[182] “Brilliant and active coruscations of the Aurora Borealis,” says Captain Back, “when seen through a hazy atmosphere, and exhibiting the prismatic colours, almost invariably affected the needle. On the contrary, a very bright Aurora, though attended by motion, and even tinged with a dullish red and a yellow in a clear blue sky, seldom produced any sensible change, beyond, at the most, a tremulous motion. A dense haze or fog, in conjunction with an active Aurora, seemed uniformly favourable to the disturbance of the needle, and a low temperature was favourable to brilliant and active coruscations. On no occasion during two winters was any sound heard to accompany the motions. The Aurora was frequently seen at twilight, and as often to the eastward as to the westward; clouds, also, were often perceived in the day-time, in form and disposition very much resembling the Aurora.”—Narrative of the Arctic Land Expedition.
[183] Faraday: On the Diamagnetic character of Flame and Gases.
[184] “The Aurora Borealis is certainly in some measure a magnetical phenomenon; and if iron were the only substance capable of exhibiting magnetic effects, it would follow that some ferruginous particles must exist in the upper regions of the atmosphere. The light usually attending this magnetical meteor may possibly be derived from electricity, which may be the immediate cause of a change in the distribution of the magnetic fluid, contained in the ferruginous vapours which are imagined to float in the air.”—Lecture on Magnetism: Young’s Lectures on Natural Philosophy, p. 533.
[185] On the supposed influence of Magnetism and Chemical Action; by Robert Hunt.—Philosophical Magazine, vol. xxxii. No. 215, 1849.
[186] Those bodies which are attracted by a magnet, as iron is, are called magnetic bodies. Those which are, on the contrary, repelled by the same power, are termed diamagnetic bodies. On these Dr. Faraday remarks:—“Of the substances which compose the crust of the earth, by far the greater portion belong to the diamagnetic class; and though ferruginous and other magnetic matters, being more energetic in their action, are more striking in their phenomena, we should be hasty in assuming that, therefore, they over rule entirely the effect of the former bodies. As regards the ocean, lakes, rivers, and the atmosphere, they will exert their peculiar effect almost uninfluenced by any magnetic matter in them, and as respects the rocks and mountains, their diamagnetic influence is perhaps greater than might be anticipated. I mentioned that by adjusting water and a salt of iron together, I obtained a solution inactive in air; that is, by a due association of the forces of a body, from each class, water and a salt of iron, the magnetic force of the latter was entirely counteracted by the diamagnetic force of the former, and the mixture was neither attracted nor repelled: To produce this effect, it required that more than 48·6 grains of crystallised protosulphate of iron should be added to ten cubic inches of water (for these proportions gave a solution which would set equatorially), a quantity so large, that I was greatly astonished on observing the power of the water to overcome it. It is not, therefore, at all unlikely that many of the masses which form the crust of this our globe, may have an excess of diamagnetic power, and act accordingly.”—On new magnetic actions, and on the magnetic condition of all matter; by Michael Faraday, D.C.L., F.R.S., &c.—Philosophical Transactions, Jan. 1846, vol. cxxxvii. p. 41.
[187] Ibid.
[188] On the Diamagnetic conditions of Flame and Gases, by Michael Faraday. F.R.S.; and On the motions presented by Flame when under Electro-Magnetic Influence, by Professor Zantedeschi.—Philosophical Magazine, 1847, pp. 401–421.
[189] On Diamagnetism; by Professor Plücker, of Bonn.—Philosophical Magazine, July, 1848.
[190] For a detailed account of the experiments of Faraday, Plücker, Becquerel, Tyndale, and Knoblauch, see De La Rive’s Treatise on Electricity in Theory and Practice.
[191] A few examples taken from Dr. Faraday’s paper will show this:—