FLUORESCENCE OF THE URANYL SALTS. 



BY EDWARD L. NICHOLS AND HORACE L. HOWES. 



I. HISTORICAL INTRODUCTION. 



The beginnings of precise knowledge concerning the luminescence of 

 the compounds of uranium are to be found in the classical memoirs 

 of George Gabriel Stokes and of Alexandre Edmond Becquerel. It is 

 true that Brewster 1 , who observed the fluorescence of chlorophyl and 

 other substances in 1833 and gave the phenomenon the name of internal 

 dispersion, mentioned a yellow glass, doubtless the "canary glass" of 

 commerce, which exhibited the same property, but it remained for 

 Stokes, 2 by means of the beautiful experiments described in his papers 

 entitled "The Change in the Refrangibility of Light," to really eluci- 

 date the phenomena and to lay the foundation for all subsequent work 

 on fluorescence. 



Having observed, by the use of suitable light-filters and by his 

 ingenious and elegant method of transverse dispersion, the unusual 

 character of the fluorescence and absorption of this glass, Stokes pro- 

 ceeded to the investigation of such compounds of uranium as he was 

 able to procure. From the nitrate he made the acetate, oxalate, and 

 phosphate; also uranates of potassium and calcium and the oxides. 

 He also obtained specimens of autunite (uranyl calcium phosphate) 

 and chalcolite (uranyl copper phosphate) . After observations of these 

 minerals he writes (Sec. 145) : 



"The intervals between the absorption bands of green uranite were nearly 

 equal to the intervals between the bright bands of which the derived spectrum 

 (i. e., the fluorescence spectrum) consisted in the case of yellow uranite. After 

 having seen both systems I could not fail to be impressed with the conviction of a 

 most intimate connection between the causes of the two phenomena, unconnected 

 as at first sight they might appear. The more I examined the compounds of 

 uranium, the more this conviction was strengthened in my mind." 



Upon reading Stokes's memoir one can not but feel that had he had 

 at his command a modern spectroscope he would infallibly have antici- 

 pated by more than half a century much of the recent work on fluores- 

 cence. He used light-filters to prevent the exciting beam from sub- 

 merging the fluorescence on the one hand and to exclude the exciting 



1 Sir David Brewster, Trans. Roy. Soc. Edin., vol. xn. 1833. 



2 Stokes, Phil. Trans., 1852, p. 463; 1853, p. 385. 



