Inference Exercise
Explain the following:
391. Dynamos have copper brushes to lead the current from the coils of wire to the line wires.
392. A megaphone makes the voice carry farther than usual.
393. Copper wire is used to conduct electricity, although iron wire costs much less.
394. A flute gives notes that differ in pitch according to the stops that are opened.
395. There are usually solid pieces of sugar around the mouth of a syrup jar.
396. You can beat eggs quickly with a Dover egg beater.
397. When ocean water stands in shallow open tanks for some time, salt begins to form before the water has all evaporated.
398. In a coffee percolator the boiling water goes up through a tube. As this water drips back through the ground coffee beans, it becomes brown and flavored, and the coffee is made.
399. Kerosene will clean off the rim of soap and grease that forms in bathtubs.
400. Beating cake frosting or candy causes it to sugar.
Section 43. Diffusion.
How does food get into the blood?
Why can you so quickly smell gas that is escaping at the opposite side of a room?
On our imaginary switchboard the Diffusion switch would not be safe to tamper with. It would be near the Solution switch, and almost as dangerous. For if you were to make diffusion cease in the world, the dissolved food and oxygen in your blood would do no good; it could not get out of the blood vessels or into the cells of your body. You might breathe all you liked, but breathing would not help you; the air could not get through the walls of your lungs into the blood. Plants would begin to wither and droop, although they would not die quite as quickly as animals and fishes and people. But no sap could enter their roots and none could pass from cell to cell. The plants would be as little able to breathe through their leaves as we through our lungs.
If gas escaped in the room where you were, you could not smell it even if you stayed alive long enough to try; the gas would rise to the top of the room and stay there. All gases and all liquids would stay as they were, and neither would ever form mixtures.
It would not make so much difference in the dead parts of the world if diffusion ceased; the rocks, mountains, earth, and sea would not be changed at all at first. To be sure, the rivers where they flowed into the oceans would make big spaces of saltless water; and when water evaporated from the ocean the vapor would push aside the air and stay in a layer over the ocean, instead of mixing with the air and rising to great heights. But the real disaster would be to living things. All of them would be smothered and starved to death as soon as diffusion ceased.
Here is an experiment that shows how gases diffuse:
Experiment 85. Take two test tubes with mouths of the same size so that you can fit them snugly against each other when you want to. Fill one to the brim with water and hold your thumb or a piece of cardboard over its mouth while you place it upside down in a pan of water. Take the free end of a rubber tube that is attached to a gas pipe and put it into the test tube a short distance, so that the gas will go up into the tube, as shown in Figure 149. Now turn on the gas gently. When all the water has been forced out of the tube and the gas bubbles begin to come up on the outside, turn off the gas. Put a piece of cardboard, about an inch or so square, over the mouth of the tube so that no air can get into it, and take the tube out of the water, keeping the mouth down and covered. Bring the empty test tube, which of course is full of air, mouth up under the test tube full of gas, making the mouths of the two tubes meet with the cardboard between them, as shown in Figure 150. Now have some one pull the cardboard gently from between the two test tubes, so that the mouths of the tubes will be pressed against each other and so that practically no gas will escape. Hold them quietly this way, the tube of gas uppermost, for not less than one full minute by the clock. A minute and a half is not too much time. Now have some one light a match for you, or else go to a lighted Bunsen burner.
Fig. 149. Filling a test tube with gas.
Fig. 150. The lower test tube is full of air; the upper, of gas. What will happen when the cardboard is withdrawn?
Take the test tubes apart gently and hold the lower one, which was full of air, with its mouth to the flame. What has the gas in the upper tube done? Now hold the flame to the upper test tube, which was full of gas. What happens? Has all the gas gone out of it?
As you well know, gas is much lighter than air; you can make a balloon rise by filling it with gas. Yet part of the gas went down into the lower tube. The explanation is that the molecules of gas and those of air were flying around at such a rate that many of the gas molecules went shooting down among the air molecules, and many of the molecules of air went shooting up among those of gas, so that the gas and the air became mixed.