Phosphorescence 345 



phors which responded well to sunlight, also responded well to the 

 electric spark. 



Theodore von Grotthus (1785-1822), a gentleman who traveled 

 widely in his youth and then settled on his hereditary estate at 

 Geddutz near Wilna, is best known as the first to realize that only 

 the light rays which are absorbed are effective in producing chemi- 

 cal change. This generalization is generally known as the Grotthus 



(1815) -Draper (1841) photochemical absorption law, since J. W. 

 Draper emphasized the fact also. The interests *^ of Grotthus in 

 chemistry were extensive, chiefly connected with effects of light and 

 electricity, and quite naturally involved phosphors. He was led to 

 his theory of phosphorescence, explained in two long papers ^° in 

 1815, from photochemical experiments and from observations on 

 the phosphorescent and thermoluminescent mineral, chlorophane 



(see Chapter IX) . Light, heat, and electricity were regarded by 

 him as practically identical, light appearing when positive and nega- 

 tive electricity combine against a resistance, while heat forms if 

 there is no resistance. Light, then, is nothing but positive and 

 negative charges traveling side by side. A photochemical change 

 was thought to be similar to electrolysis, with the reaction products 

 combining with positive and negative electricity. 



Grotthus believed that sunlight on striking the outer surface of a 

 phosphor becomes split into its electrical principles (into positive 

 and negative charges) between the poles of the elementary parts of 

 the material. The gradual combination of the separated light ele- 

 ments then results in phosphorescence. Metals and conducting 

 fluids, including water, do not phosphoresce because the combina- 

 tion of the light elements goes too rapidly. This theory, also, had 

 its difficulties, as was bound to be the case, considering the variety 

 of facts to be reconciled and the state of knowledge at the time. 



New Phosphors. The Effect of Traces of Metals 



After the period of Dessaignes, Heinrich, and Grotthus, the papers 

 on phosphorescence were mostly short communications describing 

 new phosphors and the part played by traces of metals in deter- 

 mining ability of a material to phosphoresce, or they continued the 

 experiments on excitation of phosphorescence by the electric spark, 



** See the collection of his papers in Ostwald's Klassiker, No. 152 (Leipzig, 1906), 

 " Abhandlungen iiber Elektrizitat and Licht," 198 pp. 



^"Apparently Grotthus was aware of Dessaignes' but not J. Heinrich's work, as 

 Heinrich is not mentioned in the first paper. Both men were subject to polemical 

 discussion in the second paper (Schweigger's Jour, fiir Chemie u. Physik 14: 133-192, 

 1915, 15: 171-199, 1915) of Grotthus. 



