Phosphorescence 347 



phosphors and the luminescence appears in hydrogen as well as 

 oxygen. 



On the other hand, Wach (1833) made little progress with the 

 Wilson method of calcining oyster shells with a few drops of metal 

 salts, but succeeded with heavy spar (BaS04) or celestite (SrS04) to 

 which oxides of Mg, Zn, Sn, Cd, and Sb were added. He obtained 

 phosphors whose light was very bright and lasted a long time. 



Osann and Wach were on the right track and pioneers in the 

 use of trace metals. ^^ The presence of impurities did affect the 

 behavior of phosphors and explained the diverse results of different 

 observers in studying what were presumably the same minerals, but 

 which actually contained small amounts of foreign metals. However, 

 the all important part played by trace elements ^^ was not fully 

 recognized, and the controlled preparation of phosphors with any 

 desired color, intensity and persistance of luminescence was a some- 

 what later development. 



Curiously enough, one of the most important present day phos- 

 phors, hexagonal blende (ZnS) , was not recognized as phospho- 

 rescent until 1866, when the mineralogist, Theodor Sidot, prepared 

 it by heating ZnO in a stream of hydrogen sulphide. Previous obser- 

 vations of J. G. Lehmann (1749) as well as C. G. Hoffmann (1750) 

 had indicated that samples of yellow blende (ZnS) from Scharfen- 

 berg would luminesce on scraping and some varieties of natural ZnS 

 were phosphorescent, but the hexagonal variety called Sidot blend 

 was particularly bright. 



In 1868 A. Forster published detailed directions for making a 

 number of phosphors and Balmain's paint, a CaS with Bi as im- 

 purity, was placed on the market in 1870, the first well-recognized 

 luminous paint.^^ Its light was blue in color. 



The phosphors prepared by A. Verneuil in 1886-1887 became 

 important commercial products a little later, especially the yellow 

 phosphorescent ZnS. Verneuil was probably the first to prove that 



^^ Another who attempted to improve phosphors was Jacques Mande Daguerre 

 (1789-1851) , the collaborator with Joseph Nicephore Niepce (1765-1833) in develop- 

 ment of the daguerreotype method of photography on metal plates. It seems that in 

 the many attempts to obtain a method of recording images, Daguerre had turned to 

 phosphors as a possibility. D. F. J. Arago, the French astronomer, presented Daguerre's 

 notes on a method of preparing one from barium sulphate to the French Academy 

 {Com. Rend. Acad. Sci. 8: 243-246, 1839) . 



^^ In 1881 and succeeding years, Lecoq de Boisbaudran (1838-1912) and also Wm. 

 Crookes (1832-1919) became interested in spectral analysis of phosphorescent light 

 and pointed out the connection between spectral bands and the presence of traces 

 of various metals in phosphors, particularly the rare earths, which were undergoing 

 study at that time. 



^^ A patent for " Leuchtfarben " was taken out in 1879. 



