URANIUM SALTS. 117 



absorption. At low temperatures the spheres of influence of the dynamids 

 are considered to extend to greater distances than at high temperatures, 

 and the free paths of the electrons are also greatly reduced. 



To each phosphorescent band Lenard and Klatt assign three phases: 

 An upper momentary or heat phase; a permanent phase possessing quite 

 definite temperature limits; and a lower momentary or cold phase. These 

 phases succeed each other as the temperature falls. The upper momentary 

 phase results when the dynamids do not store electrons. Whenever elec- 

 trons are stored, these return afterwards to the atom from which they were 

 expelled by the light-wave, and produce the permanent phase of the phos- 

 phorescent band. At low temperatures a few electrons return to the 

 atoms from which they were expelled and these cause the lower momentary 

 phase. 



In general the temperature of solid hydrogen is sufficiently low to 

 bring all phosphors into the lower momentary phase. Lowering the tem- 

 perature to 180 continually causes new bands to appear in the perma- 

 nent phase. Among such bands of long duration, for example, is the Ca.Ni/? 

 band, or the orange afterglow of BaCu. According to Lenard and Klatt 

 uranyl compounds show only the upper momentary phase. 



There are definite wave-lengths which in all temperature phases 

 of the centra bring into phosphorescence only the momentary phase. 

 The electrons under the photoelectric influence of these wave-lengths are 

 ejected from the metallic atoms of the centra, and then return almost 

 immediately to the metallic atom again; thus causing the emission of 

 light. Other wave-lengths cause the electrons to be ejected from the 

 same centra, but in this case the electrons are retained in the neighbor- 

 hood. These stored electrons, when they finally return to the atom 

 from which they were ejected, produce the phosphorescent band of the 

 permanent phase. 



The phenomena of luminescence are generally conceded to be due to 

 some kind of electrolytic dissociation or ionization of the dissolved sub- 

 stance in the medium about it. Among the first to hold this view were 

 Wiedeman and Schmidt. 1 The theory explains Stokes' law 2 and most of 

 the other properties of phosphorescence. Some of these other properties 

 are as follows: The distribution of intensity throughout a phosphorescent 

 band is independent of the intensity and the wave-length of the exciting 

 light; the light emitted from an isotropic medium is unpolarized; during 

 the decay of phosphorescence each band behaves as an individual unit; 

 the decay curve is dependent on the intensity and duration of excita- 

 tion; the behavior of a phosphorescent body depends upon its past history. 



Wiedeman and Schmidt have suggested that some of the ions produced 

 during phosphorescent excitation form semi-stable combinations with the 

 solvent molecules, and that after excitation these combinations are broken 

 down. Merritt 3 further discusses the theory of Wiedeman and Schmidt. 



1 Ann. Phys., 56, 177 (1895). 



2 Phys. Rev., 22, 279 (1906) 



3 Ibtd., 27, 384 (1908). 



