The Poetry of Science; or, Studies of the Physical Phenomena of Nature

by Robert Hunt

But doth suffer a sea change

Into something rich and strange,”

he painted, with considerable correctness, the chemical changes by which decomposing animal matter is replaced by a siliceous or calcareous formation.

But the gifted have the power of looking through the veil of nature, and they have revelations more wonderful than even those of the philosopher, who evokes them by perpetual toil and brain-racking struggle with the ever-changing elements around him.

The mysteries of flowers have ever been the charm of the poet’s song. Imagination has invested them with a magic influence, and fancy has almost regarded them as spiritual things. In contemplating their surpassing loveliness, the mind of every observer is improved, and the sentiments which they inspire, by their mere external elegance, are great and good. But in examining the real mysteries of their conditions, their physical phenomena, the relations in which they stand to the animal world, “stealing and giving odours” in the marvellous interchange of carbonic acid and ammonia for the soul-inspiring oxygen—all speaking of the powers of some unseen, in-dwelling principle, directed by a supreme ruler—the philosopher finds subjects for deep and soul-trying contemplation. Such studies lift the mind into the truly sublime of nature. The poet’s dream is the dim reflection of a distant star: the philosopher’s revelation is a strong telescopic examination of its features. One is the mere echo of the remote whisper of nature’s voice in the dim twilight; the other is the swelling music of the harp of Memnon, awakened by the sun of truth, newly risen from the night of ignorance.

To return from our long, but somewhat natural digression, to a consideration of the chemical phenomena connected with the atmosphere, and its curious and important element, nitrogen, we must first examine the evidence we have of the condition of the air itself.

The mean pressure exerted upon the surface of the earth, as indicated by the barometer, is equal to a column of mercury thirty inches high; that is, the column of air from the surface of the ocean to its highest limits exactly balances that quantity of mercury. If our tube of mercury had the area of one square inch, the columns would weigh fifteen pounds, which represents a pressure of fifteen pounds upon every square inch of the earth’s surface. This pressure, it must be remembered, is the compound weight of the gaseous envelope, and the elastic force of the aqueous vapour contained in it.[225] If the atmosphere were of uniform condition, its height, as inferred from the barometer, would be about five miles and a half. The density of the air, however, diminishes with the pressure upon it, so that at the height of 11,556 feet, the atmosphere is of half density; or one volume of air, as taken at the surface of the earth, is expanded into two at that height. Thus the weight is continually diminishing; but this is regularly opposed by the decreasing temperature, which diminishes the rate of about one degree for every 352 feet of ascent, although in all probability it is less rapid at great distances from the earth.

It has been calculated from certain phenomena of refraction, that our atmosphere must extend to about forty miles from the surface of the earth. It may, in a state of extreme tenuity, extend still further; but it is probable that the intense cold produced by rarefaction sets limits to any extension much beyond this elevation.

The uses of the atmosphere are many. It is the medium for regulating the dispersion of watery vapours over the earth. If there were no atmosphere, and that, as now, the equatorial climes were hot and the poles cold, evaporation would be continually going on at the equator, and condensation in the colder regions. The sky of the tropical climes would be perpetually cloudless, whilst in the temperate and arctic zones we should have constant rain and snow. By having a gaseous atmosphere, a more uniform state of things is produced; the vapours arising from the earth become intimately mixed with the air, and are borne by it over large tracts of country, and only precipitated when they enter some stratum much colder than that which involves them. There are opposite tendencies in an atmosphere of air and one of vapour. The air circulates from the colder to the warmer parts, and the vapour from the warmer to the colder regions; and as the currents of the air, from the distribution of land and sea—the land, from its low conducting power, being more quickly heated than the sea—are very complicated, and as some force is employed in keeping the vapour suspended in the air, water is less suddenly deposited on the earth than it would have been, had not these tendencies of the air and its hygrometric peculiarities been such as we find them.

The blue colour of the sky, which is so much more agreeable to the eye than either red or yellow, is due to a tendency of the mixed gas and vapour to reflect the blue rays rather than red or yellow. The white light which falls upon the surface of the earth, without absorption or decomposition in its passage from the sun, is partially absorbed by, and in part reflected back from, the earth. The reflected rays pass with tolerable freedom through this transparent medium, but a portion of the blue rays are interrupted and rendered visible to us. That it is reflected light, is proved by the fact of its being in a polarized state.[226] Clouds of vapour reflect to us again, not isolated rays, but the undecomposed beam, and consequently they appear white as snow to our vision.