BOOK XV. Aurora We now proceed to consider other igneous phenomena, none of which possess the dreadful energies of thunder. The aurora borealis, so often celebrated by Ossian, presents a spectacle equally magnificent and astonishing. In these blood-stained and fiery meteors, what poet is there who could not discover the shades of warriors, who, once conquerors of the earth, and now rulers of the air, stoop from the clouds to behold the combats of their posterity? Are not those pale and soft lights the daughters of heroes who, cut down in the early bloom of their beauty, now float upon the wings of the wind? We hear their sighs, and the rustling of their resplendent robes; and we see, rising towards the zenith, and assembling on every side the luminous columns of the wandering palace of departed spirits. But we must leave to the painter and the poet the delineation of such enchanted scenes, and confine ourselves to a more rigorous and sober description.* The aurora borealis, in our European countries, uniformly appears in the north, and inclining generally a little to the west. In Greenland, we sometimes perceive this phenomenon towards the south. In the other hemisphere it is observed, but with a feeble lustre, in the direction of the south pole. The ancients appear to have described it under the name of lamps, or burning torches, and other designations. In our age, it is a rare occurrence in the temperate zone, and becomes common only towards the 60th degree of latitude. It commences generally three or four hours after sunset, and is preceded by a sombre cloud, nearly resembling the segment of circle, of which the horizon forms the cord. This segment, seen at Upsal, for example, is of a deep black, whilst in Lapland it appears greyish, or even becomes almost invisible. Its circumference very soon borders upon a whitish light, which sometimes appears gently to expire. Most frequently the cloudy segment opens in chinks, whence issues streams and rays of light, of a yellow, a rose, * Mairan, Traité des Aurores Boréales, p. 115. Bergmaun, Geog. Phys. ii. 96-104. XV. a purple, or a sea-green colour. A general movement agi- BOOK tates all the cloudy and enlightened space; rays becoming more and more bright, shoot across each other like lightning flashing in the midst of effulgent splendour; by degrees, there is formed in the zenith a luminous crown, which seems to be the central point of all the motions of the luminous matter. After having occupied, for the space of an hour or two, almost the whole expanse of the heavens, the phenomenon contracts itself at first on the southern side, afterwards on the west and the east, and finally disappears towards the north. The rising sun invariably extinguishes these rival luminaries. The further we remove from the pole, the less distinctly do we perceive these different appearances of the aurora borealis. It generally appears in France, only like a light not much elevated above the horizon. Philosophers have given different explanations of these phenomena. That which Professor Libes lately proposed, appears to unite simplicity to probability.* This philosopher lays down the six following principles: 1st, If the electric spark be transmitted through a mixture of azotic and oxygen gas, the result is the production of nitric and nitrous acids, or of nitrous gas, according to the proportion subsisting between the oxygen and azote which compose the mixture. 2d, Nitric acid acquires additional colour and volatility from being exposed to the sun. Scheele first observed this phenomenon, and M. Libes has made frequent observations on the subject. Having placed a glass receiver over a saucer containing nitric acid, and having exposed the whole to the sun, he observed that the acid became coloured, and that the receiver was filled with reddish-looking vapours, which remained in it a long time, diffusing a lustre similar to that of the aurora borealis. 3d, In phials containing nitrous acid, we always perceive above the acid a very red and very volatile vapour, which never condenses. Traité de Physique, par Libes, tome iii. p. 309. XV. BOOK 4th, Nitrous gas, when in contact with atmospherical air, emits deep red vapours, which fly away into the atmosphere. 5th, The hydrogen gas which is disengaged from the surface of the globe, occupies, in the higher regions of the atmosphere, a place marked out by its specific gravity. 6th, The solar heat has but little influence in the polar regions. In reflecting upon these principles, it is easy to perceive, that the production of hydrogen gas being extremely scanty, if at all occurring, in the polar regions, the electric fluid in flowing back from the equator towards the pole, must pass through a space occupied only by a mixture of azotic and oxygen gas; that the electric spark fixes and combines these two gases; that this combination ought to cause the production of nitrous acid, or of nitrous gas, according to the proportion which prevails between the two component gases; and that the gas and the nitric acids thus formed, should exhale red and volatile vapours, which give rise to the phenomena of the aurora borealis. If these phenomena do not occur in the temperate zones, the reason is, that in their strongly heated atmosphere, there is always to be found a mixture of oxygen and hydrogen gas, which attracts the electric spark, thus occasioning thunder and lightning. These two phenomena are not known in the polar regions, from their deficiency in hydrogen gas. The observations of the elder Gmelin, in his travels in Siberia, tend to confirm the opinion of M. Libes. As we advance into eastern Siberia, the aurora borealis becomes more frequent and more dazzling; and it appears that the frozen regions, between Jenissa and Cape Behring, are the native country of these phenomena. Now, that is exactly the quarter of the old continent where the cold is most intense, and consequently where least hydrogen is disengaged. This explanation, however, does not account for several circumstances of the phenomenon; amongst others, for the blackish segment which forms the base. We must wait until enlightened observers have applied the principles on which it is founded to a certain number of these pheno mena. We would ask these observers, whether the aurora borealis might not be in some measure the effect of one of those optical illusions of which we have been speaking, viz. the mirage? and whether the circular segment be not the image of the terrestrial globe itself, repeated in the nocturnal atmosphere? And then the luminous rays which appear to issue from this cloudy segment, would in fact issue from the earth itself. This is a solution which we submit to the examination of philosophers. BOOK XV. Other igneous phenomena recall us towards the surface of the earth. We know that animal substances, in a state of putrefaction, always emit phosphorus, which, taking fire from the contact of the atmosphere, produces light and wandering flames. Such is probably the origin of those ignes Ignis Fafatui, which flutter at night over church-yards and fields of tuus. battle, and which have given rise to pretended apparitions of spirits in churches where it is the pernicious custom to accumulate the remains of the dead. Hydrogen gas is often combined with phosphorus; this mixture is not fit for respiration; it quickly suffocates. This is also a circumstance which seems to enter into many histories of spirits and apparitions, namely, the inflammable air disengaged from marshes, which is hydrogen gas mixed with azotic. The air which inflames on the surface of certain springs, known by the name of burning fountains, arises from the presence Burning of hydro-phosphoric gas, or as it is otherwise termed, phos- fountains, phureted hydrogen. One of these springs is met with in the parish of Saint Bartholomew, in the department of the Isere. The disengagement of inflammable gas during the summer, is so considerable, that we continually see a flame seven feet high; and travellers, when they first behold it, imagine that the whole village is on fire.* Ignes fatui, arising from the development of phosphureted hydrogen, are necessarily soon extinguished; a suc Bouvier, Journal de la Médicine Eelairées par les Sciences Physiques, tom. iii. No. 8. XV. BOOK cession of these fires will therefore appear to the spectator to be one single flame, which moves with rapidity from place to place, when we attempt to approach it. The air, driven on before us, forces the lambent flame to recede.* There are other similar fires, which appear to be immoveable when viewed from a particular spot. There was one near Rettwick, in Sweden, which was supposed to issue from the mouth of a dragon that kept watch over some hidden treasures. A simple miner ventured to sink a shaft, which discovered a cavern filled with sulphurous pyrites and petroleum, the combustion of which had occasioned the phenomenon.† Falling stars. Fire of St. Falling, or shooting stars, are appearances every where observed. They are probably the effect of hydrogen gas more or less sulphureted, for phosphorus is too rapidly inflamed, by the contact of the air, to be capable of reaching so great an elevation. What seems to prove the hydrosulphureted origin of these meteors, is the nature of the circumstances by which they are accompanied. These fires, we are assured, often fall to the ground: and nothing is found at the place of their fall but a fetid, glutinous matter, of a whitish colour, bordering upon yellow. Now, we know that sulphureted hydrogen gas holds sulphur to it in solution; that the hydrogen and the sulphur does not burn at the same moment; that, consequently, the sulphurous part may be precipitated to the earth, whilst the hydrogen, mixed with the oxygen of the air, is kindled by a slight electric spark. The Fire of Saint Elmo is generally considered as an accumulation of electric matter around a point, which moves in the air. This fire, then, may be expected to appear frequently at the top of the masts of a vessel sailing along with rapidity. The ancients observed this phenomenon. These fires, when seen in pairs, were called Castor and Pol * Derham, Philosoph. Transact. No. 411. + Mémoires de l'Académie de Stockholm, 1740. Bergmann, Géog. Phys. ii. 80. |