IV. PHOSPHORESCENCE OF THE URANYL SALTS. 



Concerning the phosphorescence of the uranyl compounds, we find 

 little on record beyond the early observations of E. Becquerel, 1 who, 

 in his classic paper of 1861, noted the brilliant and very short-lived 

 after-glow and made some observations on the law of decay. 



For the study of the phenomena of phosphorescence in these sub- 

 stances and in other cases having a duration of glow of a few thou- 

 sandths of a second, we devised a new instrument, the synchrono- 

 phosphoroscope. Indeed, for the experiments to be described in this 

 chapter, and which involved the use of surfaces of considerable size, 

 the cooling of the substance during excitation, simultaneous observa- 

 tions during fluorescence and phosphorescence, etc., none of the exist- 

 ing forms are easily adapted. The original phosphoroscope of Bec- 

 querel, 2 later modified by E. Wiedemann, 3 and also the revolving drum 

 type used successively in various forms by Becquerel, 4 Tyndall, 5 Kester, 6 

 and Waggoner, 7 afford sufficient speed, as does Merritt's 8 phosphoro- 

 scope of 1908; but none of these could be used without modification. 



The new apparatus 9 consists of a small synchronous, alternating- 

 current motor A. (7., figure 27, and a small direct-current motor D. C. 

 upon a common shaft. To one end of the shaft is attached a sectored 

 disk, WW, figures 27 and 28, with four equal open and four closed sec- 

 tors, corresponding to the four poles of the A. C. motor. On the cir- 

 cuit of 60 cycles this machine, when brought to speed by the D. C. 

 motor and released, runs steadily at 30 revolutions per second. A 

 "step-up" transformer TT, in the same alternating-current circuit, 

 produces discharges at the spark-gap, or series of gaps (E), at each 

 alternation, i. e., 120 times a second. This discharge may be reduced 

 to a single spark by proper adjustment of the resistance and capacity 

 of the circuit, or more conveniently for many purposes the discharge 

 may be confined to the peak of the wave by means of the four-pointed 

 star- wheel SS (figs. 27 and 28), which is mounted on the shaft and 

 carefully adjusted as to phase. 



The direct-current motor may also be used to drive the sectored disk 

 at other speeds, in which case the circuit of the motor A. C. is broken 

 and the discharge is derived from any convenient source capable of 

 producing a proper spark at each quarter revolution. 



1 E. Becquerel. Annales de Chimie et de Physique (3), LXII, p. 1. 1861. 



2 Ibid., LV, p. 5. 1859. 



3 E. Wiedemann, Wiedmann a Annalen, xxxix, p. 446, 1888. 



4 E. Becquerel, 1. c. 



8 Tyndall. See Lewis Wright's volume on light, p. 152. London, 1882. 



9 Kester, Physical Review (1), ix, p. 164. 



7 Waggoner, Carnegie Inst. Wash. Pub. No. 152. 



8 Nichols and Merritt, Carnegie Inst. Wash. Pub. No. 152. 



9 E. L. Nichols: Proc. Nat. Acad. of Sciences, v. 2, p. 328. 1916. Also Science, XLIII, 

 p. 937. 1916. 



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