PHYSICS. 383 



unresolved bands even at the temperature of liquid air. This is in 

 accord with Dr. Howes's^ observations on frozen solutions, a summary 

 of which was recently published, while further material of interest and 

 significance is in preparation for the forthcoming monograph. 



Phosphorescence of the Urantl Salts. 



By means of a new form of phosphoroscope, the synchrono-phos- 

 phoroscope^, a study has been made of the phosphorescence of the vari- 

 ous uranyl compounds, concerning which but httle has been recorded 

 since the original observations of E. Becquerel in 1861. The after-glow 

 in these substances, while brilliant, is very brief, the intensity falling 

 to YTTTTw of its initial value in a few thousandths of a second. It was 

 found possible, nevertheless, to estabUsh the following facts :^ 



(1) The phosphorescence spectrum is identical in structure with 

 the spectrum of fluorescence, all the narrow bands observable at 

 — 185° being present in both. 



(2) During the decay of phosphorescence all the different bands 

 decrease in intensity at the same rate, so that there is no change in 

 the relative distributions of energy, and the entire complex, in this 

 respect as in many others, is a unit. 



(3) The law of decay"* differs from that of the phosphorescent 

 sulphides and other substances hitherto investigated in that, while 

 there are three rates or processes following one another, the second 

 process is more rapid instead of being slower than the first, and the 

 third more rapid than the second. 



Experiments are planned by which to determine whether this new law 

 of decay is common to all substances for which the phosphorescence 

 is of very brief duration or is pecuUar to the uranyl salts and due perhaps 

 to their radioactivity and to the consequent presence of /3 rays. 



The Phosphorescent Sulphides. 



The change of color of the phosphorescence of the luminescent 

 sulphides is commonly attributed to the existence of two overlapping 

 bands the intensities of which die away at different rates after the 

 cessation of excitation. By the use of the synchrono-phosphoroscope 

 it has been found possible to establish the actual existence of these 

 bands and to determine their properties in some detail.^ 



In the sulphides of Lenard and Klatt the band of short duration 

 lies in the green, that of long duration partially overlapping, it so that 

 in the spectroscope the appearance is that of a single very broad band. 

 In the barium sulphides the band of long duration is on the side 

 towards the red, in the sulphides of calcium and strontium towards 



'Howes, H. L., Physical Review (2), vi, p. 192 (1915). 



-Nichols and Howes. Science, xliii. p. 937 (1916) ; also Physical Review (2), vii, p. 583 (1916). 



'Nichols, Proc. Nat. Acad. Sci., ii, p. 328 (1916). 



^Nichols and Howes, Physical Review (forthcoming). 



^Nichols, Proc. Am. Philos. Soc, lv. p. 494 (1916). 



