112 STUDIES IN LUMINESCENCE- 



its power of phosphorescence under excitation by the iron spark, but on 

 being heated it regained its former brilliancy. This led to the suspicion that 

 these simple salts are photo-luminescent only when in the anhydrous condi- 

 tion, and subsequent tests showed this to be true of all the compounds studied. 



The fact that moisture affects the photo-luminescence of cadmium salts 

 may be shown by exciting the anhydrous phosphorescent compound by an 

 ultra-violet source and allowing the light to decay for several seconds. 

 Now if a drop of water be placed on the decaying compound the residual 

 light will flash out brilliantly for an instant and then disappear very rapidly. 

 It was first thought that this phenomenon was due to the changes which 

 take place within the salt as it takes up water of crystallization. The 

 thermo-phosphorescence of the cadmium compounds, however, is quite 

 marked, and Dr. Kortright, of the Department of Chemistry, West Vir- 

 ginia University, suggested that the effect might be partly due to thermo- 

 phosphorescence brought about by the heating which accompanies the 

 process of hydration. To obtain some idea of the rise in temperature, 

 several grams of anhydrous salt w r ere placed in a test-tube with a ther- 

 mometer, and water was added. The rise of temperature was found to be as 

 much as 5 C, which would account in a large measure for the phenomenon 

 purely on the basis of thermo-phosphorescence. Alcohol, which is not a 

 solvent for cadmium sulphate, was tried, but gave no effect. 



The fact that these cadmium compounds do not show photo-luminescence 

 when in the crystalline state led the writer to try some of the original cad- 

 mium sulphate from which the compounds with manganese were made ; and 

 when the water of crystallization was driven off, the salt was found to be 

 highly phosphorescent. The color of the phosphorescence was similar to 

 that when a manganese salt had been added, but was less intense. When 

 subjected to kathode rays the sulphate showed more brilliant fluorescence 

 but less phosphorescence than when excited by the spark. 



An attempt was first made to purify the original cadmium sulphate by 

 forcing it out of water solution with sulphuric acid, but this treatment 

 increased rather than decreased the intensity of phosphorescence. 



Another method was to precipitate the cadmium with hydrogen sulphide 

 as cadmium sulphide, wash it with water till free from sulphates, then re- 

 dissolve the sulphide in sulphuric acid, crystallizing the salt formed. This 

 sulphate still gave a brilliant yellow phosphorescence under both the iron 

 spark and the kathode rays. 



The next method was to crystallize the salts in fractions from water 

 which was slightly acidified with sulphuric acid to prevent hydrolysis. 

 The mother liquor was saved in this case and the crystals redissolved and 

 crystallized out. It was found that the phosphorescence of the successive 

 crystals, as they separated out from the retained mother liquor, decreased rap- 

 idly in intensity and at the end of the seventh fractionalization was almost 

 nil, thus indicating the presence of an impurity less soluble than the cadmium 

 sulphate. This view was supported by a micro-chemical test made by Dr. 

 Wilkinson 1 showing the presence of sodium and a slight trace of zinc. 



In casting about for a cadmium sulphate free from phosphorescence 

 numerous samples were tried and it was found that the "tested purity" 

 cadmium sulphate made by Eimer and Amend was free to a remarkable 



Journal Physical Chern., xin, p. 719, 1909. 



