EXPERIMENTS IN SOUND. 



^7 



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 flut- 

 ter 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 projec- 

 tion 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 vibra- 

 tions through solids, liquids, and gases, we select — 



Experiments showixg 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 JfS. — 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 open- 

 ing 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. Pres- 

 ently you will get an opening of a size that causes an intense sound, 



