PHYSICAL BASIS OF SOLAR CHEMISTRY. 389 



ruption, from the red of the lowest pitch to the violet of 

 the highest. But suppose the player, instead of gradu- 

 ally shortening his string, to press his finger on a cer- 

 tain point, and to sound the corresponding 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 com- 

 posed of separate bright bands with intervals of darkness 

 between them. But this, though a perfectly true and in- 

 telligible analogy, is not sufficient for our purpose ; we 

 must look with the mind's eye at the oscillating atoms of 

 the volatilised metal. Figure these atoms an connected 

 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 volatilised 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. 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 



