268 



SOUND 



noting the sound produced. Then heat the flask, allowing the 

 water to boil briskly, and after the boiling has continued for a 

 few minutes remove the flame and instantly 

 close up the second opening by inserting a 

 glass stopper. Now shake the flask and 

 note that the sound is very much fainter 

 than at first. As the flask was warmed, air 

 was rapidly expelled ; so that when the 

 flask was shaken the second time, less air 

 was present to transmit the sound. If the 

 glass stopper is removed and the air is 

 allowed to reenter the flask, the loudness 

 of the sound immediately increases. 



Since the sound of the bell grows fainter 

 as air is removed, we infer that there would 

 be no sound if all the air were removed 

 FIG. 166 - Sound is car- from the flask ; that is to say, sound can- 



ried by the air. . , ., . , , , . 



not be transmitted through empty space 

 or a vacuum. If sound is to reach our ears, it must be through 

 the agency of matter, such as wood, water, or air, etc. 



252. How Sound is transmitted through Air. We saw in 

 Section 250 that sound can always be traced to the motion 

 or vibration of matter. It is impossible to conceive of an 

 object being set into sudden and continued motion without 

 disturbing the air immediately surrounding it. A sounding 

 body always disturbs and throws into vibration the air around 

 it, and the air particles which receive motion from a sounding 

 body transmit their motion to neighboring particles, these in 

 turn to the next adjacent particles, and so on until the motion 

 has traveled to very great distances. The manner in which 

 vibratory motion is transmitted by the atmosphere must be 

 unusual in character, since no motion of the air is apparent, 

 and since in the stillness .oi^night when " not a breath of air " is 



