PHYSICS. 463 



(9) Unlike kathodo-luminescence and photo-luminescence of the oxides, 

 this type of luminescence does not appear to depend upon the admixture of an 

 activating element. It is, however, sometimes profoundly modified by the 

 presence of such admixtures. 



(10) The effect is by no means confined to the blue, but may be observed 

 in other parts of the spectrum. It yields intensities very many times greater 

 than that of the black body for the same temperature and wave-length. The 

 intrinsic brightness is thus vastly greater than that of any known luminescent 

 surface at ordinary temperature. 



Kathodo-Luminescence of the Oxides. 



Another outcome of the discovery of the flame excitation of lumines- 

 cence, announced last year, is the systematic investigation by Mr. 

 D. T. Wilber and the writer of the kathodo-luminescence of the oxides, 

 such as CaO, MgO, BeO, AI2O3, Si02, which had been found active 

 in the hydrogen flame. 



The behavior of these in the kathode tube, as was to be anticipated 

 from the work of Crookes, Urbain, and others, is much more complicated 

 than their flame luminescence. We find that the various minute 

 admixtures of chromium, manganese, etc., which are incapable of 

 detection by means other than that of the character of the luminescence 

 spectrum, require different degrees of heat to render them active. 

 Thus a sample of aluminum oxide may be made to fluoresce green, red, 

 or blue, according to the precise temperature to which it is exposed. 



Luminescence of Beryllium Oxide. 



Beryllium oxide is a new member of this very interesting group, and 

 its luminescent properties, which hitherto have not been investigated, 

 are now being studied. We find this oxide to be unaffected by light 

 or ultra-violet rays, almost completely inert under exposure to X-rays, 

 finely fluorescent in the kathode tube, after heating, and highly 

 responsive to excitation by the hydrogen flame. 



Between 500° and 1200° C. it exhibits markedly the blue glow. 



Luminescent Flames. 



That the light from flames containing various salts is due to lumi- 

 nescence was suggested many years ago by E. Wiedemann, and experi- 

 mental evidence in favor of this view was given by Ebert in 1891, 

 Certain physical chemists, particularly Trautz and Bancroft and 

 Weiser, in their papers on flame reactions, have taken a similar position. 



We find by a study of the spectra of these flames abundant further 

 confirmation of this theory. In every case examined the structure of 

 the spectrum conforms to the general type of luminescence spectra, 

 being made up of series of bands having a constant frequency interval. 



Other Studies in Progress. 



Mr. J. A. Becker is determining the temperature range through 

 which substances excited to luminescence by X-rays are active and 

 is comparing the spectra of bodies thus excited with the corresponding 

 photo-luminescence. 



