240 Causes of (he Aurora Bar calls. 



pothesis of light reflected from the arctic ices is untenable, it will be 

 almost superfluous to reply to the doctrine that it proceeds from par- 

 ticles thrown off from the atmosphere of the sun ; or that sulphure- 

 ous fumes are ignited in the air ; for " if nature accomplishes her 

 wonders by varying the combination of a few simple means," we need 

 not go to other spheres for the materials which produce the phenome- 

 na of ours, nor seek in the most occult causes for the solution of such 

 as are not understood. It is not conformable to the optical phenome- 

 na of the celestial bodies, nor to the known properties of light, that a 

 substance inherently luminous, should exist in the atmosphere, in a 

 form so fugitive and irregular as the aurora, not regulated in its re- 

 turns by periodical laws, but strangely, and almost paradoxically uni- 

 form in its place of appearance, and its singular varieties. 



These anomalies may he explained by considering the aurora as a 

 m.eteorological 'phenomenon, produced by a gaseous medium, illumina- 

 ted by the refracied and reflected rays of solar light. 



A medium possessing a higher refractive power than the mixed 

 oxygen and nitrogen which compose the atmosphere, may exist in it 

 at an altitude sufficient to refract over the particles of light after the 

 direct rays of the sun have so far left the hemisphere, that they are 

 no longer refracted by the atmosphere at such an angle as to be i-en- 

 dered visible in a given latitude. This medium must be presumed to 

 be a gaseous combination, forming an embodied and impressible trans- 

 parent vapor, reflecting the light from mass to mass like a series of mir- 

 rors, which by refraction becomes visible in the darkened hemisphere. 



I shall first examine the probability of the existence of such a va- 

 por, and what may be supposed to be its properties ; and secondly, 

 the reasons for attributing its illumination to the solar light. 



I. Vapor the basis of the Aurora. 



It must first be assumed that vapor is the basis of the aurora, and 

 if the assumption coincides with ascertained laws, it may perhaps be 

 entitled to a degree of confidence, although the sources from which 

 it originates remain unknown. It probably results from a union of 

 elements previously existing in simple forms, combined by the agen- 

 cy of electricity, caloric and light, those ruling powers of the atmos- 

 phere. 



A medium, consisting of atmospheric air, holding equally diffused 

 through it more than a common portion of water, acquires a great 

 increase of transparency, provided the watery vapor is not condensed 



