256 PHYSICS. 
Many philosophers considered it to be an optical phenomenon, produced by 
the reflection of solar light from the polar snow and ice, against the concave 
surface of the higher atmospheric strata, whence the rays were a second 
time reflected to the observer. According to De Mairan, it is produced by 
the coming of the earth at stated periods into the atmosphere of the sun, 
supposed to extend to the earth’s orbit, an entirely false supposition, as 
shown by what is said of the dimensions of this atmosphere on page 237. 
According to Euler, it consists of solar rays, which, by their violent impact 
against the finer particles of the atmosphere, carry these to a height of more 
than 4000 miles, the height at which he believed them to exist. According 
to Kirwan. with whom Volta and Parrot agree, the northern light is pro- 
duced by hydrogen gas generated by decomposition on the earth, volcanoes, 
and other causes, .especially near the equator, and carried by aerial 
currents towards the polar regions, there to become inflamed. Before this, 
however, Halley considered it to be produced by the influence of magnetic 
currents from the two poles, and his hypothesis, in a somewhat modified 
form, is at present maintained by the most eminent investigators. 
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