We can perform the same experiment with a burning candle. The candle should be a short one and fastened by means of a stiff wire to the cork, so that the candle may be held upright in the bottle.

We can also carry out the experiment with burning sulphur held in a long-handled spoon (combustion spoon). The handle is put through the cork. After burning for a little while, the sulphur flame " goes out" and the bottle has in it a gas with a sharp odor. If we now put into the bottle a burning splinter, or a candle, it will not continue to burn.

25. How Much of the Air Supports Burning? — The experiments we have already performed show us that a burning body needs air. Let us now try to find out whether all, or only a part, of the air takes part in the burning.

FIG. 18.

As the candle burns, it uses up part of the air of the bottle.

a

We can use a short candle, such as

"birthday" candle (Fig. 18). Soften the bottom of the candle by warming it and then press it against a piece of tin, such as a jelly-glass cover, until it sticks. Now set the candle upright in a basin containing water to the depth of about one When it is burning briskly,

inch, and light the candle. put over it, and into the water, a bottle of fresh air. At first the candle flame burns just as it does in the outer air, but soon it dies down and goes out. When the flame goes out and the gas left in the jar becomes cool, water rises into the bottle. What does this mean? It means that there is less air in the jar after the candle has burned than there was before. The water rises to take the place of that part of the air which has been used up.

Another material that helps us in our study of burning is phosphorus, but the experiment must be performed by the teacher, since phosphorus burns with great fierceness. There are two kinds of phosphorus: one is a red powder and the other a yellow, waxy solid that comes in sticks which look like lemon stick candy. The red is much safer to handle and to keep. If the yellow is used, it must be kept under water and cut under water, or it may take fire of itself and cause serious burns. For this reason, the piece cut off for use must be handled with forceps or tongs, never with the fingers. You would never guess that such an active substance

could be a part of our bones and of the rock phosphate that is put upon soil, but it is so.

FIG. 19. The phosphorus burns until it has used up that part of the air which permits burning to go on.

Let the teacher set up the apparatus of Fig. 19. The vessels needed are a pint fruit jar and a shallow pan containing water. The phosphorus is put on a holder that reaches about halfway up the jar. The holder may be a strip of "tin " with a horizontal top about the size of a 25-cent piece, or it may be a small cork fastened to a wire and covered with a disk of "tin." The lower end of the strip or wire is put through a hole or a narrow slit in a piece of sheet lead or into a rubber stopper, so that the phosphorus holder will stand upright. A floating support for the phosphorus is often used; it may be made out of a thin, flat cork with a disk of tin laid upon it to protect it. When the support is ready, we set the phosphorus on fire, put the jar of air over it, and press the jar against the bottom of the pan.

The phosphorus will burn for a time, forming a dense, white smoke; then, although there is still some phosphorus left, it stops burning. Finally the smoke dissolves in the water, leaving the gas in the jar quite clear and transparent, like the air itself. But long before the smoke disappears, water will rise into the jar.

Let us examine the bottle in which the candle has burned, or the jar in which phosphorus has burned.

2,

What fraction of its volume is filled with water? Is it 1 ,,, or what? To find out, we can close the mouth of the bottle or jar, under water, with a card and then remove the jar from the water and set it upright on the table. If we put a burning splinter or candle into the jar, we find that the gas in the jar is inactive and does not allow burning to go on.

26. Is Air a Mixture? - By burning a candle or some phosphorus in a jar of air we find out a very wonderful thing. When the experiment is carried out with great care, the water rises into the jar until it fills about of it (nearly 21 per cent). Many other burning substances act upon air in this way; that is, after they have used up about of the air, they do not act further. How can we explain this fact? The explanation is that the air consists of at least two different gases: one that permits burning to go on and another that does not. The active gas, which unites with substances when they burn, is called oxygen. The inactive gas, making up about of the air, is nitrogen (mixed with small amounts of some other gases). The air is oxygen diluted with nitrogen, much as lemonade is lemon juice diluted with water.

27. How Can We Explain Burning? We can now understand what the strange phenomenon called burning, or fire, really is. Burning is the uniting of a body with the oxygen of the air. The burning body and the oxygen rush together so vigorously that heat and light are produced. The burning body sometimes disappears, as far as we can tell by our senses, but if we have the skill and the patience, we can prove that it has merely

united with oxygen, and we can collect the invisible materials that are formed. The white smoke formed when phosphorus burns contains both the phosphorus that was used up in burning and the oxygen that united with it.

Another name for burning is combustion. A substance like wood, coal, or fuel gas, which burns in the air is a "combustible substance," or, simply, a combustible.

28. What is a Flame? Have you ever asked yourself, as you watched the flame of a bonfire leap high into the air, what a flame really is, and how it can reach out so far from the burning body? And have you noticed that a jet of gas, or a candle, or a piece of wood burns with a flame, while a piece of burning charcoal or coke merely glows? Why is this?

To find out the reason, let us study the quiet, little flame of a lighted candle. A candle consists of wax, or tallow, and a wick. When the wick is set on fire, some of the wax is melted, and the liquid wax rises through the wick into the flame, where it is burned. In a kerosene or alcohol lamp the liquid rises through the wick in the same way.

If we look at the flame carefully, we see that it consists of a dark, central part surrounding the wick, and a burning, outer part that gives the light. How can we find out what is taking place in the central part? One way is to hold one end of a small tube in it, as shown in Fig. 20. If we then bring a burning match near the upper end of the tube, we can get a flame there, too. What does this mean? It means that an invisible, combustible gas is formed in the dark, central part of the flame and passes

up through the tube. On the wick and in the central part of the flame the wax is being turned into this gas. Why does not the gas burn on the inside of the flame? Because the air gets at the gas only on the outside. So the candle flame really consists of a burning, outer region surrounding a central region of unburned gas; a flame is a burning gas.

FIG. 20.

In the center

of a candle flame there is

a combustible

gas

that can

be drawn off through the 'glass tube and burned.

By

We can show in other ways that flames are burning gases. Suppose we "blow out" a candle flame, and at once hold a burning match just above the wick. making several trials we shall find that, even with the match quite a distance above the wick, we can still relight the candle. What is it that passes from the wick into the air and is set on fire by the match? The invisible formed out of the wax. If we give the wick the wax into a gas;

gas

time to cool, it can no longer turn

then we cannot relight the candle without heating the wick up to its kindling temperature. We can carry out

the same experiment with a kerosene lamp.

If we watch a burning piece of wood, especially a burning log, we see that gas comes out of the wood in little jets that burn with flames; often these can be lighted at some distance from the wood. Soft coal gives off a gas in the same way, and so burns with a flame. Coke and charcoal burn without flames because they have no combustible gases to give off (cf. § 36).

1. Why are paper and wood used in kindling a

2. How can a "burning glass" start a fire? Do you think that broken bottles left in the woods might act as burning glasses and cause forest fires?