4i 8 PHYSIOLOGICAL PHYSICS. [Chap. xx?<r. 



sound is propagated can be represented as composed 

 of particles so separated from one another that one 

 may move without affecting another. Between all 

 the particles of the air there is a certain elastic 

 force, owing to which one particle cannot move 

 without affecting the others, because of the changes 

 produced in the elastic force. Thus, as soon as the 

 first particle has begun to move, owing to the 

 elasticity between it and the second, the second is 

 moved also ; and similarly, as soon as the movement of 

 the second has begun, the third is also set in motion. 

 So that, by the time the first particle has reached its 

 greatest excursion from its place of rest, the forward 

 movement has been propagated for a considerable 

 distance along the line, and the general effect of the 

 movement will be to squeeze the particles, as it were, 

 nearer to one another, the first approaching the 

 second, the second the third, and so on. In fact a 

 condensation is produced. Then, when the first 

 particle recoils and starts on its backward movement, 

 the elastic force between it and the second particle 

 will be less than the force between the second and the 

 third ; the second will, therefore, follow the first, and 

 for the same reason the third will follow the second. 

 There will be a backward movement for some distance 

 along the line, the wave of condensation being passed 

 farther onwards. Then the first particle will not 

 have exhausted its force of recoil when it reaches its 

 position of rest, and it will pass this position for some 

 distance, and then return to rest. Consequently, 

 by this excursion in the direction opposite to 

 the condensation there will be increased distance 

 between the particles, a rarefaction will be pro- 

 duced. Like the condensation, the rarefaction will 

 travel in a wave ; it will affect a considerable 

 number of the particles at once, but in different 

 degrees, each particle having its maximum, one after 



