Phosphorescence 323 



while the whitish sparks indicated silver, whose symbol is the moon. 

 The argument was that a phosphor like the Bolognian stone must 

 contain silver because of its whiteness, for was not the moon goddess, 

 Diana, with her swan the whitest of all the goddesses. 



Friedrich Hoffmann and Calcium Sulphide Phosphor 



The next century was to show that the ability to shine after 

 illumination was a very general property of many substances and 

 not the rare and unique phenomenon previously supposed. It was 

 ushered in with a discovery of Friedrich Hoffmann the yoimger 

 (1660-1742), the noted German physician and professor of medi- 

 cine at Halle, who made mineral waters popular. He found (1700) 

 that gypsum (singularis species Talci) could be made into a phos- 

 phor. The statement is very brief and the method not given, al- 

 though presumably similar to that used for the Bononian phosphor. 

 The untreated Bononian stone is a BaS04, while gypsum is a CaS04, 

 and Hoffmann's phosphor a calcium sulphide. Although barium 

 and calcium were not recognized as separate elements at the time, 

 the discovery of the calcium sulphide phosphor may legitimately be 

 attributed to Hoffmann, who named the material " phosphorus 

 lucens germanicus." He adopted the sponge theory of light absorp- 

 tion in explanation of its behavior. The phosphorescence of a stron- 

 tium sulphide was first observed by J. F. John in 1817. 



De Mairan and Cohausen 



In the prize essays of de Mairan (1717) and Cohausen (1717) 

 the Bononian and Balduinian stones were both discussed. As we 

 have seen (Chapter V) , de Mairan believed that luminescence in 

 general was due to the " movement " of sulphur. This movement 

 had to be great enough to set the sulphur free from the surrounding 

 material with which it was bound. Only then would light appear. 

 He regarded fire itself, which was accompanied by light, as com- 

 posed of foreign corpuscles with a great quantity of sulphur and 

 that in the calcination of the stones to be made into phosphors, fire 

 was important in determining the character of the pores in which 

 the light was held. 



De Mairan's essay ends with an explanation of the varying colors 

 of the light of phosphors, in which this point of view is clarified. He 

 based his ideas on Newton's theory of colors, that white represents 

 all colors in proper proportions and black absence of color; that the 

 varying colors can be separated by a prism, but they cannot be con- 

 verted one into another; and that fundamentally different colors 



