PHYSICAL BASIS OF SOLAR CHEMISTRY. 257 
his finger ou a certain point, anil 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 baud 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. 
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 IE the same intervals as my puffs; it is obvious that 
