PHYSICAL BASIS OF SOLAR CHEMISTRY. 25? 



his finger on a certain point, and to sound the correspond- 

 ing note; then to pass on to another point more or less 

 distant, and sound its note; then to another, and so on, 

 thus sounding particular notes separated from each other 

 by gaps which correspond to the intervals of the string 

 passed over; we should then have the exact analogue of a 

 spectrum composed of separate bright bands with intervals 

 of darkness between them. But this, though a perfectly 

 true and intelligible analogy, is not sufficient for our pur- 

 pose; we must look with the mind's eye at the oscillating 

 atoms of the volatilized metal. Figure these atoms as con- 

 nected together by springs of a certain tension, which, if 

 the atoms are squeezed together, push them again asunder, 

 and if the atoms are drawn apart, pull them again together, 

 causing them, before coming to rest, to quiver for a certain 

 time at a certain definite 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. Tf 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. 

 Having thus far cleared our way, let us make another 

 effort to advance. 



A heavy ivory ball is here suspended from a string, I 

 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 



