262 Prof. H. E. Armstrong and Dr. T. M. Lowry. [June 11, 



constitution and fluoresence, published early in 1900,* took the im- 

 portant step of associating the appearance of fluorescence not with the 

 mere occurrence of the quininoid type of structure which one of us has 

 long contended is characteristic of visibly coloured substance, but with 

 the continued development of such a structure in other words, he has 

 regarded it as the outcome of oscillatory changes in the course of which 

 a non-quinonoid compound undergoes conversion into the isodynamic 

 quinonoid compound,! for example, in the manner represented by the 

 formula which picture the changes undergone by fluorescein : 







\/v\/ ^ 



u 



/ 



CgH 4 .COOH XX C 6 H 4 .COOH 



According to Hewitt, "all the molecules will be undergoing 

 tautomeric change continuously and frequently, and energy absorbed 

 when the molecules have one configuration will be, to an appreciable 

 extent, emitted when they correspond to the other configuration. 

 It is practically certain that the vibration frequency of fluorescein is 

 different in the two states, and hence every opportunity is offered for 

 energy of a rapid vibration frequency to be largely transformed into 

 energy of greater wave-length." 



Hewitt obviously does not regard fluorescence as a " flash phe- 

 nomenon " but as a form of colour, as it were. 



While agreeing with Hewitt, that the origin of the effect is to be 

 sought in the occurrence of reversible changes involving the produc- 

 tion of dynamic isomerides, we think that fluorescence is to be 

 regarded as something apart from colour, which, more often than not, 

 is superposed upon colour. Colour appears to be dependent on 

 selective absorption and scarcely to involve any return of the 

 absorbed energy as light. The character of the colour effect in 

 fluorescence is quite distinctive ; it is not only remarkable on account 

 of its intensity, but there is in it an indefinable qualitative difference 

 which seems to separate it from ordinary colour. If regarded as a 

 " flash phenomenon " this difficulty disappears. 



Hewitt appears to regard fluorescence as the outcome of mere 



* ' Cheui. Soc. Proc.,' 1900, vol. 16, p. 3 ; Zeit. Pliys. Chem.,' 1900, vol. 34, p. 1. 



f In this connection, attention may be called to the following passage which 

 occurs in a note by one of us on fluorescence in quinine salts (' Chem. Soc. Trans.,' 

 1892, p. 790) : " It is well known that the oxy-salts of quinine are alone fluores- 

 cent, salts like the chlorhydride not being so; it is conceivable that this is due to 

 the inferior stability of the former in solution and that, owing to the occurrence of 

 dissociation, a condition is engendered favouring the passage by dynamic change 

 from the non-fluorescent centric to the fluorescent ethenoid condition. 



