420 PHYSIOLOGICAL PHYSICS. [Ch ap . xxxi. 



the spring be released before the bell is placed inside 

 the jar, the ringing of the bell will be heard quite 

 distinctly through the walls of the jar. As one pro- 

 ceeds to exhaust the jar the sound is heard more and 

 more faintly, till, when the exhaustion is as complete 

 as possible, only one near to the bell hears it. If 

 hydrogen gas be now permitted to fill the jar the 

 sound of the ringing bell is not much intensified. 

 But on again exhausting, as completely as possible, the 

 sound is 110 longer heard, even if one's ear be pressed 

 closely against the jar's sides, and while it is evident 

 to the eyes that the hammer is still hitting the bell ; 

 as soon as even a small quantity of air is permitted to 

 enter the exhausted receiver, the sound of the bell is 

 restored. 



In ordinary circumstances, then, sound is propa- 

 gated by air. It may, however, be propagated by 

 many different media, gases, liquids, solids. In. water 

 sound is readily propagated, and everyone knows how 

 it is conducted by solid bodies like wood, how, for 

 example, the merest scrape with a pin at one end of 

 a long log of wood will easily be heard by an ear at 

 the other end. While sounds may be propagated in 

 gaseou's, liquid, and solid bodies, it is not with the 

 same velocity in each, the velocity of propagation 

 being dependent upon the elasticity of the medium in 

 relation to its density. Generally speaking, the 

 greater the elasticity the greater the velocity, and the 

 greater the density the less the velocity. The velo- 

 city of sound in air at G. is 1,090 feet, and it 

 increases 2 feet for every degree centigrade of increased 

 temperature. In oxygen the velocity is 1'040 feet, 

 in carbonic acid gas 858 feet, in hydrogen 4 - 164 feet, 

 in water nearly 5,000 feet, in copper (at 20 U G) 11,666 

 feet, in iron (at 20G) 16,822 feet, in pine wood, 

 along the fibre, 10,900 feet. In these solid substances 

 the increased velocity is not due to the increased 



