218 Mr Potter on the Aurora Borealis. 



But if we consider this light as arising from the mercurial 

 vapour, it will be asked, from whence could such a metallic 

 vapour arrive at the highest regions of the atmosphere ? In 

 answer, I would say, tell me whence come the meteoric stones, 

 and I can give as likely an origin for such a metallic vapour. 

 He would be very hardy at the present day, and I think little 

 versed in chemistry, who would venture to assert the non-ex- 

 istence of metals in such a state, even at low temperatures. 

 We know that aqueous vapours exist at temperatures below 

 the freezing point, and we see that many metals, as zinc, arse- 

 nic, mercury, &c. are easily distilled. We find also, by the 

 evidence of our sense of smelling, though so much inferior in 

 the human species to what it is in many of the lower animals, 

 an impression of the existence of a metallic vapour, when we 

 are in places where metals are worked. But for a ready ex- 

 periment, let any one take a piece of iron or steel, and rub away 

 the coating of varnish or oxide which may be on its surface 

 upon a rough stone or file, on removing it to a short distance, 

 he will readily perceive, if his olfactory sense be not particu- 

 larly dull, a peculiar metallic odour, which cannot arise other- 

 wise than from the iron, and which is only perceived in its 

 vicinity. 



I must acknowledge that there is quite as much difficulty 

 in accounting for metals in this state floating in space, as there 

 is in accounting for the origin of aerolites. But the phenome- 

 na of the aurora are undoubtedly, in my opinion, best ac- 

 counted for, on the view that the metals which form so prin- 

 cipal and characteristic a portion of meteoric stones exist also 

 in a state of vapour, moving in space ; which, arriving within 

 the influence of the earth's attraction, are brought into a state 

 of commixture with the higher portions of the atmosphere. 



The forms of the meteor, the peculiar light, and the posi- 

 tions with respect to the magnetic tendency, must then result 

 from the necessary effect of the earth's electrical or magnetic 

 state. 



The rarity of the matter which causes the aurora is quite in 

 agreement with the tenuity which must exist in a metallic 

 vapour at exceedingly low temperatures. For we find a mass 

 of forty to fifty miles in depth, hardly, in a telescope, affect 



