66 SCIENCE PROGRESS 



are excited by light of the same wave-lengths and that the wave- 

 lengths which are inactive in respect of the one are inactive in 

 respect of the other phenomenon ; again, separate components of 

 the phosphoroid which show no phosphorescence also show no 

 photo-electric effect. From the close connection of the two effects, 

 the theory that the photo-electrically liberated electron causes 

 the emission of phosphorescent light seems well established. 



J. Becquerel has carried out some very interesting experi- 

 ments, partly in collaboration with H. Becquerel and Kammer- 

 lingh Onnes, on the phosphorescence of uranyl salts. The 

 bands of the spectrum of the emitted light became very narrow 

 at low temperature, but a magnetic field did not appear to 

 influence the emitted light ; Lenard had likewise looked for a 

 magnetic effect in the phosphoroids of the alkaline earths and 

 failed to find it. A noteworthy point is that in the uranyl salts 

 no traces of foreign metal condition the phosphorescence, which 

 must be attributed to the uranium itself. Experiment indicates 

 that the " centres " of light emission are present only in relatively 

 very small numbers, as in the phosphoroids hitherto discussed, 

 only a very few of the uranium atoms being active at a time. 

 The experimenters suggest a possible connection between the 

 light-emission and the radioactivity of the uranium atom, 

 the atoms being assumed to be active only while they are 

 breaking down. The fact that the intensity of the emitted 

 light does not decrease when the temperature is lowered even 

 to 14 absolute offers some support to this theory, which is, 

 however, not very strongly upheld. 



We now pass on to the extinction of phosphorescence by 

 means of red and infra-red light of which mention has already 

 been made. The effect, although most marked with these rays, 

 is not confined to the infra-red region of the spectrum, as 

 Fommel found a short wave region (384-96 up) which could 

 also extinguish phosphorescence. Further work by Dahms has 

 shown that light of certain wave-lengths which can extinguish 

 the emission of a phosphoroid already excited can also excite 

 an unexcited phosphoroid, which shows that there is no essential 

 difference between rays which excite and those which extinguish; 

 if light of a given wave-length and intensity falls on a phos- 

 phoroid, an equilibrium is finally set up. Thus a piece of spar 

 excited by the ultra-violet showed extinction to the edge of 

 the ultra-violet, but, if previously unexcited, was excited by the 



