( 77 ) 
|changes when brought under the influence of a very low temperature, 
the same change is also to be expected in the emission spectrum of 
its phosphorescence. This^ expectation is fully supported by our 
lexperiments. The more or less broad and diffuse bands which follow 
each other in regular sequence in the spectra of the uranyl salts at 
[•ordinary temperature, are, at the temperature of liquid air, resolved 
into multiple bands, which, in some cases, are particularly fine and 
intense; in that respect, all the bands in both emission and absorption 
spectra undergo identical changes. The appearance of these spectra 
at liquid air temperature has already been described 1 ). At the Leiden 
; cryogenic laboratory we have extended the investigation of these 
phenomena to the temperature of solid hydrogen (14° K.), and the 
most important of the results thus obtained concerning emission 
spectra at very low temperatures are here published. 
§ 2. Experimental method. The apparatus here used for the in¬ 
vestigation of the spectra of phosphorescence was the same as that 
which had already served for the investigation of absorption spectra, 
and for a description of which we refer to former papers *). 
Here we have only to mention that the source of the spectrum 
was a Rowland plane grating with a lens of 1.30 m. focal length 
arranged in autocollimation,. and that the phosphorescent substances, 
enclosed in thin-walled glass tubes, were placed in a non-silvered 
vacuum vessel containing liquid hydrogen; in the latter case the 
glass containing liquid hydrogen was placed in a second vacuum 
vessel with liquid air so as to reduce the vaporisation of the hydrogen. 
The light of an arc-lamp, filtered through an ammoniacal solution 
of copper sulphate, served to illuminate the salts. 
§ 3. Behaviour o'f the spectra when the temperature is lowered. The 
displacement and the limiting position of the phosphorescence hands 
of uranyl salts. 
When uranyl salts are cooled to a temperature of 14° K. (solid 
hydrogen), the lustre which they show at ordinary temperature is 
not diminished and vanishes with the illumination. Hence, lowering 
of the temperature of uranyl salts in no way prevents nor prolongs 
or shortens 8 ) their emission of light. 
The bands which, between ordinary temperature and that of liquid 
air already undergo subdivision, become groups of much finer lines 
at the temperature of liquid hydrogen. 
h Henri Becquerel Gompt. Rend. t. GXLIV 1907 p. 459 and. p. 671. 
2 ) Proc. 29 Febr. 1908; Comm. Phys. Lab, Leiden N°. 103. Le Radium t. V, 
p. 227. 
3 ) Added in the translation 
