OF THE AURORA BOREALIS. 47 



If it be held as proved that the light of the aurora is direct and not reflected light, 

 as experiments on polarization made by Arago would indicate, it must be either 

 self-luminous, or be developed in the passage of the auroral vapor through the 

 atmosphere. The latter case is plainly conceivable; for a vaporous matter passing 

 through even the upper regions of the atmosphere, with the immense velocity which 

 the auroral vapor is known to have, would become luminous by the condensation 

 of the air before it, as tinder is set on fire in the air match. A wad, shot from a 

 cannon with such a velocity, would take fire by the heat elicited by condensation ; 

 or if it w T ere constituted of some filmy, incombustible substance, that would become 

 red-hot. 



Although I am unable to find any evidence that electricity is the cause of the 

 aurora borealis, yet that certain of the local appearances it exhibits occasionally 

 may be owing to electricity, is not incompatible with the views I entertain of its 

 origin. Among the various phenomena, there is one which has much resemblance 

 at times to the passage of electricity through rarefied air. I allude to the momen- 

 tary flashes which sometimes constitute a striking feature in auroras of the first class, 

 as was the case in that of April 6, 1848. Respecting this, I find recorded in my 

 note-book the following remark : " The appearances of the luminous flashes more 

 strikingly resemble those of the auroral tube [in which a strong electric spark is 

 transmitted through a long glass tube containing rarefied air] than anything of the 

 kind I ever witnessed before. I had recently been experimenting with a very large 

 auroral tube, and a powerful electric machine, and was, therefore, peculiarly fitted 

 to make the comparison. At 2 h. 20 m. the luminous undulations extended east- 

 ward to Aquila, covering the Swan and the Lyre, and the appearances much resem- 

 bled those of the auroral tube." In the exhibition of February 20, of the same 

 year, at about 4 o'clock in the morning, similar appearances were seen, which 

 possibly might, and perhaps did, result from the passage of electricity through the 

 upper regions of the atmosphere ; but I regard these appearances rather as one of 

 the effects than as the grand cause of the aurora borealis. They may arise from 

 a disturbance of the electric equilibrium in the auroral columns, but cannot be 

 regarded as producing the columns themselves. 



The term " nebulous matter" is not well defined, but is applied, in general, to a 

 kind of matter found in the celestial spaces in a state of extreme diffusion, compos- 

 ing a cloud of dust lighter, to any extent, than atmospheric air. Of such a material 

 we conceive to be constituted not only the nebuloB proper, but also comets' tails, and 

 the zodiacal light; and of such a material we have evidence that the aurora borealis 

 is constituted. We deem it, therefore, allowable to call this material nebulous 

 matter, and to denominate a collection of it existing in space a nebulous body. 

 The number of separate nebula? that have been discovered among the heavenly 

 bodies, and the vast extent which they severally occupy, and the immense volume 

 which comets' tails sometimes fill as well as the zodiacal light, are facts which 

 strongly indicate the great prevalence of nebulous matter in the celestial spaces ; 

 nor would it seem a violent presumption to suppose that more or less of this matter 

 may be diffused in the planetary spaces, even were we not assured of its presence 

 there by the zodiacal light, and its descent in the form of meteoric showers. 



