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.