4(3 ON THE RECENT SECULAR PERIOD 



period, in the neighborhood of each other, and then gradually separating, not return 

 to the same relative position until after a C}*cle of years. Indeed, it would not be 

 difficult to give to the nebulous body such an orbit as to make it always remain, as 

 is the fact with the zodiacal light, near to the plane of the earth's orbit, and to 

 assign to it such an extent as to render it practicable for extreme portions of it at any 

 time to enter the earth's atmosphere, and produce auroral appearances, on a dimi- 

 nished scale at least, while the peculiar positions relative to the earth, which occur 

 only after certain cycles, are essential in order to exhibit the higher forms of the 

 phenomenon. 



The foregoing considerations proceed no further than to show simply that the 

 material out of which the aurora borealis is formed, is derived from the planetary 

 spaces ; in other words, that its origin is cosmical and not terrestrial ; and this is 

 the main point we have in view, and this conclusion will not be affected by any 

 opinions we may entertain respecting the nature of the material itself, or respecting 

 its mode of existence before it came into the earth's atmosphere. Still, it may be 

 important to the cause of truth, to determine, if we can, what sort of matter the 

 auroral vapor consists of, in what form it existed in space, and what relation it 

 sustained to the solar system. 



First, what is the nature of the auroral vapor itself? It is exceedingly light and 

 rare, since the stars are seen through it; those of the first magnitude, and the pla- 

 nets with no perceptible diminution of lustre; and though small stars are rendered 

 invisible by it, yet it is difficult to decide whether this is a real obscuration, or 

 merely the loss of contrast. There is often, however, especially in the earlier 

 stages of the exhibition, an accumulation of auroral vapor near the northern horizon, 

 which is so dense as entirely to obscure the stars. 



With respect to the specific nature of the matter itself, the idea was long since 

 advanced, both by Dalton and Biot, that it is metallic. Dalton goes further, and 

 considers it ferruginous. This opinion has had but few advocates, and some have 

 thought it not deserving of the least attention. 1 But what inconsistency is there 

 in supposing it ferruginous vapor? Iron, when sublimed by a high heat, as that of 

 the oxyhydrogen blowpipe, assumes the form of an exceedingly attenuated vapor. 

 Moreover, it is a striking fact that the matter which comes to us from the regions 

 of space is, to a great extent, ferruginous, as in the case of meteoric stones; nor does 

 there seem to me any great improbability in supposing that the matter which com- 

 poses the tails of comets is iron, in a state of extreme diffusion. Although iron is 

 not the only substance susceptible of magnetism, yet an unknown substance exhi- 

 biting this property is so commonly ferruginous that, without special evidence to 

 the contrary, it is inferred to be so. Now here is a substance which arranges itself 

 in obedience to all the laws of magnetism : conforming to them in respect to the 

 magnetic pole, the magnetic meridian, and the pole of the dipping needle, and pro- 

 ducing the most striking changes in the motions of the compass needle. The inference 

 that the substance is iron is so natural, that it would require special evidence to 

 the contrary to assume that it was anything else. 



Becqucrel, Vol. VI. 



