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THE AIR WE BREATHE.
currents, which tend to mix and blend together the different gases of which the air consists, all gases, by a law of nature, tend to diffuse themselves through each other, and to intermix more or less speedily, even where the utmost stillness prevails, and no wind agitates them. Hence a light gas, like hydrogen, does not rise wholly to the utmost regions of the air, there to float on the heavier gases; nor does a heavy gas, like carbonic acid, sink down so as to rest permanently beneath the lighter gases.
In obedience to this law, carbonic acid in all places slowly rises, or slowly sinks, and thus a nearly uniform purity is maintained in the air we breathe. Its presence in the atmosphere may be shown by the formation of a white film of carbonate of lime on the surface of lime-water when it is exposed to the air.
The watery vapor varies in quantity with the climate and temperature of the place. It is less in cold seasons and climates generally than in such as are hot. It seldom forms more than one sixtieth, or less than one two-hundredth of the bulk of the air. Its presence may be discovered in the hottest days by pouring ice-cold water into a tumbler, when the vapor of the air will rapidly condense on the outer surface of the vessel, in the form of dew-drops.
From every breath of air which the animal draws into its lungs, it extracts quantity of oxygen. The oxygen thus obtained is a part of the natural food of the animal, which it can obtain from no other natural source, and new supplies of which are necessary to it every moment. The oxygen of the atmosphere, therefore, is essential to the very existence of life in the higher orders of animals.
The candle burns also, and all combustible bodies kindle in the air, only because it contains oxygen. This gas is a kind of necessary food to flaming and burning bodies; so that were it absent from the earth's atmosphere, neither light nor heat could be produced from coal, wood, or other combustible substances. The proportion, also, in which oxygen exists in the air is adjusted to the existing condition of things. Did the atmosphere consist of oxygen only, the lives of animals would be of most brief duration, and bodies once set on fire would burn so fast as to be absolutely beyond control. The oxygen is therefore mixed with a large proportion of nitrogen. This gas harmlessly dilutes and weakens it, and prolongs its action on the system, as water dilutes wine or spirits, and assuages their too fiery influence upon the animal frame.
Every green leaf that waves on field or tree sucks in, during the sunshine, carbonic acid from the air. It is as indispensable to the
THE AIR WE BREATHE.
life of the plant, as oxygen is to the life of the animal. Remove carbonic acid from the air and all vegetable growth would cease. It must, therefore, be a necessary constituent of the atmosphere of our earth. But carbonic acid is poisonous to animals. It is for this reason that the proportion of this gas contained in the air is so very small. Were this proportion much greater than it is, animals, as they are now constituted, could not breathe the atmosphere without injury to their health.
On the other hand, that growing plants may be able to obtain a sufficiently large and rapid supply of carbonic acid from a gaseous mixture which contains so little, they are made to hang out their many waving leaves into the atmosphere. Over the surface of these leaves are sprinkled countless pores or mouths, which are continually employed in separating and drinking in carbonic acid gas. The millions of leaves which a single tree spreads out, and the constant removal of the moving air in which they are suspended, enable the living plant to draw an abundant supply for all its wants, from an atmosphere already adjusted to the constitution of living animals.* This constant action of the leaves of plants is one of the natural agencies by which the proportion of carbonic acid in the lower regions of the atmosphere is rendered less than it is in the higher regions.
So, also, the watery vapor of the atmosphere is not less necessary to the maintenance of life. The living plant consists of water to the amount of nearly three fourths of its whole weight, and from the surface of its leaves water is continually rising into the air in the form of invisible vapor. Were the air absolutely dry, it would cause this water to evaporate from their leaves more rapidly than it could be supplied to them by the soil and roots. Thus they would speedily become flaccid, and the whole plant would droop, wither, and die.
The living animal, in like manner is made up for the most part of water. A man of one hundred and fifty-four pounds' weight contains one hundred and sixteen pounds of water, and only thirty-eight pounds of dry matter. From his skin and from his lungs water is continually evaporating. Were the air around him perfectly dry, his skin would become parched and shriveled, and thirst would oppress his feverish frame. The air which man breathes from his lungs is loaded with moisture. Were that which he draws in en
* A common lilac tree, with a million of leaves, has about four hundred thousand millions of pores, or mouths at work, sucking in carbonic acid; and on a single oak tree, as many as seven millions of leaves have been counted.
“LET THERE BE LIGHT.”
tirely free from watery vapor, he would soon breathe out the fluids which fill up his tissues, and would dry up into a withered and ghastly mummy. It is because the simoom and other hot winds of the desert approach to this state of dryness, that they are so fatal to those who travel on the arid waste. Thus the moisture which the atmosphere contains is also essential to the maintenance of the present condition, both of animal and vegetable life.
Simple as the air appears, its scientific history as a whole is somewhat complicated. The adjustment of its several substances involves many interesting particulars, and the arrangements by which the constant presence of its essential constituents is secured, both in kind and quantity, are very numerous ; yet we can not fail to perceive both a physical beauty, and a wise contrivance in them all.--Chemistry of Life.
"LET THERE BE LIGHT."
BY PROF. N. W. BENEDICT.
WHEN, from the plastic hand of God, the earth
None but Jehovah's eye that makes as light
“ LET THERE BE Light."
Awoke his thund'ring base and rose to meet,
Hark! from the throne that voice whose power creates,
High o'er the battlements of heaven appear,
But now the pure ethereal blaze sweeps o'er
And usher in the dawn of endless day.
refracting telescope in the world. It is called the “ Craig Telescope,” after the Rev. Mr. Craig, under whose directions and at whose expense it was constructed. The object-glass of this instrument is two feet in diameter, and the focal length seventy-six feet. The tube in which it is mounted is made of sheet-iron riveted together like a steam-boiler, and is thirteen feet in circumference at the largest part.
This telescope weighs about three tons. It is supported by a tower of brick sixty-four feet in height, and fifteen feet in diameter. The different floors of the tower are loaded so as to make it as steady as possible ; and its entire weight is estimated at two hundred and twenty tons. The telescope is suspended at the side of
* We are indebted to the kindness of Mr. F. J. Huntington, of this city, for the engraving which so faithfully illustrates this article. It is one of the many valuable illustrations in Mattison's “ High-School Astronomy.”