Page:Popular Science Monthly Volume 5.djvu/402

386 of repose, its time of wakefulness and time of sleep; the other never sleeps till death, but keeps up its incessant action; the beating of the heart, the introduction of air by breathing, involving millions of movements which never fatigue us, and of which we are indeed, for the most part, unconscious. And, now, who would suppose that these the highest and noblest results of a far greater mechanician than man, are ultimately connected with the return of the spring; and that, in fact, the continuance of the life of man is indissolubly linked with the putting forth of the buds of a tree?

Yet so it is; and surely we cannot spend an hour more profitably than in tracing that connection. Such studies are appropriate to all intelligent men. And, when another spring revisits us, we shall not find that this hour has been entirely lost. The reflections it may suggest will, perhaps, increase the pleasure with which we view the return of that great natural phenomenon.

In thus explaining to you the connection subsisting between the animal and vegetable kingdoms, I shall have, in the first place, to introduce an account of the great scientific discovery of the last century—the discovery of oxygen gas—an event rivaling in importance the establishment of the doctrine of universal gravitation by Sir Isaac Newton, in the preceding age.

Until the middle of the last century an opinion universally prevailed that the atmospheric air is a perfectly homogeneous and undecomposable body—that there is but one kind of air, that which we breathe, and though in mines, wells, and other deep and solitary places, substances somewhat analogous occur, they are in reality nothing more than vitiated forms of atmospheric air, which has gathered poisonous qualities from mineral exhalations. From the remotest times these opinions had prevailed. Many of the Greek philosophers looked upon the Olympian Jupiter as only an emblem of the atmosphere, and little suspected that the day would come when that great god of antiquity would be anatomized, dissected, and his various parts and qualities displayed. How often do things which have struck one generation with awe become commonplace affairs in another!

It so happened that, though, from time to time, after the thirteenth century, different gaseous substances were accidentally encountered, they all possessed the quality of extinguishing the light of a candle, and were therefore incompetent to support combustion, and when breathed were destructive of animal life. The doctrine that these were only vitiated forms of the atmosphere seemed very plausible, and this interpretation was received until the middle of the last century, when the capital discovery was made by Dr. Priestley that the air is not a simple substance, and that there is a great family of analogous bodies, each of the members of which possesses peculiar properties. He completely broke down the ancient doctrine of the elementary nature of the atmosphere.