( 85 ) 
compared with that of the other emission bands* is much modified 
by this property. 
The experiment concerning the reversal of certain bands of the 
uranyl salts is the first proof that certain emission bands occupy 
exactly the same position as the absorption bands. Further, the irre¬ 
versible homologous bands of the other groups correspond with 
vibration-frequencies of the absorbed light separated from these bands 
by constant differences. 
There seems, therefore, to exist an immediate and simple relation 
between absorption and emission in the uranyl salts. 
We may recall that the uranosalts, which do not phosphoresce, 
have absorption spectra in which the bands with marked regularity 
follow the law of succession of the emission bands of the phosphor¬ 
escent uranyl compounds ; ). 
§ 8. Phosphorescence of the sulphur compounds at low temperatures. 
We have made but very few observations of the phosphorescence 
at low temperatures of phosphorescent substances other than the 
uranyl salts. As regards sulphur compounds of the alcaline earths we 
could add nothing to the masterly research of Lenard and Klatt*). 
Zinc sulphide is very luminous when excited by violet light at 
80 K. and remains luminous for a long time afterwards; 4 or 5 
seconds after excitation ceases, the stronger radiation comes to an 
end, and then for a long time afterwards the zinc sulphide shines 
with a soft glow. From the moment that warming begins as it 
returns to ordinary temperature, zinc sulphide shows a particularly 
strong lustre. 
At hydrogen temperatures the light emitted during excitation, and 
by warming after excitation, becomes less. 
The colour of the light coming from zinc sulphide seems to be 
the same at various temperatures, and between 80° K. and ordinary 
temperature no change can be seen in the spectrum, which was not 
unexpected seeing that the spectrum was continuous. 
$ 9. Conclusion. The uranyl salts must be regarded as a class by 
themselves sharply differentiated from other classes of phosphorescent 
substances. 
From the researches of Lecocq de Boisbaudran, Lenard and Klatt, 
and Urbain on the rare earths, the sulphur compounds of the alkaline 
1 ) Henri Becquerel Compt. Rend. t. Cl. 1885, p. 1252. 
2) P. Lenard and V. Klatt, Ann. d. Phys. IV. 15. 1904, p. 225—282 and 
425—672. 
