382 Prof. E. Wiedemann on the 



p times as many molecules B are changed into A, as in the 

 second case. 



This must consequently also be the case at all subsequent 

 equal times. The brightnesses i x and i 2 observed at equal times 

 in the two series of experiments must therefore in fact always 

 stand to each other in the same ratio. 



If we obtain by interpolation, from the data given above, 

 the brightnesses i x and i 2 for the times i in the series of expe- 

 riments I. and II., we obtain the values of i 2 ji\ given in the 

 following Table : — 



t 4". 15". 25". 40". 1'. 



i x 27-8 13-3 6-2 4-2 3-2 



i 2 16-8 86-5 447 205 14-8 



ijh 6-0 6-5 7-2 5-0 4"7 



The ratio i 2 \i x is as constant as can be expected with such 

 necessarily difficult observations. Whilst the brightness sinks 

 to ■£ of the original value, the ratio i 2 /i x varies between 6'0, 

 7*2, and 4*7. When the brightness becomes small, the diffi- 

 culties of the observation increase, and consequently the 

 uncertainty of the determination. 



From this theory, that the transformation of a modification 

 B into another modification A produces the phosphorescence, 

 we have also the explanation of the fact that particular rays 

 of the spectrum produce a phosphorescence more vivid, 

 but of shorter duration. It is evidently the rays absorbed by 

 the modification B which, because they excite the molecules 

 of B, accelerate their transformation with more active evolu- 

 tion of light. 



In this lies the explanation of the following phenomenon. 

 M. H. Becquerel* determined, on the one hand, the wave- 

 lengths X, X 1? X 2 , . . . of those rays in the violet which ex- 

 cite phosphorescence in different sulphur compounds of the 

 metals of the alkaline earths of analogous composition, and 

 then the wave-lengths X, X/, X 2 ' . . . in the infra-red of those 

 rays which strengthened it ; then the order of the X"s and of 

 the X's was the same, only that for those bodies to which in 

 the violet a greater X corresponded, a smaller X / was found in 

 the infra-red. The modifications B, . . . B„ will of course be 

 similarly marked for the different bodies, as the modifications 

 of A L . . .A n from which they were formed. The wave-lengths 

 of the absorption-bands of the modifications formed from the 

 different analogous compounds show, however, the same order 

 as the wave-lengths of the absorption-bands of the original 

 substances. Therefore the absorption-bands of the modifica- 



* Comptes Rendus, xcvi. p. 1853 (1883). 



