Page:Popular Science Monthly Volume 79.djvu/207

Rh It has been shown by the work of physicists, and particularly by the recent researches of Professor Richardson, of Princeton University, that when a body is very hot an immense number of negatively charged corpuscles or ions are given forth from the body. Air containing free ions becomes a conductor of electricity, hence we have in a meteor rushing through the atmosphere a condition extremely like a very long electrical discharge tube containing a gas at low pressure. The passage of the burning meteor through the atmosphere must form a column of highly ionized air thirty or forty miles in length. Moreover, at a certain altitude, corresponding to a pressure near two-tenths of a millimeter of mercury pressure, or about from one two-thousandth to one four-thousandth of one atmosphere pressure, the conditions are precisely right for the formation of phosphorescence in the meteor track. If at different levels in the upper atmosphere the air is at different electrical potentials, discharges must certainly occur in the meteor's track, and the burning meteoric mass thus may readily play the part of an incandescent electrode in a very long discharge tube, the column of ionized air being a ready conductor of electricity. When the meteor nucleus has been consumed, all that remains visible in the dark sky is the body of phosphorescent gas in the part of the track where the gas pressure conditions were correct for the formation of the persistent glow.

Under the above circumstances, it is not surprising that luminous effects are produced in the meteor train zone of the upper atmosphere where the density of the air is apparently the same as that at which luminous effects can be produced in vacuum tubes in the laboratory by even very weak electrical discharges.

It is not certain that electrical discharges takes place in the meteor track, but they may not even be essential for the formation of the phosphorescence. It has already been pointed out that the flight of a meteor through the atmosphere at the rate of twenty to thirty miles a second produces an exceedingly high temperature immediately about the meteorite, probably a matter of many thousands of degrees. The air thus heated and highly ionized by the burning meteorite, a condition which is sure to occur, may readily suffer chemical or physical changes in its composition which on gradually reverting to its original state gives out a long-enduring phosphorescent glow, just as is apparently the case in the formation of gas phosphorescence. Thus it is not unlikely that the production of the phosphorescent light of the meteor train is connected directly with the highly ionized state of the air and that this condition is produced by the outpouring from the intensely heated meteor of electrons, those electrically charged minute particles discovered by Sir J. J. Thomson which are supposed to play the important rôle in all electrical phenomena.