Inside the tube is a small quantity of very fine precipitated silica, probably the lightest powder known. Hold the tube in an horizontal position and blow the whistle. The silica-powder springs up into groups of thin vertical plates, separated by spots of powder at rest, as in the figure. This is a very beautiful and striking experiment.
Fig. 25.
Experiment 33 a.—The following experiment shows that the sound is caused by the vibrations of the column of air in the tube and whistle, and not by the vibrations of these solid bodies. Grasp the tube and whistle tightly in the hands. These bodies are thus prevented from vibrating, yet the sound remains the same.
The breath driven through the mouth of the whistle strikes on the sharp edge of the opening at the side of the whistle, and sets up a flutter or vibration of air. The air within the glass tube now takes part in the vibrations, the light silica-powder vibrates with it, and makes the vibrations visible.
To exhibit this experiment before a number of people, lay the tube carefully on the water-lantern before the heliostat, and throw a projection of the tube and the powder on the screen. When the whistle is sounded, all in the room can see the fine powder leaping up in the tube into thin, upright plates.
From Chapter VI., which is on the transmission of sonorous vibrations through solids, liquids, and gases, we select—
Experiments showing that the Air is constantly vibrating while Sonorous Vibrations are passing through it.—We must now add to our apparatus an open metal A pipe about seven and a half inches (nineteen centimetres) long, shown at C in Fig. 27. This pipe the organ-builder will accurately tune to your "A-philharmonic" fork.
Experiment 43.—Get a glass tumbler about two and a half inches in diameter and about three and a half inches deep, though any tumbler will do. Take a piece of window-glass about three inches square and place it over the tumbler. The glass must touch the edge of the mouth of the tumbler all around. Now slowly slide the glass so that the opening into the tumbler gets larger and larger, while the vibrating fork is held all the time over this opening, as shown at A in Fig, 27. Presently you will get an opening of a size that causes an intense sound,
