heated wires men provide each circuit with a fuse through which the current of that circuit must pass. These fuses are generally placed in a metal box (see Fig. 183), or a box lined with asbestos board. The "switch," through which the current from the outside, "service" line enters the house, is usually in the same box. Find the fuse box and switch in your house and examine them carefully. When we open the switch, we cut off the current from the house. We ought always to do this when any work is being done upon the electric system of the house. Examine a "fuse plug"; it contains a short piece of wire made of an alloy of metals that will melt at a rather low temperature. When any circuit becomes too hot, because of the extra load it is carrying, the fuse on that circuit melts, and the current will not flow until a new fuse is put in. 259. How are Gas and Electricity Measured? — Do you know how the amount you owe for electricity and gas is calculated? Possibly you do, for you may have seen your meters read. A man comes once a month for this purpose. You pay for the amount of electricity or gas which has run through the meter since the last reading |