vessel, we must give the air a way of getting out; if we want to pour the water out of a vessel, we must let the air go in to take its place:

10. Does Air Have Weight? When we have grown

used to the idea that air fills vessels that we think of as "empty," we are ready to believe that air has weight. How can we find out if this is

subtracting the weight of the cup from the weight of the cup and water, we get the weight of the water alone. We write down the results in this way:

You may have seen this method used when you have bought butter in a crock or honey in a pail. The weight of the butter or honey alone is spoken of as the " net weight, while the weight of the material and the container together is the " gross" weight.

When we weigh an "empty" cup, crock, or pail, we make no account of the air which it contains. But when we try to find out whether or not air has weight, we must first get the weight of a vessel which is really

empty, that is, without even air in it. One way to do this is to attach the vessel, such as a flask (Fig. 6), by means of rubber tubing, to an air pump and then to remove all the air we possibly can. Then, while the flask is still attached to the air pump, we close the stopper in the glass tubing. We can now weigh the flask really empty; then we open the stopper, let air enter, and weigh the flask again. The flask and air together weigh more than the empty flask, so air must have weight.

FIG. 6.

A flask from which we can remove the air.

11. Is Air Matter?

Since air takes up

room and has weight, it is a form of matter, or a substance, just as water and sugar are substances.

One cubic foot of air ordinarily weighs about 1 ounces; how can we find out how much the air of your schoolroom weighs?

Suppose that the room is 24 feet long, 20 feet wide, and 10 feet high. The volume, in cubic feet, will be 24X20X10, or 4800 cubic feet. Since 1 cubic foot of air weighs about 1 ounces, the air of the room must weigh about 4800×11, or 6000 ounces. To get the weight in pounds we divide 6000 ounces by 16. So the schoolroom holds about 375 pounds of air.

Measure the living-room of your house and find the volume and weight of the air it holds.

12. How Do Heating and Cooling Affect Air? - Suppose we set an "empty" flask upside down in a shallow dish of water (Fig. 7) and carefully heat the flask with a burner, or pour hot water over it; what happens? We can also try warming the flask by means of our hands.

Ice Water

As the air in the flask is warmed, it expands, so that its volume is too great for the flask, and some of it escapes through the water. If we pour cold water over а second flask of air, the air contracts in volume. Since the outside air cannot get into the flask, water is forced up instead. Think of some cases you have seen in which air expands and contracts.

13. Can Air Be Compressed? We saw in the

FIG. 7.

Heating the air expands it, while cooling makes it shrink, or

contract.

last section that if air is cooled, it shrinks in volume. This is the same as saying that it occupies

a smaller space. We can also force air to occupy a smaller

pressure upon it.

space if we increase the Thus, a popgun is a tube having one end closed by a cork and the other end closed by a piston. As we force the piston into the tube, we compress the air inside. Finally its pressure becomes great enough to force the cork out with a "pop."

We can see how the pressure of water compresses air, if we put a glass of air, upside down, under water. The deeper the water, FIG. 8. the smaller the volume of the air will become.

The air in the vial is com

pressed by the

pressure of the

water in the jar.

We can see this better if we fasten a glass vial or flask (Fig. 8) to a rod of glass or metal and put it into a deep vessel of water..

The apparatus of Fig. 9 also shows how, by increasing the pressure, we decrease the volume of a gas. If we begin with a certain volume

Air

Mercury

FIG. 9. — The air in

the closed arm of the tube is compressed by the weight of the mercury in the longer arm.

of air in the closed, shorter arm of the bent tube, and add portions of water or mercury (quicksilver) to the longer arm, the air will be forced into a smaller and smaller space. Mercury is used because it is so much heavier than water; a column of mercury 1 inch high will compress the air as much as a column of water 13.6 inches high, because mercury is 13.6 times as heavy as water.

14. For What Can Compressed Air Be Used? Have you ever thought of some of the important uses man has made of compressed air? To pack air into a smaller space we commonly use a compression pump. The bicycle and automobile pumps (Fig. 10) are the most common of the compression pumps; they are used to crowd a great deal of air into the tire. The compressed air, as well as the rubber tire, acts as an elastic cushion and protects the car from jolts. We can also use a bicycle pump to crowd air into The a basketball or football.

Air

To bicycle

FIG.

10.

bicycle

com

pump
presses a great

the tire.

How do you suppose men lay the founda- deal of air into tions of bridges in the bottom of a river, or do other work under water? We learned in the last section that if a vessel of air is pushed, mouth downward, under water, the pressure of the water compresses

the air. If we are to keep the vessel entirely full of air, we must force a supply of air into the vessel. This is what is done in the diving bell (Fig. 11), in which men work under water. Compressed air is forced into the "bell" at such a pressure that it keeps the water from rushing in and furnishes fresh air for the diver. The air is kept flowing, so that

it bubbles out around the edge of the bell.

A caisson is a diving bell which men use in placing the foundations of large buildings, when water rushes into the excavations (cf. § 133).

Compressed air

FIG. 11.

You would not think that a submarine boat is a device for using compressed air, yet this is so. It has compartments, or spaces, which can be filled with either water or compressed air. When water is allowed to push the air out of these spaces, the boat as a whole is heavier and sinks under water; when compressed air is used to force the water out of the spaces, the boat rises to the surface. The compressed air apparatus also keeps the sailors supplied with air for breathing.

Air is kept flowing through the bell, so that the workman can lay a foundation upon the bottom.

There are many other inventions that use compressed air. Have you ever heard the rapid pounding of the pneumatic hammer on a steel bridge or building? It is used to rivet together large pieces of steel, and increases greatly the speed with which steel structures can be built.

The air brake, invented by George Westinghouse, is another important piece of apparatus that uses compressed air. It sets the brakes against the wheels of railroad cars and stops the train. The air is compressed and stored in the locomotive.