their hulls meeting, according as the inverted image is above or below the other. Dr. Wollaston has shown that the production of these images is owing to the refraction of the rays through media of different densities. Looking along a red-hot poker at a distant object, two images of it were seen, one erect and the other inverted, arising from the change produced by the heat in the density of the air. A singular instance of lateral mirage was noticed upon the Lake of Geneva by MM. Jurine and Soret in the year 1818. A bark near Bellerive was seen approaching to the city by the left bank of the lake; and at the same time an image of the sails was observed above the water, which, instead of following the direction of the bark, separated from it, and appeared approaching by the right bank the image moving from east to west, and the bark from north to south. When the image separated from the vessel, it was of the same dimensions as the bark; but it diminished as it receded from it, so as to be reduced to one-half when the appearance ceased. This was a striking example of refraction, operating in a lateral as well as a vertical direction.

6. Ignis fatuus. This wandering meteor, known to the vulgar as the Will-o'-the-Wisp, has given rise to considerable speculation and controversy. Burying grounds, fields of battle, low meadows, valleys, and marshes, are its ordinary haunts. By some eminent naturalists, particularly Willoughby and Ray, it has been maintained to be only the shining of a great number of the male glow-worms in England, and the pyrausta in Italy, flying together-an opinion to which Mr. Kirby, the entomologist, inclines. The luminosities observed in several cases may have been due to this cause, but the true meteor of the marshes cannot thus be explained. We have but a few authentic notices of its appearance in this country of a recent date, probably owing to an extended system of drainage, and the careful cultivation of the soil. The following instance is abridged from the Entomological Magazine:-"Two travellers proceeding across the moors between Hexham and Alston, were startled, about ten o'clock at night, by the sudden appearance of a light, close to the road side, about the size of the hand, and of a well-defined oval form. The place was very wet, and the peat-moss had been dug out, leaving what are locally termed 'peat-pots,' which soon fill with water, nourishing a number of confervæ, and the various species of sphagnum, which are converted into peat. During the process of decomposition, these places give out large quantities of gas. The light was about three feet from the ground, hovering over the peat-pots, and it moved nearly parallel

with the road for about fifty yards, when it vanished, probably from the failure of the gas. The manner in which it disappeared was similar to that of a candle being blown out." The ignis fatuus has not become so strange in various continental districts as with We have the best account of it from Mr. Blesson, who examined it abroad with great care and diligence.

us.

"The first time," he states, "I saw the ignis fatuus, was in a valley, in the forest of Gorbitz, in the New Mark. This valley cuts deeply in compact loam, and is marshy on

its lower part. The water of the marsh is ferruginous, and covered with an iridescent crust. During the day, bubbles of air were seen rising from it, and in the night blue flames were observed shooting from and playing over its surface. As I suspected that there was some connection between these flames and the bubbles of air, I marked during the day-time the place where the latter rose up most abundantly, and repaired thither during the night; to my great joy I actually observed bluish-purple flames, and did not hesitate to approach them. On reaching the spot they retired, and I pursued them in vain; all attempts to examine them closely were ineffectual. Some days of very rainy weather prevented farther investigation, but afforded leisure for reflecting on their nature. I conjectured that the motion of the air, on my approaching the spot, forced forward the burning gas, and remarked that the flame burned darker when it was blown aside; hence I concluded that a continuous thin stream of inflammable air was formed by these bubbles, which, once inflamed, continued to burn, but which, owing to the paleness of the light of the flame, could not be observed during the day. On another day, in the twilight, I went again to the place, where I awaited the approach of night: the flames became gradually visible, but redder than formerly, thus showing that they burnt also during the day: I approached nearer, and they retired. Convinced that they would return again to the place of their origin when the agitation of the air ceased, I remained stationary and motionless, and observed them again gradually approach. As I could easily reach them, it occurred to me to attempt to light paper by means of them; but for some time I did not succeed in

this experiment, which I found was owing to my breathing. I therefore held my face from the flame, and also held a piece of cloth as a screen; on doing which I was able to singe paper, which became brown-coloured, and covered with a viscous moisture. I next used a narrow slip of paper, and enjoyed the pleasure of seeing it take fire. The gas was evidently inflammable, and not a phosphorescent luminous one, as some have maintained. But how do these lights originate? After some reflection, I resolved to make the experiment of extinguishing them. I followed the flame; I brought it so far from the marsh that probably the thread of connection, if I may so express myself, was broken, and it was extinguished. But scarcely a few minutes had elapsed when it was again renewed at its source (over the air-bubbles), without my being able to observe any transition from the neighbouring flames, many of which were burning in the valley. I repeated the experiment frequently, and always with success. The dawn approached, and the flames, which to me appeared to approach nearer to the earth, gradually disappeared. On the following evening I went to the spot and kindled a fire on the side of the valley, in order to have an opportunity of trying to inflame the gas. As on the evening before, I first extinguished the flame, and then hastened with a torch to the spot from which the gas bubbled up, when instantaneously a kind of explosion was heard, and a red light was seen over eight or nine square feet of the marsh, which diminished to a small blue flame, from two and a half to three feet in height, that continued to burn with an unsteady motion. It was therefore no longer doubtful that this ignis fatuus was caused by the evolution of inflammable gas from the marsh."

The ignis fatuus of the churchyard and the battle-field we may conclude to arise from the phosphuretted hydrogen emitted by animal matter in a state of putrefaction, which always spontaneously inflames upon contact with the oxygen of the atmosphere; and the flickering meteor of the marsh may be referred to the carburetted hydrogen, formed by the decomposition of vegetable matter in stagnant water, ignited by a discharge of the electric fluid, or by contact with some substance in a state of combustion. This wandering light has often been a source of terror to the ignorant, and has frequently seduced the benighted traveller into dangerous bogs and quagmires, under the impression that it proceeded from some human habitation.

"Drear is the state of the benighted wretch,

Who then, bewilder'd, wanders through the dark,
Full of pale fancies, and chimeras huge;
Nor visited by one directive ray,
From cottage streaming, or from airy hall.
Perhaps impatient as he stumbles on,
Struck from the root of slimy rushes, blue,
The wild-fire scatters round, or gather'd trails,
A length of flame deceitful o'er the moss:
Whither decoy'd by the fantastic blaze,
Now lost and now renew'd, he sinks absorb'd,

Rider and horse, amid the miry gulf:

While still, from day to day, his pining wife
And plaintive children his return await,
In wild conjecture lost."

The production of inflammable gases is one of the processes in constant action in the great laboratory of nature, and extraordinary disengagements of combustible elements occur, though we are quite ignorant of the cause. In the middle of the last century the snow on the summit of the Apennines appeared enveloped in sheets of flame; and in the winter of 1693 hay-ricks in Wales were set on fire by burning gaseous exhalations.

CHAPTER XVII.

GEOGRAPHICAL DISTRIBUTION OF PLANTS.

AVING reviewed the arrangements and phenomena of inanimate nature, we proceed to consider the globe as the habitation of innumerable organised beings, beginning with the lowest division in the world of living existence, the diversified vegetable forms which clothe the surface of the earth, adorn its scenery, and contribute to the sustenance of its animal and human races. The productions of the vegetable kingdom are among the most useful and interesting objects we contemplate. They are associated with the earliest and some of the purest pleasures of mankind; for every one will vividly recollect the delight experienced in his childhood by the appearance of the harbingers of the vernal season-the flowers of the snowdrop, crocus, primrose, and violet, peeping up above the greensward, or from the hedgerows, proclaiming, in an obvious and impressive manner, "Lo, the winter is past, the rain is over and gone, the flowers appear upon the earth, the time of the singing of birds is come." The cultivated flora of the garden, and the wild flora of the field, are among our first and best instructors, conveying, by their external configuration, lessons of purity and of grace to the mind in the age of its awakening susceptibilities. This is a moral and intellectual discipline, silent and unostentatious in its process, but of great importance in its effect as a source of valuable directive influence to the thoughts and feelings. But to man, in mature life, the larger plants and timber trees are essential. His existence and civilisation depend upon them. They furnish, with unbounded prodigality, the food which satiates his hunger and gratifies his taste; supply many of the medicines that allay his sickness; afford him materials for an habitation; yield the means of transporting himself and his property across the land, and of accomplishing the passage of the ocean; besides being the chief ornament of his walks during the period of their growth.

It belongs to Botany, Physiology, and Agricultural Chemistry, to investigate the structure of plants; to unfold the riches of the vegetable kingdom with its different organisations, and the means by which their development and fructification are secured. The department of the physical geographer is simply to notice the general disposition of the vegetable tribes, and the circumstances which seem to regulate their distribution. We have no more striking evidence of contrivance than in the wide dispersion of vegetable life, and in the different conditions under which it exists, at the extremes of terrestrial elevation and depression, of cold and heat, of light and shade, of solidity and solution. Forests of beech wave in the Himalaya at a greater height than that reached by the Finsterhorn of the Alps, or more than fourteen thousand feet; and there also shrubs flourish upon sites which are above the altitude of the hoary-headed Mont Blanc, the monarch of

the European mountains. On the other hand, the bottom of the sea is clothed with an

endless variety of green, red, and purple alge; and, in the dark caverns of the ocean, the vine-leaved fucus produces its enormous fronds, with a hue as green as that of grass. Melville Island, with its long dreary night, and nine or ten months of rigid winter, has in various places an abundance of moss, lichen, grass, saxifrage, the dwarf-willow, poppy, and sorrel; and Captain Parry found a ranunculus in full flower, in a sheltered spot, during the second week of June. In a diametrically opposite condition, some plants of the confervæ tribe thrive, living in hot springs, where the temperature is that of boiling water; and, at the commencement of the seventeenth century, a shrubbery flourished in the crater of the Vesuvian furnace. The rich black loam of the plains sustains luxuriantly an appropriate vegetation; while creeping plants derive sustenance from the hardest rocks, and alternate beautifully with the naked projections of mountain masses. Within the tropics magnificent trees grow up under the direct action of the solar beams; and in dens and caves of the earth, which have never been visited with the light of day, vegetable life defies the perpetual darkness. In the great cavern of the Guacharo, to the south-east of Cumana, Humboldt beheld with astonishment the progress of subterranean vegetation, after having passed a considerable distance beyond the point to which the daylight penetrates. The seeds which the birds carry into the cave to feed their young, spring up wherever they fix in the mould that covers the calcareous incrustations; and blanched stalks were noticed, which had risen to the height of two feet, with some halfformed leaves. During the visit of the traveller, these traces of organisation amid darkness forcibly excited the curiosity of the native Indians, who examined them with the aid of their torches in silent meditation and fear, as if the subterraneous vegetables, pale and disfigured, had been phantoms banished from the face of the earth. To Humboldt, the scene recalled one of the happiest periods of his earliest youth-a long abode in the mines of Freiburg, where he had found plants growing in the complete darkness, green as well as blanched. The entire failure of moisture seems to be the only insurmountable obstacle to the growth of plants; for the sandy desert will "rejoice and blossom as the rose" wherever a very scanty supply of humidity gains access to it. De Candolle makes