HISTORICAL. 7 



tion with that of the afterglow, Becquerel reached the very important 

 conclusion that the fluorescence and phosphorescence are identical. 

 This point finds ample confirmation in the present work. 



In 1872, E. Becquerel returned to the study of the uranyl salts. 

 The following are the conclusions reached in this investigation: 1 



(1) The salts of the protoxide of uranium are inactive. 



(2) Many, but not all, salts of the sesquioxide (uranyl salts) are active. 



(3) Five, six, and sometimes seven bright bands, or groups, are visible; 



lying between the Fraunhofer lines C and F. 



(4) The positions of the bands vary for different salts, but are always 



the same for a given salt. 



(5) The acid of composition determines the disposition of both bright 



and dark bands. 



(6) In double salts of the same acid the composition of individual groups is 



the same, but their position is not the same for the different salts. 



(7) In a given substance the distance between bands, as viewed in the 



spectroscope, increases from red to violet; but the differences of 

 wave-length decrease. The ratio of the above distances to the 

 square of the mean wave-length is nearly constant throughout the 

 spectrum, and this ratio (d/2) is the same for the various salts. 



(8) No simple relation is apparent between the location of homologous 



bands in different compounds and the chemical properties of the 

 compounds. 



(9) The absorption spectra also differ for the various compounds and the 



absorption bands seem to form a continuation of the fluorescence 

 series. 



(10) The location and character of these spectra being fixed and definite 



for each compound, we have the basis for an analytical method 

 similar to but less general than ordinary spectrum analysis. 



In 1873, Henry Morton and H. ,Carrington Bolton published an 

 account of extended studies of the fluorescence and absorption of the 

 uranyl salts. 2 Their list contains 85 substances, chiefly of their own 

 preparation, including 17 double acetates; but not all of these com- 

 pounds were found to be fluorescent. Readings were made on the 

 Bunsen scale in vogue at that time, and some of these, for comparison 

 with our own determinations, will be found, reduced to approximate 

 wave-lengths, in Chapter III. 



Figure 1, which is reproduced from the paper of Morton and Bolton, 

 gives an excellent general view of some of the most interesting of their 

 observations. The unshaded portions are fluorescence bands, the 

 shaded regions are the bands of absorption. The partial resolution 

 of the bands in several cases is clearly shown and the breaking-up into 

 distinct groups of the uranyl ammonium chloride ; also the coincidence 

 in certain cases of absorption and fluorescence in what in this mono- 

 graph we shall term the reversing region. 



1 E. Becquerel, Comptes Rendus, LXXV, p. 296. 1872. 



2 Morton and Bolton, Chem. News, pp. 47, 113, 164, 273, 244, 257, 268. 1873. 



