earths, and the oxides of the earth metals, it is known that these 
substances in the pure state do not phosphoresce; and it appears 
that the influence of impurity, as was first noticed by Edmond 
Becquerkl, is essential to the formation in those substances of complex 
molecules, Lenard’s “centra”, which, as distinguished from the molecules 
of the substances themselves, have the power of emitting light. 
It is completely different with the salts of uranyl. These compounds 
always phosphoresce, even when quite pure, and each of them is 
characterised by a specifically distinct emission spectrum. 
And in other respects the uranyl salts differ from those just 
mentioned as phosphorescent only by the influence of admixture. 
Even at a temperature so low as that of liquid hydrogen, in intensity 
me emitted light does not alter and vanishes with the illumination 1 * j. 
he uranyl salts do not exhibit thermoluminescenee. Finally, the 
spectra of these salts are of remarkable structure, while an immediate 
relation exists between the emission and absorption spectra. 
These optical phenomena at low temperatures, by which conside¬ 
rable light is thrown on the characteristic properties of the phosphor¬ 
escence of uranyl salts, thus lead to a distinction between two kinds 
of phosphorescence, which seem to be of different origins. On the 
one and, in the case of the rare earths, the sulphur compounds of 
the alkaline earths, and the oxides of the earth metals, according to 
the beautiful theory of Lenard ■), the electrons, which, under the 
photo-electric influence of the absorbed light, are ejected by the 
atoms of the metal and by their return to these cause phosphorescence 
are supplied by that metal of which a small quantity is added to 
the experimental substance so as to cause it to phosphoresce by the 
formation of “centra”, of which the structure of the molecular com- 
p exes play an important part in the phosphorescence. On the other 
t»? n k ° * n track of the same theory, the explanation of 
the phenomena exhibited by uranyl salts must be sought in the motion 
ol the electrons contained in the uranium atom itself and in the 
structure of the molecules of the substance itself. 
n is case especially considering the small obstruction cold offers 
he return of the electrons 3 ) the question obtrudes itself if, perhaps, 
16 W 0 e process of phosphorescence takes place within the uranium 
aom, or at least within the uranyl group, and electrons, under the 
*°, ° e /^, nC in ^ uence of the absorbed light, are projected from a 
° IS atom to other parts, remain there temporarily, and, by 
J ) Added in the translation. 
3 ) The^miT’ Verh \ d * ^Naturhist. Med. Ver Heidelberg 5 Febr. 1909. 
sion vanishing with the illumination. (Added in the translation). 
