PHYSICAL METABOLISM 233 



esisted various degrees of molecular connection 

 from mere casual juxtaposition to the closest 

 chemical union. 



Thus a compound molecule may vibrate as a 

 whole, or by virtue of its connections with adjacent 

 molecules, or it may vibrate by itself in the manner 

 of an isolated vibrating plate or rod, and between 

 these extreme limits we may conceive various inter- 

 mediate modes of vibration. The resultant effect 

 would naturally be of the most irregular type, and 

 the spectrum appear to be continuous, as is found 

 to be the case with all fluorescent and phosphor- 

 escent substances, except with chlorophyll, when 

 the spectrum is homogeneous and no actual 

 change of refrangibility occurs. 



Indeed, it may be shown experimentally by the 

 method of crossed prisms, originally used by 

 Newton and employed by Stokes in his experi- 

 ments, that when the illumination is feeble the 

 fluorescent light never seems to be homo- 

 geneous. By this we should now understand that 

 it never appeared to be homogeneous, on account 

 of the feebleness of the illumination ; our own 

 opinion being that a continuous spectrum is not 

 an essential part of the phenomenon of fluorescence, 

 because when the light is sufficiently intense we 

 have seen what appeared to be a continuous 

 spectrum at lower intensities become resolved into 

 a line or fine band spectrum, but this only in 

 the case of gases ; still, the line spectrum, as 

 the brightness diminished, likewise assumed the 



