PHYSICS. 361 



definite modifications of the flame-spectrum are observable. In general the 

 bands of flames containing Ca, Sr, and Ba are rendered narrower and less hazy 

 when the flame is exposed to the exciting light; black grounds tend to become 

 blacker; narrow components of some of the broader bands, which may be 

 scarcely discernible when unexcited, come out more clearly; the dark lines 

 between the crests of double bands become deeper and, in a word, there is a 

 tendency towards better resolution of the spectrum. 



A very striking and instructive example of photo-excitation, unique as yet 

 in our experience, is the green line of thallium. When excited by an amalgam 

 mercury arc in quartz this line increased m brightness more than 50 per cent 

 (in one instance many times). This unusual sensitiveness was found to be due 

 to the presence of the thallium lines in the amalgam arc. To these wave- 

 lengths in particular the thallium in the flame is opaque and capable of absorb- 

 ing the energy necessary to excite luminescence. 



Miscellaneous. 



Professor Frances G. Wick has completed her studies of the luminescence of 

 certain fluorites (Year Book 21). She finds that by heating these crystals to 

 fusion the rather broad-banded spectrum is converted into a spectrum of 

 exceedingly fine lines, the narrowest perhaps as yet known in fluorescence 

 spectra. There are the lines of Samarium, Europium, Dysprosium, and other 

 rare earths, to the presence of traces of which the luminescence of these 

 fluorites is due. Heating also brightens and prolongs the phosphorescence of 

 the crystals. 



The work on the effects of great pressure upon absorption and fluorescence, 

 which Miss Wick did in collaboration with Professor Bridgman, is being 

 pul)lished by the American Academy of Arts and Sciences. 



Mr. Paul Bayley has just completed an investigation of the fugitive colora- 

 tion of the salts of Li, Na, K, Cs, and Rb by the action of X-rays and of the 

 phosphorescence of these salts under X-ray excitation. 



Some recent experiments by Miss Dorothy Waugh afford valuable data for 

 the development of the theory of the "dark current" in photo-active electro- 

 lytic cells. 



Plans are being developed to attack the important general question as to 

 whether all cases of luminescence depend upon the presence of an admixture of 

 an activating metal or element. Preliminary experiments are already under 

 way by Mr. Wilber to investigate the most important exception, that of the 

 uranyl salts. Tests of the platino cyanides are also being devised. 



