TRANSACTIONS OF SECTION B. 629 



The ultimate cause of fluorescence has naturally attracted attention. Stokes * 

 •was inclined to attribute a peculiar sensibility to the molecules of substances 

 exhibiting this phenomenon. Lommel - started with the assumption that light of 

 a certain frequency may give rise to vibrations of varying amplitudes in the mole- 

 cules of a substance. If the frequency depends on the amplitude, the emitted 

 light will not be homogeneous and the substance maybe considered as fluorescent. 

 Two grave objections to Lommel's theory are, that there seems to be no possi- 

 bility of a source of light remaining homogeneous whilst it fades in intensity, and 

 that all coloured substances should be fluorescent. Both deductions are at 

 variance with actual facts. 



Fluorescence must of necessity attract the attention of organic chemists, chiefly 

 on account of the fact that so many fluorescent substances are organic compounds 

 of known constitution. Richard Meyer ^ attempted to connect the fluorescence 

 of organic dyestufls with the presence of certain atomic groupings which he termed 

 ' fluorophors.' Amongst such fluorophors, the pyridine, pyroue, and paradiazine 

 rings may be mentioned. For fluorescence to be developed it is necessary that 

 the fluorophor be attached to heavy carbon groups, usually aromatic nuclei. 

 Meyer's theory gives no explanation of the influence of solvents and of the differ- 

 ences frequently observed in the case of isomeric compounds. 



The present author * has started from a fundamentally different point of view, 

 which may be stated as follows. If in the case of a tautomeric compound the 

 passage from one to the other configuration can be effected by two equal but 

 opposite atomic displacements, the molecules will vibrate between the two extreme 

 positions of less symmetry, passing through the intermediate more symmetrical 

 configuration. Energy absorbed when the molecules possessed one configuration 

 could then be emitted when they had the other configuration ; and as the two 

 configurations would certainly correspond to different vibration frequencies, one 

 has the necessary conditions for the exhibition of fluorescence. 



Consider the fluorescence phenomena in the case of the following compounds : — 



I. Fluoran,'- Cj„H,,0^ 



II. 'd-Q-Dihydroxijfluomn or Fluorescein, G.^f^^S>„. 



III. Tetrabromofluorcscein {Eusine), CgllgBr^Os^ 



IV. Tetraiodofluoresce'in, Cj^H^I^Oj. 



V. i-o-DoiitroJi/orescein/ C.,oH j^^NO,,) A- 

 VI. 4-o-Z)/«iYro-2-7-«?/6ro?nq^zw;-escei«,'"C.,oH^(NO,>,Br,0-. 

 VII. 2-7-Dimtro-i-b-dibvomoJluorescem,'' C'j,H^(NOj)JBr.p^. 



Of these substances, I. is colourless, and in neutral solvents gives colourless, 

 iion-fluorescent solutions. It fluoresces, however, if dissolved in strong sulphuric 

 acid. II. III. and IV. all fluoresce, especially in alkaline solution. 



The alkaline solutions of V. VI. VII. do not fluoresce at all. Meyer's theory 

 gives no explanation of these differences. The theory now brought forward agrees 



• Phil. Trans., 1852, 463. "- Wied. Annalen, 3, 268. 

 ^ Zeitschr. physihal. Ch. (1897), 24, 468. 



' Proc. Ch. Sue. (1900), 16, .3 ; Zeitschr. physikal. Ch. (1900). 34, 1-19. 



" Berichte (1891), 24, 1412; (1892). 25, 1385; Annalm (1882), 212, 319. 



* J. Chem. Soc. (1900) 77, 1324 ; (1902), 81, 893. 

 ' Ibid. (1902), 81, 893. 



