THE AURORA. 13 



mistaking. It teaches us that the light of the aurora 

 is due to luminous vapour, and we may conclude, with 

 every appearance of probability, that the luminosity of 

 the vapour is due to the passage of electric discharges 

 through it. It is, however, possible that the position 

 of the bright line may be due to the character of the 

 particles between which the discharge takes place. 



But the view we are to take must depend upon the 

 position of the line. Here a difficulty presents itself. 

 There is no known terrestrial element whose spectrum 

 has a bright line precisely in the position of the line in 

 the auroral spectrum. And mere proximity has no 

 significance whatever in spectroscopic analysis. Two 

 elements differing as much from each other in character 

 as iron and hydrogen may have lines so closely approxi- 

 mating in position that only the most powerful spectro- 

 scope can indicate the difference. So that when 

 Angstrom remarks that the bright line he has seen lies 

 slightly to the left of a well-known group of lines be- 

 longing to the metal calcium (the principal ingredient 

 of common chalk), we are by no means to infer that he 

 supposes the substance which causes the presence of the 

 bright line has any resemblance to that element. Until 

 we can find an element which has a bright line in its 

 spectrum absolutely coincident with the bright line der 

 tected by Angstrom in the spectrum of the aurora,* all 

 speculation as to the real nature of the vapour in which 



* Other green lines have since been discovered in the auroral 

 spectrum ; and occasionally a red line is seen. 



