Fluorescence 405 



During the last quarter of the century the effect of concentration 

 of dissolved fluorescent material on color and intensity of fluores- 

 cence and the duration of fluorescent light have been minutely 

 studied. Solids and pure liquids have also been investigated. The 

 decay time, even though short, was important and challenged meas- 

 urement. After E. Becquerel's pioneer work, E. Wiedemann (1888) 

 developed a phosphoroscope that would measure a few millionths 

 of a second, but found that the fluorescence of solutions was even 

 shorter than could be detected by his instrument. Actual measure- 

 ment of time intervals of the order of 10"^ seconds was a product 

 of the twentieth century, when experimental work continued with 

 new theories and new names. The effect of different temperatures,^® 

 of different solvents," and of the state of aggregation ^^ of the fluo- 

 rescent substances and other changing conditions were investigated 

 in great detail during the eighties and nineties, but space prevents 

 any extensive discussion of the results. Only one important factor 

 in fluorescence emission can be mentioned, that of quenching. 



QUENCHING OF FLUORESCENCE 



Among the many observations of Stokes (1852) was the fact that 

 addition of NaCl or HCl (and other halogens) to quinine sulphate 

 solution reduced the intensity of fluorescence, a phenomenon now 

 known as quenching, since the effect is reversible, and chemical 

 changes do not occur in the solution. E. Buckingham (1894) , who 

 interpreted fluorescence in terms of Ostwald's dissociation theory, 

 showed that quinine ions were fluorescent. The effect of CI ions 

 on quinine sulphate is not the result of optical absorption. The 

 situation is also quite different from that where pH changes de- 

 crease or increase the fluorescence intensity of a compound by chang- 

 ing ionization. Quinine chloride is not fluorescent because the CI 

 ions completely suppress fluorescence of the quinine ion. Later, 

 G. C. Schmidt (1900) also emphasized that the halogen ions acted 

 by their mere presence, like a catalyst. 



Most of the studies on quenching have been made in the twentieth 

 century, but the phenomenon introduced a secondary effect into 



^* Stokes established the fact that fluorescent glasses were affected by temperature 

 but that temperature change had practically no influence on solutions within the 

 limits of temperatures then available. Later work, already mentioned in Chapter VIII 

 has indicated how very low temperatures change a fluorescence to a phosphorescence. 



'^'^ Stokes studied the effect of different solvents and the later work of V. Pierre 

 (1862-1866), E. Hagenbach (1872), H. Morton (1872-1875), F. Stenger (1886), B. 

 Walter (1889) , and O. Knowblanch (1895) has indicated that both color and inten- 

 sity of fluorescence depend on solvent. 



^« See especially the studies of E. Hagenbach (1872, 1874) . 



