PHYSICAL METABOLISM 249 



have ventured to describe, the intensity and the 

 absorption would depend upon both the number 

 and the duration of the periods thus produced ; 

 and it is this which I think the change of 

 absorption in fluorescence most distinctly proves. 



We may now perhaps turn our attention to the 

 obvious bearing of this phenomenon on Balfour 

 Stewart and Kirchofi''s law. 



M. Ch. Ed. Guillaumei and M. A. Cotton 2 

 have given much attention to this subject. At 

 first sight, indeed, it may seem as though the 

 change of absorption during fluorescence is a direct 

 consequence of this law, another instance merely 

 of the general rule that all bodies absorb the 

 radiations which they can emit. But this pheno- 

 menon, however, on closer examination is not really 

 one of the direct consequences of the law. 



In general, at a given temperature, there are in 

 a given body, for a particular kind of radiation, an 

 emissive power and an absorptive power which are 

 equal. 



But in the case of fluorescence, the absorptive 

 power is changed like the emissive power without 

 any appreciable change in the temperature of the 

 body ; and this is due to the fact that under the 

 influence of the external source the fluorescent 

 body emits a new kind of radiation, which is ex- 

 tinguished in the course of time when the exciting 

 source is removed and the molecular structure of 



1 Nature, March 3rd, 1898. 



2 Astrophysical Journal, April, 1899. 



