PHOSPHORESCENCE SPECTRA. 



51 



expected with the uranyl salts, where the spectrum, in spite of its 

 complexity of structure, is a unit. It was deemed of interest, however, 

 to determine the effect of temperature upon the latter. 



For this purpose a specimen of the uranyl ammonium nitrate was 

 mounted within a cylindrical Dewar flask with unsilvered walls and its 

 decay of phosphorescence was determined with a synchrono-phos- 

 phoroscope at a temperature a few degrees above that of liquid air 

 (about 180) at +20 and at 4-60. The last-named temperature 

 was maintained during the run by means of an electrical heating-coil. 



2.8 



5.2 



~ 



URANYL RUBIDIUM NITRATE 

 VARYING EXCITATION 



11.9 



50 



40 



30 



20 



sec. 



FIG. 41. 



The principal change consists in a marked retardation of decay with 

 lowering temperature (see fig. 39), but this is not a universal charac- 

 teristic of the uranyl compounds. Uranyl ammonium sulphate, for 

 example (fig. 40), is but slightly influenced in its rate of decay by 

 change of temperature and the curve for 180 is intermediate between 

 those for +20 and +60. 



