By the time supper is over or even before, during a large portion of the year daylight has gone. Our grandmothers would have brought out the candles. Perhaps we shall use a candle to light our way while we carry the butter and food into the cool cellar.

The candle flame although small in area is typical of all flames. Flame indicates the burning of a gas for solid substances in burning simply glow and do not burn with flame. When wood and soft coal burn, gases are set free by heat and these gases burn over the bed of fuel, giving the flames.

The general form of the candle flame is a cone widest above the base, or about at the top of the wick. If it is examined carefully it will be seen to consist of three layers. Fig. 21. The interior part is dark, giving out no light. The second is yellow and is the luminous part, and surrounding this and most easily seen at the base, is a very thin blue layer.

Experiment. If a small splint of wood or a match be placed across the lower part of the flame near the wick for a moment, it will be charred where the outer layers of the flame have touched it, but the centre will not be changed. Press a piece of card board quickly down on the flame from above and remove it before it is set on fire, and a ring of scorched paper will show the shape of the hot part of the flame.

The candle consists of hydrocarbons (compounds of carbon and hydrogen). When a match is applied to the wick, the hydrocarbons are melted and the liquid rises on the wick by capillary attraction. The heat changes this to gas (or vapor) which is set on fire, since at the high temperature it easily unites with the oxygen of the air. There is plenty of oxygen present, but it is all seized upon by the carbon and hydrogen in the outer parts of the column of gas rising from the wick, so that none reaches the centre. The gas diffuses outward toward the oxygen continually, so that the inner cone may be regarded as a gas factory. The yellow light is caused by the incandescence or glowing of small particles of carbon, heated to "white heat." These are set free from the compounds where the flame is very hot and they are not yet united with oxygen.

Flames "smoke," that is, throw off unburned carbon when there is an insufficient supply of oxygen. Any device which constantly renews a steady supply of air (with oxygen) will make the flame burn better. The chimney of a lamp does this by protecting the flame from wind and by making, enclosing, and directing upward a current of air. The chimney makes the lamp "draw," as the chimney of the house makes the stove "draw."

When the air is mixed with an inflammable gas and the temperature of any part is raised to the kindling point of the gas, as happens if a light is brought into such a mixture, an explosion takes place. The flame spreads through the whole and combination ensues everywhere almost instantly. Great heat is produced and the gases expand suddenly and with violence. If the gases are confined, the enclosing walls may be broken by the pressure. Contraction follows this expansion and air rushes in, producing a second sound. The sounds occur so near together as to give the impression of one.

In a mixture of inflammable gas and air there must be a certain proportion of each to give conditions which will produce an explosion. A very small amount of gas in the air will not explode under any conditions, as when there is an odor of coal gas in the room from which no explosion follows even though a light be present. On the other hand, a mixture containing a large proportion of inflammable gas and a little air will not explode. The proportion of air to gas in an explosive mixture varies in different cases, but in general ranges from about twelve to five parts of air to one part of gas. It is, of course, never safe to rely on the chance of the correct proportions of gas and air not being present.

Fig. 21. Flame of a Candle

Fig. 21. Flame of a Candle

Lighting 36

Explosions sometimes occur by unwise use of kerosene in kindling a fire in a stove. If the kerosene is poured upon a fire already burning, enough vapor of kerosene may be produced to give a disastrous explosion. Soaking wood or paper in kerosene for use as kindlings and then lighting would produce no such dire results.

Explosions in mines are usually caused by a gas called fire-damp and composed of carbon and hydrogen. When this escapes from the coal and becomes mixed with air, it is very explosive. If a miner brings a naked flame into the mine, the fire-damp will ignite and disaster results. A safety lamp was devised by Davy for use in such dangerous places. It was found that a gas is cooled below its kindling temperature in passing through a fine wire gauze. Lamps surrounded by such a gauze may be taken into a mine with comparative safety. Fig. 22.

The action of the wire gauze upon the gas may be studied by holding over a gas jet a piece of fine wire netting, such as is used in window screens, and then lighting the gas above the netting. Fig. 22a. It will be seen that the gas below the netting is very slow • in igniting, since it does not readily become sufficiently heated, the wire netting cooling it below its kindling point.

The kerosene lamp gives light by the principle already described. The reservoir of the lamp corresponds to the cup of melted tallow at the top of the candle. The oil is drawn to the top of the wick by capillary attraction, where the hear vaporizes it; so that vapor and not oil is what really burns. The structure of the flame is precisely like that of the candle, although its shape differs, because of the shape of the wick.

Safety Lamps

Kerosene Lamps

Illuminating gas is today the source of light in most city houses. There are two kinds of gas now furnished for this purpose. Coal gas is obtained from the destructive distillation of soft coal. Receivers or retorts of iron or fire clay are filled with soft coal and heated to 1100° or more. From these retorts tubes lead up into a large pipe called the hydraulic main, through which water is kept flowing. As the coal becomes heated, a number of different substances are given off, which at this high temperature are in the gaseous state. Some of them dissolve in the water of the hydraulic main, but those needed for illuminating gas are not soluble and passing out of the main, they travel through several hundred feet of vertical pipe called the condenser, where more water removes any impurities which may have escaped from the hydraulic main.