DYNAMICS OF LUMINESCENCE 147 



oxidizers which are themselves unaffected at 100°. If we 

 are to connect reaction velocity with intensity of lumi- 

 nescence we must conclude that the evolution of light is 

 dependent rather on an optimum than a maximum reac- 

 tion velocity. 



Quite a number of instances are known in which in- 

 creasing the mass of reacting substances leads not to an 

 increase but to an actual cessation of luminescence. This 

 fact does not confirm the theory that reaction velocity is 

 a determining factor in luminescence. The conditions for 

 the luminescence of white phosphorus are most interesting 

 and unusual. (See van't Hotf, 1895 ; Ewan, 1895 ; Centners- 

 zwer, 1895 ; Eussell, 1903 ; Scharff, 1908.) Phosphorus will 

 only begin to luminesce at a certain small pressure of 

 oxygen. This ^'minimum luminescence pressure'' of oxy- 

 gen is very low, so low that earlier observers, failing to 

 remove traces of oxygen, thought that luminescence might 

 occur in absence of oxygen. Curiously enough there is also 

 a ** maximum luminescence pressure" of oxygen above 

 which no luminescence occurs. Phosphorus will not lumi- 

 nesce in pure oxygen. Between the minimum and max- 

 imum is an ^^ optimum luminescence pressure" where 

 luminescence of the phosphorus is brightest. The exact 

 values of these pressures vary with degree of water vapor 

 present and with temperature. According to Abegg's 

 Handhuch der anorganiscJien Cliemie, the maximum lum- 

 inescence pressure with water vapor present, is 320 mm. 

 Hg at 0° and increases 13.19 mm. Hg for each degree rise 

 in temperature. This means that for a definite tempera- 

 ture, say, 20°, phosphorus will not luminesce with an oxy- 

 gen pressure of 583 mm. Hg, but will luminesce with 

 pressures under this. If, however, we raise the tempera- 



