RECENT ADVANCES IN SCIENCE 543 



once destroyed, no longer returning. This irreversibility is to 

 be explained, according to Perrin, as a result of definite chemical 

 reaction. The substances examined are all ring compounds, 

 and it is probable therefore that the mechanism of fluorescence 

 is the same for all. In the case of anthracene it is well known 

 that exposure to light brings about polymerisation of the single 

 molecule to the double molecular form ; consequently it is 

 probable that a similar process occurs in other cases. Perrin 

 concludes that it is only at the moment of destruction of a 

 fluorogene molecule, that is, at the instant at which the chemical 

 change takes place, that fluorescent light is emitted. This is in 

 general agreement with what we would expect on the basis 

 of the electronic structure of atoms and molecules, such as that 

 of Rutherford as developed by Bohr. 



A further point substantiated by Perrin 's experiments is 

 the independence of fluorescence in respect of temperature 

 change. Even at liquid air temperatures the effects appear to 

 be the same as those observed at ordinary temperature. It is 

 evident from this that although the fluorescence itself may be 

 due to a true chemical process, it must be very different from 

 ordinary or thermal chemical changes. It is well known that 

 photochemical changes as ordinarily carried out are scarcely 

 affected by temperature and to an even greater extent is radio- 

 active change independent of temperature. Perrin draws 

 attention to the analogy between fluorescence and photo- 

 chemical change and radio-activity. 



Another phenomenon associated with fluorescence is con- 

 sidered by Perrin, namely the existence of an optimum concen- 

 tration. With extremely dilute solutions the fluorescence is 

 necessarily weak. With very concentrated solutions the 

 fluorescence is likewise feeble, the effect being most marked at 

 some intermediate concentration which varies somewhat from 

 one substance to another. To explain this Perrin introduces 

 the idea of the fluorescent power possessed by a single molecule, 

 such fluorescent power being a function of the environment or 

 concentration, in the sense that it diminishes as the concentra- 

 tion increases. At great dilution the fluorescent power P 

 reaches a finite maximum value P per molecule. Further 

 dilution has no effect on the value of P , but it will naturally 

 have an effect upon the total intensity of fluorescence owing 

 to the diminution in the number of fluorogenes or fluorescent 



