( 171 ) 
such a manner that it cannot be used by any other band” 1 ), and 
also the storing of electrons (the permanent phase) in the different 
bands of the same “phosphor” is often bound to widely differing 
temperatures (“1904” p. 761); this has also been further confirmed 
by our experiments at the lowest temperatures. The material sur¬ 
rounding the centra, which is the same for all kinds of centra in 
the same “phosphor”, cannot' therefore have anything to do with 
the accumulation of electrons, only the molecule called the centrum 
can play a part in that process. 
At first there seemed to be some foundation for the supposition 
that atoms that had passed over from the surrounding material to 
the centrum would be able to cause storing of electrons. If, however, 
the result that has begun to appear probable from the more detailed 
investigations of P. Lenard, viz. that the additional substance has 
no influence upon the intensity nor, independent of that circumstance, 
upon the duration of the bands, will be confirmed, then in the first 
place the sulphur atoms in the centra are to be taken into account 
as responsible for the efficient and lengthy storing of electrons. For 
it is only substances containing sulphur, “phosphors” of the alkaline 
earths, and zinc sulphide, that show persistent phosphorescence. 
Whatever those places in the molecule which we have called 
centrum may be which have the power of storing electrons, we 
shall begin by accepting, for trying an explanation, that the electron 
which is ejected from the metallic atom must in every case reach 
one of those places before it can be retained. By no means every 
wave-length of the exciting light seems suitable for this purpose. 
But not even the most suitable wave-lengths, corresponding with the 
places of maximum duration in the excitation-distribution, can in 
consequence of the heat motions of the molecules of the “phosphors” 
and their atoms that cause the individual centra to simultaneously 
assume different configurations, be the most suitable for all centra 
at the same time for ejecting the electron just in the direction of 
the place in which it can be stored. 
This is the explanation of the circumstance that in “1904” (p. 281, 
477, 475) it was found, that in order to develop the afterglow to 
the full and in the same manner to have good storing of electrons 
in the lower momentary phase long exposure to, or at least great 
intensity of the exciting light was necessary. For duration and 
*) See also the comprehensive resume “1909” p. 14. In P. Lenard and Sem 
Saeland Ann. d. Phys. 28. (1909). “the surrounding of the centrum” is inadver¬ 
tently spoken of twice instead of “the surrounding of the metallic atoms” as the 
place where the electrons are stored. 
