CHAPTER XIII 



PHYSICAL METABOLISM 



The phenomenon of fluorescence — Discovered by Herschel — 

 Change of refrangibihty discovered by Stokes — Work of 

 Lommel — Change of absorption — Description of experi- 

 ments on this — Views of C. E. Guillaume and A. Cotton — 

 Negative results — The experiments of NichoUs and 

 Merritt — Eelation between absorption and the intensity of 

 fluorescence — Inference to be drawn from these obser- 

 vations and the change of refrangibihty, the fluorescence 

 being independent of the presence of the exciting light — 

 Muorescence a building up and breaking down of mole- 

 cules — Physical metabolism. 



Sir William Herschel first observed the effect 

 of light upon certain substances such as sulphate 

 of quinine, parafiin oil, &c., which shine on 

 the surface when exposed to ordinary daylight 

 and give out what he called epipolised light : 

 whilst Sir George Stokes investigated the matter 

 more fully and discovered the law of change of 

 refrangibility of light known as Stokes' Law : 

 the change of frequency of light from higher 

 to lower. This he called Fluorescence. As far 

 back as 1852 he was led by his experiments to 

 remark that it is possible to conceive and at 

 the time it seemed probable to him that there 



