THE ECLIPSE OF 1870. 71 



of vapour ; for, although it is exceedingly probable 

 that iron forms one of the chief constituents of the 

 coronal substance, yet, in the first place, we have no 

 reason for believing that a degree of heat intense 

 enough to vaporize iron would exist where we see the 

 corona ; and, in the second, other elements must also 

 be present in the coronal substance, and they also would 

 be vaporized, whereas we find none of the lines due to 

 other known elements. 



The idea suggested by Professor Young and others 

 seems more likely to be the correct explanation of the 

 matter. For bizarre and fanciful as the idea may seem 

 that the corona is a perpetual solar aurora, it must 

 not be forgotten that General Sabine and Dr. Stewart 

 propounded, some years since, in explanation of known 

 terrestrial phenomena, the theory that the coloured 

 prominences are solar auroras. This idea has been 

 shown, indeed, to be erroneous, but the reasoning on 

 which it was based was sufficiently sound, and the 

 observed facts would be equally well explained by 

 supposing the corona, instead of the prominences, to 

 form a perpetual solar aurora. 



When we remember that the zodiacal light a 

 phenomenon which holds a position midway between 

 the terrestrial aurora and the solar corona has been 

 shown to give a spectrum closely resembling both the 

 auroral and the coronal spectra, the idea does certainly 

 seem encouraged that all three phenomena are inti- 

 mately associated. We might thus not unreasonably 

 regard the zodiacal light as the outer and very much 



