356 FRAGMENTS OF SCIENCE 



nite rate determined by the strength of the spring. Now, 

 the volatilized metal which gives us one bright band is 

 to be figured as having its atoms united by springs all of 

 the same tension; its vibrations are all of one kind. The 

 metal which gives us two bands may be figured as having 

 some of its atoms united by springs of one tension, and 

 others by springs of a different tension. Its vibrations 

 are of two distinct kinds; so also when we have three 

 or more bands we are to figure as many distinct sets of 

 springs, each capable of vibrating in its own particular 

 time and at a different rate from the others. If we seize 

 this idea definitely, we shall have no difficulty in dropping 

 the metaphor of springs, and substituting for it mentally 

 the forces by which the atoms act upon each other. Hav- 

 ing thus far cleared our way, let us make another effort 

 to advance. 



A heavy ivory ball is here suspended from a string. 

 1 blow against this ball; a single puff of my breath moves 

 it a little way from its position of rest; it swings back 

 toward me, and when it reaches the limit of its swing I 

 puff again. It now swings ' further ; and thus by timing 

 the puffs I can so accumulate their action as to produce 

 oscillations of large amplitude. The ivory ball here has 

 absorbed the motion which my breath communicated to 

 the air. I now bring the ball to rest. Suppose, instead 

 of the breath, a wave of air to strike against it, and that 

 this wave is followed by a series of others which succeed 

 each other exactly in the same intervals as my puffs; it 

 is obvious that these waves would communicate their mo- 

 tion to the ball and cause it to swing as the puffs did. 

 And it is equally manifest that this would not be the case 

 if the impulses of the waves were not properly timed; for 



