1881.] Discontinuous Phosphorescent Spectra in High Vacua. 207 



of solution of ammonia. The precipitated hydrate of alumina is 

 filtered, washed, ignited, and tested in the molecular stream. It 

 phosphoresces of the same crimson colour, and gives the same spectrum 

 as the ruby. 



Alumina in the form of ruby glows with a full rich red colour, and 

 when examined in the spectroscope the emitted light is seen to be dis- 

 continuous. There is a faint continuous spectrum ending in the red 

 somewhere near the line B ; then a black space, and next an intensely 

 brilliant and sharp red line, to which nearly the whole of the intensity 

 of the coloured glow is due. The wave-length of this red line, which 

 appears characteristic of this form of alumina, is, as near as I can 

 measure, X 689*5 m.m.m. This line coincides with the one described 

 by E. Becquerel as being the most brilliant of the lines in the spectrum 

 of the light of alumina in its various forms, when glowing in the 

 phosphoroscope. 



This coincidence is of considerable inlerest, as it shows a relation 

 between the action of molecular impact and of sunlight in producing 

 luminosity. The phosphorescence induced in a crystal of ruby by the 

 molecular discharge is not superficial, but the light comes from the 

 interior of the crystal, and is profoundly modified according as its 

 direction of vibration corresponds or makes an angle with the axis of 

 the crystal, being quenched in certain directions by a Nicol prism. 



Sunlight falling on the ruby crystal produces the same optical phe- 

 nomena. The light is internally emitted, and on analysis by a prism, 

 is seen to consist essentially of the one brilliant crimson line, X 689"5. 

 This fact may account for the extraordinary brilliancy of the ruby, 

 which makes it so highly prized as a gem. The sun not merely 

 renders the red-coloured stone visible, as it would a piece of coral, but 

 it excites the crystal to phosphorescence, and causes it to glow with a 

 luminous internal light, the energy of which is not diffused over a 

 broad portion of the spectrum, but is chiefly concentrated into one 

 wave-length. 



The crimson glow of alumina remains visible some time after the 

 current ceases to pass. When the residual glow has ceased it can be 

 revived by heating slightly with a spirit-lamp. 



After long experimenting with chemically pure alumina precipitated 

 from the sulphate as above described, a curious phenemenon takes 

 place. When sealed up in the vacuum two years ago it was snow 

 white ; but, after being frequently submitted to the molecular discharge 

 for the purpose of exhibiting its brilliant phosphorescence, it gradually 

 assumes a pink tinge, and on examination in sunlight a trace of 

 the alumina line can be detected. The repeated molecular excitation 

 is slowly causing the amorphous powder to assume a crystalline form. 



Under some circumstances, alumina glows with a green colour. 

 Ammonia in large excess was added to a dilute solution of alum. The 



