spurs that rotate in opposite directions (Fig. 172). To be efficient these spurs must make close contact with the case

of the pump and with each other. The water comes in through the inlet and passes

to the right and left as it is inclosed between the outer case and the spurs of A and B. As the wheels rotate the water is forced upward and through the outlet. Rotary pumps have the advantage of having no valves. Various forms of centrifugal and rotary pumps are used in vacuum cleaners (Fig. 174).

185. Absorption of Gases by Solids. Some porous solids have the property of absorbing gases to a great extent, a given body of the solid absorbing many

times its own volume of the gas.

Trim a piece of charcoal

that it will slip easily
Invert the tube and

Demonstration. about an inch long so into a large test tube. fill it with ammonia gas. Pour mercury into a dish and place the piece of charcoal on its surface. Bring the test tube down over the charcoal and fix it in such a position that its mouth will be a little below the surface of the mercury. In a short time, the charcoal will absorb the ammonia, and the mercury will rise in the tube, as in Fig. 173. This property of charcoal is of value in the absorption of noxious gases.

FIG. 173

186. Absorption of Gases by Liquids. — Liquids also absorb gases more or less freely. The bubbles of air rising from a glass of water when it is placed under the receiver of an air pump and the air is exhausted, afford evidence that air is absorbed by water. It is on account of the absorbed air that fish are able to live in water. Water at normal pressure and temperature absorbs nearly twice its vol

C

ume of carbon dioxide and more than a thousand times its volume of ammonia

gas.

Demonstration. - Fit two flasks with rubber stoppers, one having two holes and the other one (Fig. 175). Draw out a glass tube to a jet and thrust it into the upper flask A after having filled A with ammonia gas. Thrust two tubes of the forms shown in the figure through the other stopper. Fill the lower flask B nearly full with a solution of litmus reddened with a few drops of acid. Press in the stopper and connect the two straight tubes with a short piece of rubber tubing having a clamp at C. Loosen C and force a little water into A by blowing through the pipe D, and the water will continue to flow until the flask A is nearly full. The quantity of water that goes into A will measure roughly the gas absorbed. Notice the change in the color of the water in A. What proof is there that there was not a vacuum in the upper flask?

B

FIG. 175

The escape of bubbles from water under the receiver of an air pump not only proves the existence of the absorbed air, but also proves that the amount absorbed depends upon the pressure. This fact is made use of in charging a soda fountain. This is done by letting a quantity of carbon dioxide flow from a high-pressure tank into a cylinder partly filled

with water, and rocking the cylinder vigorously until the gas is absorbed. The large amount of gas absorbed under pressure is shown by the effervescence of the water when it is drawn out into a glass.

187. Diffusion of Gases. If a closed vessel containing a gas is connected with another containing any different gas, no matter what its density, the gases will diffuse completely.

Demonstration. — Fit a large rubber stopper with one hole into the open end of a porous cup, such as is used in small battery jars. Put one end of a glass tube about 2 ft. long through the stopper, and hold the tube inverted with the open end below the surface of water in a dish. Bring over the porous cup a jar filled with hydrogen, or with common illuminating gas, and bubbles will be seen to rush from the end of the tube and rise through the water, showing that gas has passed into the cup. After the bubbles stop rising, remove the jar and notice what follows.

FIG. 176

It has been proved that a given volume of a gas contains the same number of molecules as the same volume of any other gas at the same pressure and temperature, and hence that the molecules of a light gas have a greater velocity than those of a heavy gas. Since the velocity of the molecules of the gas in the outer jar is much higher than that of the air molecules in the inner cup, the number that strike the outside of the porous cup is correspondingly greater than the number that strike the inside. This means that a greater number will pass through the pores of the cup from the outside to the inside than in the opposite direction, hence the excess has to pass

out of the lower end of the tube. As the number of gas molecules increases within the cup, the number of impacts on the inside finally becomes equal to the number on the outside, and no further bubbles will escape. When the outer jar is removed, the conditions and results are reversed.

Questions

1. How does a gas differ from a liquid?

2. How does the elasticity of a gas differ from the elasticity of a spring?

3. Why is it possible to pump air from a closed vessel?

4. When will an air pump stop taking air from a receiver? Is it possible to take it all out?

5. What will take place if the tip of an incandescent lamp bulb is broken off under water? Why?

6. Why does an arrow with a cup-shaped rubber tip, stick to the wall when shot against it?

7. To what volume must a cubic foot of any gas be to make its elastic force three times as great?

FIG. 177

compressed

9. What advantage is there in having a pneumatic tire on an automobile?

10. Why does a deep sea fish look bloated when brought to the surface? 11. Suppose a piece of rubber hose is used to siphon water from the barrel into the pail (Fig. 177). Will the water run faster when the barrel is full or when nearly empty?

12. Suppose a hose is used to siphon water from a pit in the side of a hill, how far below the opening of the pit will it draw the water?