The Nineteenth Century 211 



rays which it absorbs, just as a photochemical reaction is only pos- 

 sible as a result of absorption of certain wave-lengths. The effect is 

 obvious, inherent in Stokes' law. It might, perhaps, be designated 

 Lommel's law, although that name does not actually appear to have 

 been applied. The more detailed history of fluorescence will be 

 found in Chapter XI. 



RADIOLUMINESCENCE 



Earlier in this chapter, attention was called to the fact that one 

 direction of phosphorescence study during the nineteenth century 

 embraced new methods of excitation. It was established that spark 

 discharges were especially active in producing phosphorescence of 

 minerals, and the effect was traced to the ultraviolet light in the 

 spark. Another course of inquiry concerned the peculiar lumines- 

 cence of the glass which appears when electric discharges pass 

 through rarified gases. From the early observations (1858) of J. 

 Pliicker and E. Becquerel, the effect was traced to cathode rays, 

 later found to be made up of a beam of electrons. Cathodolumines- 

 cence was thus recognized as a form of light emission. Somewhat 

 later, anode rays of positive particles were discovered (1886) by 

 E. Goldstein (1850-1931) and their effect was called anodolumi- 

 nescence. 



The production of higher and higher vacua in glass tubes through 

 which an electric current is passed led to discovery of the X-rays of 

 Wilhelm Konrad Rontgen (1845-1923) in 1895, immediately fol- 

 lowed by experiments of Henri Becquerel (1852-1908) with ura- 

 nium (1896) and those of the Curies, Pierre (1859-1906) and 

 Marie (1867-1934) with radium (1898). The two elements proved 

 capable of emitting various types of rays, including gamma rays, 

 which also excite luminescence in appropriate substances. In gen- 

 eral the effects of X-rays, gamma rays, cathode and anode rays are 

 referred to as radioluminescences, thereby adding additional types 

 of luminescence to the classification based on the nature of the 

 excitation. The details of these discoveries will be considered in 

 Chapter XII. 



CHEMILUMINESCENCE 



The light of the element phosphorus is usually thought of as 

 the prime example of chemiluminescence. Although the first to be 

 prepared artificially, every living luminous organism produces light 

 by chemiluminescent reactions proceeding within its living cells or 

 in a liquid secreted to the exterior. The luminescence of organisms 

 always comes from some organic compound in solution. 



