Mat 5, 1911] 



SCIENCE 



699 



due to change in eliemical composition. 

 The examples selected are uranyl sulphate 

 (TT0,S0^-3H,0) and uranyl potassium sul- 

 phate (U0,sb,KS0.-2H,0). 



That this relation is not an accidental one, 

 but that these various bands are probably due 

 to the oscillations of one and the same com- 

 plex mechanism may be almost conclusively 

 established by observing the spectrum under 

 various conditions of excitation and of tem- 

 perature. When we subject accidental combi- 

 nations due to different substances in a fluo- 

 rescing mixture to such changes the independ- 

 ence of the various components is readily 

 shown, but in the case of the uranyl bands 

 when excited by the monochromatic light of 

 various lines of the mercury arc from .4538/" 

 to .2536/i it is found that the relation be- 

 tween the intensities of individual bands 

 remains very nearly the same. When the sub- 

 stance is gradually cooled to the temperature 

 of liquid air and observations are made at 

 intermediate temperatures, it is found that all 

 of the bands shift towards the violet and that 

 their gradual movement as the temperature 

 changes is of the same character for all. The 

 relation of intensities in this case does not re- 

 main the same, but it is significant that the 

 changes are such as to produce a modification 

 in the curve of relative intensities precisely 

 similar to that which is known to occur in the 

 case of the ordinary broad continuous fluo- 

 rescence bands when the excited substance is 

 cooled. It would therefore appear that we 

 have in the case of the fluorescence of the 

 uranyl compounds a single fluorescence band 

 of the usual type but broken up into several 

 easily separable components. It is easy to 

 imagine that a continuance of the change 

 which occurs in the fluorescence of these sub- 

 stances as the temperature rises from that of 

 liquid air to + 20° C. would result in the 

 merging of these components into a continu- 

 ous band in which they could no longer be 

 distinguished from one another. 



In the case of some of the uranyl salts, in- 

 deed, the overlapping of the bands is consid- 

 erable under ordinary conditions, as may be 

 seen in Fig. 3, which is from measurements of 



the three strongest bands of uranyl nitrate 

 at 20° C. 



Fig. 



■ii-Sf^ .So SZ -^f 



?'IG. 3. Diagram showing the overlapping of 

 the three principal bands in the fluorescence spec- 

 trum of uranyl sulphate. 



The relation may be carried still further 

 when we consider that the broad continuous 

 bands of fluorescent substances are always as- 

 sociated with a broad absorption band usually 

 overlapping the fluorescence band on the side 

 toward the violet and that the absorption 

 spectrum of the uranyl salts consists of a 

 series of bands precisely similar as regards 

 their arrangement and number to the bands 

 of the fluorescence spectrum. This group of 

 bands according to E. Becquerel is to be 

 regarded as a continuation of the series of 

 fluorescence bands. It has been shown by H. 

 and J. Becquerel and Onnes" to overlap the 

 group of fluorescence bands. According to 

 their photographs the bands nearest the red in 

 the absorption group when observed at the 

 temperature of liquid air are nearly if not 

 quite coincident with the last ones of the 

 fluorescence group towards the violet. 



As in the case of the fluorescence bands of 

 the uranyl salts, the absorption bands increase 



'^ Becquerel and Onnes, \. c. 



