104 FLUORESCENCE OF THE URANYL SALTS. 



a degree or two, light vibrating horizontally is, in this arrangement, 

 less absorbed than light vibrating vertically. As to the direction of 

 vibration of the white light within the crystal, it can be said to be more 

 nearly parallel to the (a) crystallographic axis than to the (b) axis. 

 Rubidium chloride crystallizes in long six sided plates. As mounted, 

 plane polarized light was transmitted most freely when the direction 

 of vibration was parallel to the (a) crystallographic axis. Potassium 

 and ammonium chlorides crystallize in thin plates which approximate 

 more nearly the hexagon in form. Examination of the transmitted 

 light with the aid of a Nicol shows that the same relations exist between 

 the directions of vibration and the crystallographic axes as for the 

 rubidium chloride. 1 



Two polarized fluorescence spectra are always present, provided the 

 crystal is mounted as previously described. It is a remarkable fact that 

 these spectra remain unchanged, whether the exciting light is unpolar- 

 ized or is polarized in a white or green direction, or any other direction. 

 Their character, moreover, appears to be independent of the direction 

 from which the exciting light enters the crystal. This is in agreement 

 with a general principle established by the study of fluorescence 

 spectra, 2 that the character and location of a fluorescence band is 

 independent of the nature of the excitation. Changes in the polarized 

 spectra occur, however, as might be expected, if different crystallo- 

 graphic faces are placed at right angles to the axis of the collimator. 



Although visual observations were made, the spectra were mapped 

 for the most part from the photographic plates. Occasionally, the 

 fluorescence and a portion of the absorption spectrum could be photo- 

 graphed simultaneously to advantage, but more often different screen- 

 ing and various times of exposure were necessary in order to bring out 

 different regions of the absorption. The exposures varied in time from 

 30 seconds to an hour. 



A STUDY OF TYPICAL GROUPS OF BANDS FROM THE FLUORESCENCE 

 AND ABSORPTION SPECTRA AT +20 C. 



In the study of the spectra of the double chlorides described in Chap- 

 ter V, it has been shown that each group consists of 5 members and that 

 each of these bands is double. Since polarization effects a resolution 

 or separation, we should expect in general to find 5 components in each 

 polarized fluorescence group. In that chapter the bands of one fluores- 

 cence group have been designated as B, C, D, E, and A, and the same 

 nomenclature will be employed here. 



A typical fluorescence group for each of the four salts is indicated 

 in figure 71 . The bands above the horizontal line are the green polar- 

 ization components (B g , C g , etc.) ; those below, the white polarization 



1 The excellent specimens which were finally utilized in this investigation we owe to the per- 

 sistent and skillful efforts of Mr. D. T. Wilber. 



2 Nichols and Merritt, Physical Review, series i, 27, p. 373. 1908. 



