Fluorescence 407 



might contribute very appreciably to the light of the corona, and 

 they advised astronomers to be on the lookout for such effects. 



These studies aroused an interest in light emission from atoms 

 and molecules in their simplest condition, the vapor state, and 

 paved the way for the concept of " resonance radiation," demon- 

 strated by Robert Wood (1863-1955) in 1905 for sodium vapor. 

 The study of metal vapor fluorescence was greatly expanded in later 

 years of the twentieth century, particularly the resonance radiation 

 of mercury vapor in the ultraviolet (2735 A) , and gave rise to the 

 important concept of sensitized fluorescence. This was very clearly 

 demonstrated by G. Carlo and J. Franck (1922) for mercury and 

 thallium vapor. In photosensitization of photographic plates, the 

 sensitizer molecule (certain dyes) can initiate chemical reactions 

 in other molecules (silver halides) which do not absorb that par- 

 ticular wave-length. In the case of Hg (sensitizer) and Th (emitter) 

 vapor, illuminated by the mercury resonance wave-length 2537 A, 

 the fluorescence spectrum contains a number of thallium lines, 

 despite the fact that thallium absorbs no 2537 A wave-lengths. The 

 mercury atoms absorb the energy and transfer it to the thallium by 

 collision; the thallium then emits a green fluorescence. 



This phenomenon of sensitization may be as widespread in lumi- 

 nescence phenomena (see Chapter XIII) as it is in photochemistry. 

 A striking example of what was called photodynamic sensitization 

 in connection with living organisms was discovered in the winter 

 of 1897-1898 by O. Raab (1900) , a student of H. von Tappeiner of 

 Munich. He found that the infusorian, Paramecium, was killed by 

 the dye, acridin, when in the light but not in the dark. According 

 to the book of H. von Tappeiner and A. Jodlbauer, Die sensi- 

 bilizierende Wirkung fiuorescierender Snbstanzen (1907) , the sensi- 

 tizing substances are fluorescent, although it is not the fluorescent 

 light which has a toxic action, but the contact of the dye with 

 living cells plus daylight. A great deal of research has been carried 

 out in this field in the twentieth century. 



Books on Fluorescence 



A survey of fluorescence study since the time of Stokes reveals 

 an enormous collection of experimental data, practically all appear- 

 ing as original articles in various journals. One exception is the 

 115-page booklet of F. J. Pisco, Die Fluorescence des Lichtes, pub- 

 lished at Vienna in 1861. Pisco's contribution explains the findings 

 of Stokes and his methods of detecting fluorescence in some detail, 

 and also includes the new papers which had appeared in Poggen- 



