Fluorescence 391 



Stokes himself held that a phosphorescence increased in light inten- 

 sity on heating, whereas a fluorescence did not. Phosphorescence is 

 usually associated with impurities while fluorescence depends on 

 fluorophore groups in the molecule, but this is not always true; 

 pure substances may show phosphorescence. Fluorescence obeys 

 Stokes' law while phosphorescence occasionally does not, but again 

 there are exceptions. Modern theory does recognize certain funda- 

 mental differences between fluorescence and phosphorescence, but 

 the terms will be retained as convenient expressions to indicate time 

 of light decay rather than to designate strictly separate phenomena ^ 

 Despite the difficulties of definition, it is particularly the fluores- 

 cence of liquids and solutions, with a few solids of very short dura- 

 tion which will be considered in this chapter. 



Lignum Nephriticum and Athanius Kircher 



Observation of fluorescence ^ in solution is very old, although the 

 phenomenon was not understod, merely regarded as a rather re- 

 markable example of varied colors. The first recorded instance of 

 a fluorescent solution appears to have been made by Nicolas Mon- 

 ardes (died 1578) a Spanish physician and botanist who wrote on 

 medicines of the New World. He described a wood, called " lignum 

 nephriticum," because of its supposed value in treating kidney dis- 

 eases. When made into cups, and filled with water, a peculiar blue 

 tinge can be observed. In the seventeenth century such cups were 

 highly esteemed gifts, fit for royalty and very important persons. The 

 physician Francisco Hernandes (sixteenth century) also described 

 the wood in his book on the natural history * of New Spain (1615) . 



Monardes' statement concerning the wood ^ is rather brief. It will 

 be found in " JoyfuU Newes out of the Newe Founde Worlde " 

 (1577) , an English translation ^ by John Frampton of the first three 



" Modern theory indicates that duration of the light is not necessarily a distinction 

 between phosphorescence and fluorescence. A better test is to determine whether the 

 excited state is paramagnetic, when phosphorescence is indicated. If both phosphores- 

 cence and fluorescence excited states occur in one molecule, the phosphorescence emis- 

 sion (observed at low temperature) is of longer wave-length and of longer duration. 



^ See the historical account by H. Konen in H. Kayser's Handbuch der Spectroscopic 

 4: 843-909, Leipsig, 1908. 



* Francisco Hernandez, Quatro libros de la naturaleza y virtudes de las plantas y 

 animales que estdn recevidos en el uso de medecina en la Nueva Espana, etc., Mexico, 

 1615. 



^ The tree is Eysenhardtia polystacha, the palo duke of Mexico. Cups were also 

 made from Pterocarpus indica wood, from the Philippines. See W. E. Safford, Ann. 

 Rep. Smithsonian Inst., 272-298, Washington, 1916, for an excellent account of the 

 history of lignum nephriticum. 



«The book was reprinted in 1925 (2 v., New York, A. A. Knopf) . See 1: 46-47. 



