ORIGIN OF LUMINOUS METEOR TRAINS 203 



It has been shown by the work of physicists, and particularly by 

 the recent researches of Professor Richardson, of Princeton Univer- 

 sity,, that when a body is very hot an immense number of negatively 

 charged corpuscles or ions are given forth from the body. Air con- 

 taining 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. More- 

 over, 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 dif- 

 ferent 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 per- 

 sistent 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 phos- 

 phorescence. 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 condi- 

 tion 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 appar- 

 ently 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 im- 

 portant role in all electrical phenomena. 



