56 FLUORESCENCE OF THE URANYL SALTS. 



(9) In certain salts, such as uranyl ammonium nitrate, decay is 

 retarded by cooling; in other cases the temperature effect is slight. 



(10) Uranyl nitrates with 2, 3, and 6 molecules of water of crystal- 

 lization vary greatly in the rate of decay, but the changes in crystalline 

 form appear to be more important in this respect than the amount of 

 water. 



(11) In the case of the polarized spectra of the double chlorides, both 

 components decay at the same rate and no change in relative bright- 

 ness can be detected throughout the range covered by observation. 



PHOSPHORESCENCE OF LONG DURATION. 



While comparing the spectra of uranyl salts under excitation by 

 kathode rays and under photo-excitation, in 1917, Misses Wick and 

 McDowell discovered that certain salts continued to glow for several 

 minutes after bombardment in the vacuum tube, at the temperature 

 of liquid air. 



Many uranyl compounds are unstable in vacuo, and of those which 

 are not decomposed rapidly, some, notably the chlorides, are prac- 

 tically inactive under the kathode rays. The following salts, which 

 were prepared by Mr. Wilber in the form of fairly large, well-formed 

 crystals, gave bright fluorescence and were fairly stable : 



Uranyl potassium nitrate, K2UOz(NOs)4 (crystallized from 10 to 30 per cent nitric 



acid). 

 Uranyl potassium nitrate, K2U02(NO 3 )4 (long crystals from 2 to 3 per cent nitric 



acid). 



Uranyl potassium nitrate, KUO2(NO 3 )3 (water form). 

 Uranyl potassium nitrate, KUO 2 (NOs)3 (anhydrous). 

 Uranyl potassium sulphate. 

 Uranyl potassium sulphate (with 2 molecules of water). 



An examination was made of all of this group. They were found 

 to exhibit phosphorescence in varying degree. Some showed no phos- 

 phorescence of noticeable duration. The following, which were among 

 the brightest, were selected for study: 



(1 and 2) K^UO^NOsV The first form, A, was crystallized from 

 a 10 to 30 per cent solution of nitric acid, and the second form, B, from 

 a 2 to 3 per cent solution. Although the crystallographic form is 

 identical, form A crystallizes in short, thick crystals and form B in 

 long, slender crystals. There appeared to be a slight difference in the 

 phosphorescence of the two forms. It is possible, however, that the 

 difference observed might have been due to some variation in the 

 conditions under which the phosphorescence was produced. 



(3) K 2 U0 2 (S04)2. To ascertain whether, as the result of exposure 

 to the kathode rays, the surface layer of the crystals had undergone 

 some change which rendered them capable of persistent phosphores- 

 cence under photo-excitation, they were alternately illuminated by the 

 light of a carbon arc and bombarded by the kathode rays. To accom- 



