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SCIENCE 



[N. S. Vol. XXXIII. No. 85S 



garded as an electric oscillator, tlie period of 

 vibration of which depends upon inductance 

 and capacity. By modifying the dielectric con- 

 stant of the solvent it was found that the posi- 

 tion of the fluorescence band could be altered 

 at will and Pauli succeeded in producing 

 preparations having bands in the ultra-violet 

 only and in other cases only in the infra-red. 



The Complexity of the Fluorescence Spectra 

 of Solids. — The complexity of fluorescence 

 spectra which was first so strikingly exhibited 

 in Wood's studies of sodium vapor is by no 

 means confined to gases. Not only do we have 

 solids whose spectra are made up of overlap- 

 ping bands due to the presence of several fluo- 

 rescing constituents, as in the ease of the 

 sulphides of Lenard and Klatt, but there are 

 also substances of definite chemical composi- 

 tion, notably the uranyl salts, the fluorescence 

 spectra of which consist of a group of several 

 bands arranged in a manner which suggests 

 the banded spectra of gases. E. Becquerel" in 

 1872 studied these fluorescence bands and 

 discussed their relations to the corresponding 

 absorption spectra. The subject has since 

 received the attention of numerous investiga- 

 tors; the most recent contribution being that 

 of Jones and Strong." Henri and Jean Bec- 

 querel," working at Leiden with Kamerlingh 

 Onnes, have studied the fluorescence spectra 

 of uranyl salts; photographing the same at 

 288° of the absolute scale, 80° (liquid air), 

 20° (liquid hydrogen) and 14° (solid hydro- 

 gen). Under these conditions the already 

 unusually narrow bands are reduced to mere 

 lines in the spectrum and each band is found 

 to consist of a group of such lines. 



The relation of the narrow fluorescence 

 bands of the uranyl salts to the broad bands in 

 the fluorescence spectra of other solids and 

 liquids is not at first sight obvious. They ap- 

 pear to be intermediate between the broad 

 bands usual to solids, of which sometimes only 



" E. Beequerel, Mernoires de I 'Academie des 

 Sciences, XL., 1872. 



"Jones and Strong, American Chemical Journal, 

 XLIIL, p. 37. 



"H. and J. Beequerel and Onnes, "Leiden 

 Communications," No. 110. 



one exists, and the complicated groups of lines 

 observed in the case of vapors. Some recent 

 studies of the uranyl bands appear, however, 

 to throw some light on this relationship." 

 Hitherto attention has chiefiy been directed to 

 the location of the bands in the spectrum. 

 When, however, we measure the relative in- 

 tensities of such a group of bands we find them 

 to be related in a very interesting manner. 

 The curve showing the relation between wave- 

 length and intensity of the seven strongest 

 bands in the fluorescence spectrum of such a 

 salt has a form which is essentially identical 

 with that of the typical curve for the distri- 

 bution of energy in the broad single bands of 

 ordinary fluorescence spectra. The shape is 

 the same for the various uranyl salts; being 

 shifted slightly in wave-length with the chem- 

 ical composition or with the presence or ab- 

 sence of water of crystallization. 



Pig. 2 will serve to indicate the charac- 

 teristic form of these curves and the shift 



.if-iju. .Si. -fi, .60 



Fig. 2. Eelative intensities of the seven prin- 

 cipal bands in the spectra of two uranyl salts. 



" Nichols and Merritt, ' ' On the Fluorescence 

 Spectra of the Uranyl Salts and on the Structure 

 of Fluorescence Bands in General. ' ' A paper 

 presented at the Minneapolis meeting of the Amer- 

 ican Physical Society, December 28, 1910. 



