a dish of linseed oil (Fig. 24) and leave it for a day or two. The oil will rise part way up the tube and then stop; why? What substance does linseed oil take out of the air?

Common paint consists of linseed (flaxseed) oil and turpentine, mixed with "white lead" or "zinc white" and perhaps a colored substance. White lead and zinc white are used to give the paint "body," or covering

FIG. 24. Linseed oil unites with

the oxygen of the What gas remains?

air in the test tube.

power." The turpentine not only thins the paint, but assists in drying the oil.

When paint dries, its linseed oil is oxidized by the air to a hard gum which does not dissolve in water and so resists the "washing of the rain; this gum forms a durable coating for the outside of a house as well as for inside woodwork.

Considerable heat is given off as paint dries; hence heaps of painters' cloths sometimes take fire without our knowing why. The phenomenon is called spontaneous combustion, or spontaneous ignition, because it seems to take place "of itself." But it is a case of oxidation as truly as the burning of a match. Cloths containing linseed oil or paint should never be left about a building except in covered metal boxes or cans.

Do you sup

34. What are Rusting and Decay? pose that rusting has anything to do with oxidation? Carry out this experiment:

Wet the inside of a test tube with water and drain off most of the water. Then put some iron filings into the moist tube and shake them around in the tube. Pour out the filings that do not stick to the tube. Now set the test tube, mouth downward, into a dish of water and let it stand for a day or two. What happens? The iron rusts and

at the same time water rises into the tube. Finally no further change takes place. What does the iron take out of the air of the tube?

Iron rust is iron oxide; the rusting of iron is really the slow oxidation of iron. Heat is given off, as in the burning of iron, but so slowly that we cannot usually notice it. Many other metals tarnish, or rust, in the air.

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Decay. What becomes of the dead bodies of animals and plants and of dead leaves, wood, and fruit? We say they “decay " or rot." Decay is another case of slow oxidation. Some kinds of the tiny plants we call bacteria (cf. § 204) bring about the decay; they use up oxygen in doing so.

Have you ever seen a "hotbed," in which vegetables may be grown during the winter or in early spring? The hotbed is a large, shallow box with a glass cover; the lower part of the box is surrounded by manure. The heat given off by the decay of the manure keeps the ground and air inside the box warm, so that vegetables can be grown there even in freezing weather.

Thus we see that oxygen is not only the supporter of combustion, but the world's great purifier. It turns the remains of former life into harmless substances, so that the life of the present time may have a better opportunity.

35. Exercises. - 1. What names should you give to the substances formed when tin and zinc burn in the air?

2. What substance is used to polish stoves? Why?

3. What metals are used to cover iron to keep it from rusting? What is "galvanized" iron?

4. Why is the soil in some places red?

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5. Of what are tin" dishes made? Test one with a magnet.

6. Examine an old washboard; what is the color of zinc rust? Why is zinc used for washboards instead of iron or copper?

7. Covering wood with an oxide protects it against fire. Why? 8. Let an empty "tin " can rust without losing any material. Does the rusted can weigh less, or more, than the original can? Why?

Burning
Gas

CHAPTER VI

CARBON AND CARBON DIOXIDE

36. Why Do Some Substances Char? Have you thought, as we studied about fire and oxidation, how strange it is that the active, colorless gas oxygen is hidden away in the red iron rust and the white potassium chlorate, and is hidden so successfully that we would never guess that it is there, until we study science? Well, the oxygen is no more skillfully hidden than is carbon, the black solid that we see in coal and in the "black lead," or graphite, of our pencils. Is it easy for you to believe that the brilliant diamond, the hardest substance known to man, is made up of the same black carbon that is present in coal? Yet this is true; the diamond is nearly pure carbon.

THeat

FIG. 25. The baking-powder

box has a hole in

its cover.

When

the box is heated, a combustible gas

escapes through the hole; this gas comes from the wood.

But there are more common hiding places for carbon than in diamonds.

Light

a match or splinter, and after a few seconds blow it out and examine the partly burned end. The end is now a black substance, no longer wood; we call it charcoal. Charcoal is a form of carbon. If we heat wood in a deep dish, such as a test tube or a bakingpowder box (Fig. 25), and apply a burning match to

the open end of the dish, we find that the heated wood is giving off a gas that can be set on fire. When the heating is over, charcoal remains. When soft coal is heated in

the same way, coke remains.

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Nearly all plant and animal substances behave like wood: when they are heated, they char, or turn to carbon." This is a short way of saying that heating

breaks up the delicate substances formed by animals and plants; that a large portion of these substances escapes as a gas, but that a part remains behind as solid carbon.

Men make charcoal on a large scale (Fig. 26) by piling wood in heaps, covering the heaps with sod, and setting the wood on fire. Small openings are

FIG. 26. Charcoal is made by the heating of covered heaps of wood.

left to allow a little air to enter and the gases to escape. The part of the wood that burns gives off the heat which is needed to drive off the gases from the wood that is near it. This is exactly what happens in the half-burned match.

If a ton of wood were "turned into charcoal," would the charcoal weigh more or less than a ton? Why?

37. What Is Formed When Carbon Burns? We are now ready to see how carbon burns in air or oxygen and to learn what substance is formed. Fasten a piece of charcoal to a wire, or hold it in a combustion spoon (cf. Fig. 23, §31) and heat the charcoal until it glows. If we then hold it in a bottle of oxygen, the charcoal

burns with a brilliant light, much more fiercely than in air. After the charcoal glow has gone out, put about a tablespoonful of clear limewater into the bottle and shake the limewater and gas together. A strange thing happens; the limewater becomes white, or milky." The milky appearance is caused by a multitude of white particles which are formed in the limewater. If we let the bottle stand, the particles settle to the bottom of the liquid.

This interesting change took place because the limewater found carbon dioxide gas in the bottle in which carbon had been burned, and united with this gas to form the white particles. Carbon dioxide is formed when the carbon burns in oxygen, just as iron oxide is formed when iron burns in oxygen (cf. § 31). We use limewater to test for carbon dioxide; that is, limewater helps us to know if a gas is really carbon dioxide or something else. For the same reason we used a glowing splinter to test for oxygen.

If we burn wood and coal in a bottle of oxygen or air, and make the limewater test, we find that carbon dioxide is formed in these cases also. If we burn a candle in air or in oxygen, the same thing is true. Men have even burned diamonds in oxygen and have proved that they produce carbon dioxide.

38. How Can We Make Carbon Dioxide? - We have already had one answer to this question: we can make carbon dioxide by burning carbon in oxygen. But if we want to use several bottles of it, we can get the gas more easily by putting some marble in a bottle and