1879.] 



Molecular Pliysics in High Vacua. 



481 



which show that glass obstinately retains at even a red heat a com- 

 pound of hydrogen — probably water — which is only driven completely 

 •off by actual fusion. 



The permanent deadening of the phosphorescence of glass is shown 

 by projecting the shadow of a metal cross on the end of a bulb for a 

 considerable time. On suddenly removing the cross, its image 

 remains visible, bright upon a dark ground. 



One of the most striking of the phenomena attending this research 

 is the remarkable power which the molecular rays in a high vacuum have 

 of causing phosphorescence in bodies on which they fall. Substances 

 known to be phosphorescent under ordinary circumstances shine with 

 great splendour when subjected to the negative discharge in a high 

 vacuum. Thus Becquerel's luminous sulphide of calcium has been 

 found invaluable in this research for the preparation of phosphorescent 

 screens whereon to trace the paths and trajectories of the molecules. 

 It shines with a bright blue-violet light, and when on a surface of 

 several square inches is sufficient to faintly light a room. 



The only body which the author has yet met with which surpasses 

 the luminous sulphides, both in brilliancy and variety of colour, is the 

 diamond. Most diamonds from South Africa phosphoresce with a 

 blue light. Diamonds from other localities shine with different 

 colours, such as bright blue, apricot, pale blue, red, yellowish-green, 

 orange, and pale green. One very beautiful diamond in the author's 

 collection gives almost as much light as a candle when phospho- 

 rescing in a good vacuum. 



ISText to the diamond alumina and its compounds are the most 

 strikingly phosphorescent. The ruby glows with a rich full red, and 

 it is of little consequence what degree of colour the stone possesses 

 naturally, the colour of the phosphorescence is nearly the same in all 

 cases ; chemically prepared and strongly ignited alumina phosphoresces 

 with as rich a red glow as the ruby. The phosphorescent glow does 

 not therefore depend on the colouring matter. E. Becquerel* has 

 shown by experiments with his phosphoroscope, that alumina and 

 many of its compounds phosphoresce of a red colour after insolation. 



ISTo thing can be more beautiful than the effect presented by a mass 

 of rough rubies when glowing in a vacuum ; they shine as if they were 

 red hot, and the illumination effect is almost equal to that of the 

 diamond under similar circumstances. 



Masses of artificial ruby in crystals, prepared by M. Ch. Feil, behave 

 in the vacuum like the natural ruby. 



In the spectroscope the alumina glow shows one intense and sharp 

 red line less refrangible than the line B, and a faint continuous spec- 

 trum ending at about B. The wave-length of the red line is 6895. 



* " Annales de Chiniie et de Physique," 3rd series, vol. lvii, p. 50. 



