Into a deep glass jar ("battery jar ") place, upside down, a large funnel filled with spinach or parsley; then almost fill the jar with the carbonated water. The stem of the funnel must be entirely under water. Now fill a test tube with water, close its mouth with your thumb, set the test tube mouth downward into the battery jar, and slip it carefully over the stem of the funnel. Then set the whole apparatus in bright sunlight for an hour or two.

When green leaves act on carbonated water

in the sunlight, they break up the carbon dioxide and set oxygen free.

If you watch carefully, you will find that bubbles of gas rise into the test tube. When enough gas has been collected, remove the test tube carefully from the funnel, close the tube, under water, with your thumb, and then turn the tube right side up. Now find out what the gas is by putting into it a splinter with a glowing tip. The gas is oxygen. Does this experiment help us our question as to dioxide is removed

to answer

how carbon
from the air?

The answer is that

green plants use up carbon dioxide and give back oxygen to the air. Thus, in spite of all the ways in which oxygen is used up, we still have it in the air.

How is the carbon dioxide removed in winter? Does all the earth have winter at the same time?

44. How Do Plants Help Animals? — In thinking of the ways in which plants help animals, we must consider that plants are not only the food of animals, but that they prepare and purify the air for the use of the animals, as we have just learned. This careful balance between the action of plants, which remove carbon dioxide from

the air and give back oxygen, and the action of the animals, which do the opposite, is one of the most interesting facts man has discovered. You can study this balance in an aquarium. If you have some small animals, such as goldfish, in the aquarium, and the right quantity of water plants, the water will not grow stale, but the plants will use up the carbon dioxide given off by the animals and will return the oxygen to the animals for their future use.

45. Exercises. 1. How may a stove make the air of a room unfit to breathe? Is there any danger in sleeping in a room having a stove? 2. When soda is put with tomatoes, a foaming takes place; why? 3. If a candle is lowered into an old well and goes out, what gas may be in the well? How did it get there? Should a man go into such a well?

4. Is there any soda in soda water? What causes its frothing?

5. Name six white substances which are partly made up of carbon. Name two transparent ones.

6. Coal and diamonds are each largely carbon; why the great difference in their values?

7. If you have a bottle of oxygen and one of carbon dioxide and neither bottle has a label, how can you tell which is which?

8. Would a carbon dioxide fire extinguisher work better with a fire near the floor or with one near the ceiling?

CHAPTER VII

THE AIR WE BREATHE

46. Why Do We Breathe? Have you ever thought of the interesting act we call breathing and asked yourself how and why we breathe? Of course all of us know what breathing is. It is the taking of outside air into the lungs and then expelling the used air from the lungs. Is air changed by breathing? To answer this question, carry out the following experiment:

Provide a bottle with a cardboard cover and make a hole in the cover. Put a glass tube or lemonade "straw" through the hole and into the bottom of the bottle. Hold your breath as long as possible and then blow it through the tube into the bottle. Now test the air in the bottle with a burning stick. Does the stick continue to burn? What has the air lost? What has it gained?

Men have found that while the air which rushes into the lungs is more than one-fifth oxygen and contains very little carbon dioxide, the air which leaves the lungs has lost a part of its oxygen and has gained carbon dioxide. Thus we see that not only in supporting fire is oxygen used and carbon dioxide formed, but also in supporting life.

Why then do we breathe? The answer seems to be to get oxygen into the body and to remove carbon dioxide from the body. We must remember, too, that the oxygen

we breathe does not stop in the lungs, but passes through the walls of the lungs into the blood vessels; it is then carried by the blood to all parts of the body. The whole process by which oxygen gets to the cells of the body is called respiration.

A grown man takes into his lungs about 350 cubic feet of air in a day. How much of this is oxygen? Why," you will ask, "do we need to take in so much oxygen?" The answer is that we need oxygen to oxidize the food we eat. The digested food, like the oxygen, is carried by the blood to all parts of the body. The oxidation of our food is a change somewhat like the burning of coal in a stove; at any rate, it produces heat, and carbon dioxide is formed, just as when a candle burns in a bottle of air (cf. § 24). It is only by the oxidation of our food that we get the power to move and to do work. The heat produced by this same oxidation keeps our bodies at about 98.6° Fahrenheit, winter and summer, as long as we are well.

Water animals need oxygen to oxidize their food and make movement possible, just as land animals do. Such creatures as fishes, clams, and tadpoles depend upon the oxygen dissolved in the water in which they live. Their gills act in place of our lungs in taking up oxygen and giving off carbon dioxide.

47. Why Do We Need Ventilation? Do you know what ventilation means? It comes from a word meaning "wind," and means the bringing of fresh air into our houses and the taking out of the used air. Do you realize how bad it is to breathe the same air over and over again? Doctors know that people who have good health

in warm weather, when they keep their doors and windows open, have colds and other troubles of the nose, throat, and lungs almost as soon as cold weather comes and their houses are tightly closed. The sealing up of the openings keeps the fresh air out and the foul air in. This is especially true when storm windows and weather strips are used to "keep out the cold."

It has been calculated that in one hour a healthy man makes about 4000 cubic feet of air unfit to breathe. This means that if one man were put into a perfectly tight room 20 feet square and 10 feet high, he alone would make the air bad in one hour. Of course rooms are not air-tight and so a good deal of fresh air gets in through cracks around doors and windows, even when we do not try to ventilate. Schoolrooms, since they have a large number of persons in a small place, need special care to make them safe. According to a Massachusetts law, each pupil should get at least 1800 cubic feet of air every hour. If a person coming in from out of doors notices that the room has an odor, the air has been breathed too often. The class work in such a room is probably dull and slow.

48. Fresh Air and Tuberculosis. — Most of us probably know some one who has, or has had, tuberculosis, or consumption." It is called It is called the "Great White Plague," because it causes the death of so many people. Persons suffering from this disease are now generally given the fresh-air treatment. They live in tents in the open air, winter and summer, day and night. Of course they must be warmly clothed and must have very nourishing food. Nearly all who begin the fresh-air treatment when they are first attacked by the disease are able to cure themselves. Many healthy persons keep themselves well by sleeping out of doors winter and