402 History of Luminescence 



(Liebermann's rule). J. Dewar (1880), C. Knecht (1882), H. E. 

 Armstrong (1888-1893), and W. N. Hartley (1892) found similar 

 relations. 



Chemical groups with which fluorescence tended to be associated 

 were called fluorophores in the extensive paper of R. Meyer (1897) 

 on this subject. The term is analogous to chromophore, for groups 

 whose presence in an organic molecule is associated with color, a 

 word applied by O. N. Witt in 1876. A fluorophore is often a six- 

 membered heterocyclic ring, but its presence does not inevitably 

 confer the ability to fluoresce on the molecule. Substitution groups 

 are also important, some decreasing and others increasing the fluores- 

 cence intensity of a compound. When the substitution of a par- 

 ticular group in a fluorophor results in a fluorescent compound, it 

 is called a fluorogen. 



By the end of the nineteenth century the number of known fluo- 

 rescent compounds had greatly increased. In 1887, a book was 

 published by K. Noack (born 1856) , professor in Giessen, contain- 

 ing a list of substances arranged according to the color of their 

 fluorescent light. Some 660 compounds were described, with refer- 

 ences to the literature, making a book of one hundred pages. No 

 comparable list appeared during the remainder of the century. 



Much of the research on chemical constitution, color, and fluo- 

 rescence occurred in the 1890's, by W. N. Hartley (1892) and early 

 1900's by H. Kauffmann and others. Kauffmann introduced the 

 term " luminophore " for atom groups capable of radiating, and he 

 designated the benzol group as the one most widely distributed. 

 Early in the next century the book of 102 pages by Kauffmann, Die 

 Beziehiing zivischen Fluorescenz und chemische Constitution (Stutt- 

 gart, 1906) , summed up the accumulated knowledge at that time. 

 In 1908 the article on fluorescence by H. Konen in Kayser's Hand- 

 buch der Spectroscopic (vol. 4) devoted 105 pages to a list of 1,700 

 fluorescent compounds, arranged alphabetically, with references to 

 the literature. Such lists have served as the basis for relating fluores- 

 cence to chemical constitution. 



PROPERTIES OF FLUORESCENT LIGHT 



Stokes' monograph was remarkably complete and started many 

 new lines of investigation. Only the more important later work 

 can be mentioned. One of the first systematic studies of fluorescence 

 of various crystals, especially those of uranium and platinum double 

 salts, were undertaken by W. Joseph Grailich (1829-1859) , a crystal- 

 lographer and Privatdocent at the University of Vienna. His prize 

 essay, Krystallographish-optische Untersuchungen, of 226 pages, was 



