238 THE ORIGIN OF LIFE 



manent ones in the substance, which are merely 

 roused up by the more refrangible rays, that 

 agitate the molecules and thus give rise to their 

 natural modes of internal vibration. This in fact 

 is what Stokes believed fluorescence to be due to. 



To illustrate our purpose, we may imagine the 

 molecule to be a hollow sphere and a particle 

 near its centre to be connected with the surface 

 by elastic springs. Such a molecule, if violently 

 shaken, will in addition to the period of this 

 vibration radiate light of another period, namely, 

 that of the internal particle, which will simply 

 be roused up by the general disturbance. 



Thus a phenomenon very similar to fluorescence 

 should result. But, however, as the period of 

 vibrations of the internal particle is a natural full 

 period of the molecule, and thereupon always one 

 of its permanent and possible modes of vibration, 

 it will respond to vibrations of the same period 

 when a wave of that frequency passes through it. 

 Thus the body, although not fluorescing, will ab- 

 sorb the fluorescent light from another similar 

 body of the same substance. Fluorescent bodies, 

 however, when inactive, are generally transparent 

 to the light of the wave-lengths they emit, which 

 disproves the idea that the fluorescence is due 

 merely to the excitation of previously existing 

 periods ; whilst, on the other hand, we have seen 

 that it cannot be a forced vibration, as its period 

 is independent of that of the exciting light. 



How then are we to reconcile these apparently 



