Page:Popular Science Monthly Volume 79.djvu/204

200 "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 composed of very finely divided smoke-like particles arising from the volatilization, 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.

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 similar to, the phosphorescent light given out by insect larvæ, like the glowworm, 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 electrical discharges. The production of the phosphorescent light or afterglow 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 phosphorescence. 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 extracted, 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 determined, 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