274 



NICHOLS— EFFECTS OF TE^IPERATURE 



[April 22, 



balanced the light of the comparison lamp as determined from the 

 appearance of the contrast field of the photometer, a record was 

 made on the same chronograph sheet on which the closing of the 

 shutter had been automatically registered. 



This procedure was repeated with the comparison lamp at vari- 

 ous positions on the bar. Typical results obtained in this way are 



6, Me.. 



'f-**^. 



S..i*<. 



-/6o' -/ia" -go' -^° 0' 



Fig. /. 



-Mo" -no' -Bo' -fyo" 

 Fig. S. 



given in Figs. 5 and 6. Fig. 5 shows the curves of decay of phos- 

 phorescence in a sample of ZnS at -f- 20° and at — 125°. In Fig. 6 

 similar in curves for a phosphorescent barium sulphide prepared by 

 the method of Lenard and Klatt and having as its active metals bis- 

 muth and potassium are given for -|- 20° and — 105°. 



The more rapid decay exhibited by both of these substances at 

 the low temperatures does not necessarily indicate lower initial in- 

 tensity of phosphorescence. Indeed in the case of the Ba Bi K 

 compound the initial intensity at — 105° which may be estimated 

 from the decay curves by plotting these in the manner already de- 

 scribed by Professor INIerritt and myself^'' in which the reciprocal of 

 the square root of the intensity is taken as ordinates is several 

 times higher than the intensity at -|-20°. 



" Nichols and IMerritt, Physical Review, Vol. XXII., p. 280, 1906. 



