68 



FLUORESCENCE OF THE URANYL SALTS. 



immersed more or less deeply in the liquid air by raising or lowering 

 the Dewar flask. To preclude the gathering of frost or moisture on the 

 surface of the crystal, it was kept during the entire experiment at a 

 sufficient distance below the lip of the flask, where it was surrounded 

 with the dry atmosphere above the slowly evaporating mass of liquid 

 air. Measurements of the temperature were by means of a small coil 

 of fine copper wire mounted at the same level as the crystal, so as to 

 have always, as nearly as possible, the temperature of the latter. 

 Changes in the resistance of the coil were indicated on the sheet of a 

 Callender recorder, carefully calibrated to read directly in degrees 

 centigrade and adjusted for a range from +20 to 200. 



A 



X 



B C D E A 



-185 



iA 



FIG. 56. 



B? C Cf D D EE A A 



FIG. 57. 



The crystal was mounted so as to cover a transverse slot in the 

 copper rod. It could thus be illuminated either from the front, as 

 shown above, or from behind by light transmitted through the slot. 

 The latter arrangement was employed especially in the study of the 

 absorption spectrum. 



When the substance, excited to fluorescence in the manner already 

 described, was gradually cooled to the temperature of liquid air and 

 the spectrum was observed through the Hilger spectrometer, the fol- 

 lowing changes were noted: 



(1) The bands become narrower and better defined until at the 

 temperature of liquid air they correspond in appearance to the usual 

 line-like bands characteristic of the fluorescence spectra of the uranyl 

 salts at low temperatures. 



(2) As the temperature falls the bands are gradually resolved into 

 doublets. One component of each doublet becomes rapidly brighter, 

 while the other frequently becomes more indistinct and sometimes 

 disappears. The general effect is that of a shift toward the violet 



