2oo THE POPULAR SCIENCE MONTHLY 



" dull reddish." Apparently the color of many of the self-luminous 

 trains observed are recorded as various shades of green, gradually 

 changing to white as the train fades, and the remainder either bluish, 

 silver or white. The residue of the meteorite is evidently a cloud com- 

 posed of very finely divided smoke-like particles arising from the vola- 

 tilization, combined with gases liberated at the same time. This cloud, 

 if seen in daylight or illuminated in twilight by the sun, is usually red, 

 yellow, or silver colored, much like the ordinary clouds as they appear 

 just after sunset. The greenish luminosity of trains seen at night may 

 be due to continued brush-like electrical discharges near the border of 

 the train, but it is probable that it is the phosphorescence of the rarefied 

 atmospheric air in or around the train. This hypothesis is suggested 

 by the following experiments. 



Experiments on Phosphorescent Gases in the Laboratory 



An attempt has been made to reproduce the phenomenon of the 

 meteor train in the laboratory. One of the interesting effects produced 

 under certain conditions in a vacuum tube by electrical discharges is 

 the phosphorescence of the particles of gas which still remain in the 

 tube. This after-glow has been the subject of careful study. This 

 type of luminescence is, of course, not the same as the oxidation of the 

 element phosphorus, and is apparently different from, though very simi- 

 lar to, the phosphorescent light given out by insect larvse, like the glow- 

 worm, and insects such as the firefly. Phosphorescence is named for 

 its likeness to the slow burning of phosphorus, but in the case of solids 

 and gases, according to the best authorities, it is due to an unstable 

 chemical condition, brought about by the excitation by light or by elec- 

 trical discharges. The production of the phosphorescent light or after- 

 glow is supposed to take place while the substance is returning to the 

 former stable chemical state. 



In one of the illustrations a photograph is shown of a vacuum tube 

 containing a gas at low pressure, which is illuminated by gas phospho- 

 rescence. The gas was previously excited by an electrical discharge, 

 but at the time the picture was taken all electrical discharges had been 

 entirely cut off. Thus the photograph was taken by means of the 

 phosphorescent light of the gas alone, no other light and no electrical 

 discharge directly contributing to the formation of the picture. The 

 gas which is glowing is atmospheric air with most of the oxygen ex- 

 tracted, and therefore is chiefly nitrogen. This gas phosphorescence has 

 many points of resemblance to the meteor train, the chief of which are 

 first, it is formed at a density of the air that, as near as can be deter- 

 mined, is the same as the computed density of the atmosphere at heights 

 where meteor trains occur. Second, the comparison of observations of 

 the spectra of meteor trains and those of gas phosphorescence apparently 

 show that both are the same kind of spectra, although the location of 



