406 History of Luminescence 



the older work, making interpretation difficult. In general, halogen 

 and heavy metal ions, certain oxyacids, and some aromatic com- 

 pounds have a quenching effect on fluorescence, as a result of which 

 the fluorescence yield is changed. Quenching is also an important 

 factor in the light intensity of chemiluminescence in solution, and 

 of any type of luminescence in a gas. 



Another kind of quenching results from too many molecules of 

 the fluorescent substance itself, a phenomenon noticed by Boyle 

 (1664) , the so-called " concentration quenching" or " self quench- 

 ing." If the fluorescence yield is constant, fluorescence intensity 

 should increase with increasing concentration of fluorescent mole- 

 cules although not proportionally, because of light absorption in 

 the more concentrated solutions, and then remain constant. Actually 

 the fluorescence intensity decreases above a certain concentration. 

 This peculiarity was noticed by Stokes, and gave rise to numerous 

 arguments and polemics during nineteenth-century study of the 

 laws relating concentration and fluorescence intensity. It follows 

 that fluorescence is proportionately more intense in dilute solutions 

 of pure compounds, not only because light can penetrate further 

 without absorption, but also because there is no self-quenching. 

 The ability to detect minute traces of material by means of fluores- 

 cence has been commented on many times by various authors ever 

 since this type of luminescence has been recognized. 



Vapor Fluorescence and Resonance Radiation; Sensitization 



A particularly important discovery of Lommel was the orange- 

 yellow fluorescence of iodine vapor, made in 1883. This effect in a 

 gas had been overlooked by Stokes, but was studied in some detail 

 by its discoverer, who recorded the spectrum and the nature of the 

 exciting rays. Lommel was impressed by the fact that iodine vapor 

 appeared to be one material whose fluorescence is not excited by 

 violet rays in solution; dissolved iodine is not fluorescent. Later 

 discoveries concerning vapor fluorescence were reported by E. 

 Wiedemann (1890) and in Wiedemann and Schmidt's paper, " Ober 

 Lichtemission organische Substanzen im gasformigen, fliissigen, und 

 festen Zustand," in 1895. This investigation dealt with fluorescent 

 organic vapors like those of anthracene, and later contributions of 

 Wiedemann and Schmidt (1896, 1897) reported the fluorescence 

 of sodium and potassium vapor, with an account of their spectra 

 and behavior under electrical excitation. These authors believed 

 that the green fluorescence of sodium and the red of potassium, as 

 well as similar fluorescence of other metals in the sun's atmosphere. 



