64 SCIENCE PROGRESS 



in these phosphoroids the sulphur atom is responsible for the 

 storage of the light energy ; correspondingly the momentary 

 centres would seem to be free from sulphur. In these, oxygen 

 may very well take the place of sulphur, as oxides containing 

 traces of active metal were long ago shown by Lecoq de 

 Boisbaudran and Crookes to give a phosphorescence of short 

 duration. 



On Lenard's theory the light is emitted by the atom of 

 active metal on the return of an electron previously photo- 

 electrically liberated by the exciting light. In the unexcited 

 state, the atom possesses its normal complement of electrons; 

 in the excited state all the electrons which can be liberated 

 from the atom by the action of light or cathode rays escape 

 from it to other parts of the centre ; whilst the intermediate 

 condition, in which the electrons return to the atom, is the 

 occasion of the light-emissions. In the excited state, the 

 escaping electrons are probably stored in the sulphur atom in 

 the case of the sulphide phosphoroids. 



The broad bands of which the emitted light is made up are 

 formed by the superposition of spectral lines of varying position, 

 as it may be supposed that the period of the emitted light will 

 vary within limits, both from centre to centre and from time to 

 time in the same centre, in consequence of the variation in the 

 immediate surroundings of the different centres in amorphous 

 substances and the molecular agitation. In support of this 

 view, it has been observed that on decreasing the molecular 

 movements by lowering the temperature of the phosphoroid, 

 the bands become much narrower. By cooling with liquid 

 and solid hydrogen — to about 14 absolute — Lenard and his 

 collaborators have succeeded in getting the bands very sharp : 

 they still remained bands, however, whose intensity would not 

 support a strong dispersion. A line spectrum could, perhaps, 

 hardly be expected even at these low temperatures in amorphous 

 substances, owing to the above-mentioned local variations in 

 the arrangement of the molecules surrounding the centres ; 

 there seems more likelihood of such an emission spectrum in 

 crystalline substances. The influence of the immediate sur- 

 roundings of the centres on the period of the light emitted 

 by them has been beautifully demonstrated in Lenard's experi- 

 ments on the spectral position of a given emission band in 

 phosphoroids made with sulphides of the different alkali metals. 



