LENARD'S RESEARCHES ON PHOSPHORESCENCE 6 7 



whole spectrum up to the infra-red. The experiments of Dahms 

 referred to the whole of the emitted light, as at the time of his 

 work little was known of the separate bands of which this light 

 is made up. 



Lenard, studying the effect of infra-red illumination in ex- 

 tinguishing the bands, found that it was in all respects similar 

 to that produced by heat. As already observed by Becquerel, 

 when the phosphoroid is exposed to the extinguishing light, 

 it first of all lights up brilliantly during a short time and then 

 rapidly loses in intensity, the light becoming extinct. The 

 effect of both infra-red light and heating is thus to accelerate 

 the emission of the stored energy and consequently the phos- 

 phoroid becomes non-luminous more rapidly. Recent measure- 

 ments by Lenard have shown that the light-total — the time sum 

 of the light energy emitted as the after-glow of a given band — 

 is the same whatever the rate at which the light is emitted, 

 whether normally or accelerated by heating or irradiation by 

 the red rays. Another example in which the irradiation by 

 the "extinguishing" rays has the same effect as heating the 

 whole phosphoroid is supplied by the effect called by Lenard 

 the " actinodielectric effect." It is found, namely, that if a 

 phosphoroid be subjected to the infra-red rays, its conductivity 

 is temporarily improved, an effect which is also produced by 

 heating the phosphoroid. 



After quenching by heat, infra-red radiation can produce 

 no further momentary illumination, and vice versa. The effect 

 of rise of temperature is to bring out each permanent band 

 as the temperature of the permanent state for that band is 

 reached : the bands then emit very rapidly and die out : if 

 the initial temperature be above that of the permanent state, 

 neither heating nor infra-red produce any effect. The thermo- 

 metric temperature of the phosphoroid is not appreciably raised 

 by infra-red radiation, but we may assume that the local molecular 

 temperature 1 of the centres rises and that this produces the 

 same effect on the light-emission as heating the whole phos- 

 phoroid. The conception of a raised local temperature is quite 

 reasonable if we consider the excited centres as resonating to 



1 It is doubtful if it be altogether advisable to refer to a local agitation of this 

 kind as temperature ; as the vibrations are forced, there is a regularity about them 

 which is essentially lacking in true temperature agitations. However, in this 

 particular case it is hoped that confusion is avoided. 



