Atmospheric Origin^of_ the Aurora, ^x. 151 



spread more rapidly than a small one, advancing like a wave, 

 overtaking and absorbing those waves which had begun to spread 

 from a smaller nucleus. This induction, or the influence of a 

 crystalline mass, in disposing particles and small crystals which 

 are in its vicinity, but at some visible distance from it, to unite 

 with each other, was still more evident from observing on the 

 screen the existence and motions of scattered clusters composing 

 a darkly dotted border or penumbra, skirting the darker image of 

 the general crystalline mass already formed, and regularly advan- 

 cing before it across the screen. Perhaps we should hardly be 

 justified in calling such phenomena magnetic ; yet it would be 

 easy to show that these and many other phenomena exhibited by 

 microscopic crystals, are regulated by laws strikingly analogous 

 to those of magnetic induction.' 



The above phenomena may be shown with great distinctness 

 in tincture of camphor, sufficiently diluted to make the process 

 slow. 



If the electricity of crystallizing water is ever connected with 

 magnetism, it must be during the perfect crystallization in the 

 elevated regions of auroral action, where the circumstances are 

 favorable to the perfection both of the process and the products. 

 The rarity of the vapor there is favorable to a regular aggregation 

 of the molecules, and the cold is intense. During crystallization, 

 the temperature of the crystal might rise to 32°, by the evolution 

 of latent heat, and soon afterwards sinking perhaps 100°, to the 

 original temperature of the vapor. For such immense and instan- 

 taneous changes, a less elevation in the air is requisite in the 

 higher latitudes ; and there, it appears from observation, that the 

 aurora itself is less elevated. It is unnecessary to cite the numer- 

 ous authorities which exist, to prove the occasional lowness of the 

 aurora in high latitudes. Mr. Trevelyan observed, that in Faroe 

 and the Shetland islands, it was often seen not more than forty 

 or fifty feet above the sea, and learned, that in both countries it 

 is frequently heard. One person had perceived in it, when red, 

 an electrical smell.* 



In our latitude, the aurora is usually at great heights. On this 

 subject the author's views seem to have been misapprehended. 

 Some of the intimate connections which he has proved to exist, 



* Edinb. Pliilos. Jour, vii, 182, 



