PHYSICAL BASIS OF SOLAR CHEMISTRY. 337 



the note would rise in pitch by a regular gradation; 

 there would be no gap intervening between note and 

 note. Here we have the analogue to the continuous 

 spectrum, whose colours insensibly blend together 

 without gap or interruption, from the red of the lowest 

 pitch to the violet of the highest. But suppose the 

 player, instead of gradually shortening his string, to 

 press his finger on a certain point, and to sound the 

 corresponding note; then to pass on to another point 

 more or less distant, and sound its note; then to an- 

 other, and so on, thus sounding particular notes sepa- 

 rated 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 anal- 

 ogy, is not sufficient for our purpose; we must look 

 with the mind's eye at the oscillating atoms of the vola- 

 tilised metal. Figure these atoms as connected to- 

 gether 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 particu- 

 lar time and at a different rate from the others. If 



