60 THE CHEMISTRY OF THE SUN. [CHAP. 



In the case of many compounds of carbon alone, we are able to 

 trace them through their entire range of existence, with others only 

 partially so. In all cases the complex groups formed from simpler 

 ones have related but not identical properties. In other sub- 

 stances this range of condition of existence passes gradually out of 

 our reach, but the phenomena remain the same in kind. Atomic 

 combination or condensation is probably a cause of what is termed 

 the " allotropic state " of some forms of matter. It is strikingly in 

 analogy with polymerization, but its investigation is at present out 

 of our range. 



Let us now dwell for a little on the principle by the appli- 

 cation of which the great stride we have chronicled has been 

 rendered possible. 



While in the giving out of light we are dealing with mole- 

 cular vibration taking place so energetically as to give rise to 

 luminous radiation, absorption phenomena afford us evidence 

 of this motion of the molecules when their vibrations are far 

 less violent. 



The molecules are so apt to vibrate each in its own period, 

 that they will even take up vibrations from light which is 

 passing among them, provided always that the light thus passing 

 among them contains the proper vibrations. 



Let us try to get a mental image of what goes on. There 

 is an experiment in the world of sound which will help us. 



If we go into a quiet room where there is a piano, and sing 

 a note and stop suddenly, we find that note echoed back from 

 the piano. If we sing another note we find that it also is 

 re-echoed from the piano. How is this ? When we have sung 

 a particular note we have thrown the air into a particular state 

 of vibration. One wire in the piano was competent to vibrate 

 in harmony with it. It did so, and vibrating after we had 

 finished, kept on the note. 1 



1 The following extract will show that this has been long known : 

 " How doth musicke amaze us, when of sound discords she maketh the sweetest 

 Harmony ? And who can show us the reason why two Basons, Bowles, Brasse 



