PHYSICAL BASIS OF SOLAR CHEMISTRY 355 



ors between red and blue by the intermediate rates. The 

 solid incandescent coal-points give us a continuous spec- 

 trum; or, in other words, they emit rays of all possible 

 periods between the two extremes of the spectrum. Color, 

 as many of you know, is to light what pitch is to sound. 

 When a violin -player presses his finger on a string he 

 makes it shorter and tighter, and thus, causing it to 

 vibrate more speedily, heightens the pitch. Imagine such 

 a player to move his fingers slowly along the string, short- 

 ening it gradually as he draws his bow, 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 colors in- 

 sensibly 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 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 purpose; we must look with the mind's 

 eye at the oscillating atoms of the volatilized metal. Fig- 

 ure these atoms as 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 com- 

 ing to rest, to quiver for a certain time at a certain defi- 



