STUDIES OF PHOSPHORESCENCE OF SHORT DURATION. 113 



degree. The phosphorescence, under the iron spark, of the anhydrous 

 powder was so small that it was necessary to stay in the dark room a long 

 time before the eye became sensitive enough to detect the phosphorescence 

 at all ; and even then one could not be sure of the color. The analysis of this 

 material given on the bottle was as follows: " Cd, 52.84 per cent; S0 3 , 47.15 

 per cent; As, none; Zn, none." This material was tested by the kathode 

 rays and it was found to show neither fluorescence nor phosphorescence. 



That a zinc salt, when added to a cadmium salt, or vice versa, would 

 produce a phosphorescent compound has been pointed out by Wiedemann 

 and Schmidt. 1 This fact led to the suggestion that the presence of small 

 traces of zinc in cadmium salt, or vice versa, might be detected by the 

 photo-phosphorescence which these compounds show, and some rough 

 tests were made showing photo-phosphorescence with less than one part 

 of zinc sulphate in 10,000 parts of the phosphorescent-free cadmium sul- 

 phate. The phosphorescent color was green, but the intensity was not 

 so great as when a sodium salt is added to a cadmium salt. 



The pure cadmium sulphate is very susceptible to a small impurity; 

 how susceptible will appear from the following experiment: Some water, 

 which was twice distilled in an all-glass still, was placed in a bottle and 

 allowed to stand over night. The following day this water was used to 

 dissolve some of the pure cadmium sulphate and was then driven off. 

 When the cadmium sulphate was tried under the ultra-violet source it was 

 found to be highly phosphorescent. 200 c.c. of this same water when 

 evaporated down in a platinum dish left a residue so small that it could 

 not be seen except when the platinum dish was at red heat. 



Cadmium Sulphate-Sodium Compounds. Starting with the phosphor- 

 escence-free cadmium sulphate, a series of preparations was made by the 

 addition of various salts of sodium. The method used was that already 

 described. It was to dissolve the crystals of sodium salt and cadmium 

 sulphate in water; slowly evaporate the solution to dryness, and test the 

 compound for phosphorescence, when it reached the temperature of the 

 room, by exciting by the iron spark. The compound, from the original 

 crystals to the final test, was kept in a large platinum dish and every pre- 

 caution was taken to prevent impurities from entering the solution. 



The water was twice distilled in an all-glass still and used immediately 

 in order to prevent it from taking up sodium from the glass container. 

 To produce an even and easily regulated temperature a small electric furnace 

 was made by wrapping "nichrome" ribbon around a porous battery cup; 

 this proved to be a very satisfactory arrangement, for the platinum vessel 

 could be placed inside of this furnace and covered over lightly to prevent 

 impurities entering from the air while evaporation was taking place. 



Only readily soluble salts of sodium were used ; and each one was tested 

 in the anhydrous condition for the presence of phosphorescence. In one 

 or two cases it was necessary to recrystallize the salt to free it from the 

 impurity causing the phosphorescence. Several different percentages of 

 cadmium and sodium were tried and it was found that the intensity of phos- 

 phorescence depended somewhat upon the percentage of sodium present; 

 but so far as the tests were carried no change was produced in the color 



>Wied. Ann., vol. 56, 1895. 



