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CHAPTER VII.

The beneficial effects of the atmosphere in the system of

nature,

This subject presents an immense field of contemplation, which it would require several volumes fully to illustrate; and, therefore, a few general statements and illustrations can only be given.

i. In the first place, air is essentially requisite to the germination and growth of plants; and, therefore, to the influence of atmospheric air, all the beauties of the vegetable creation are to be chiefly ascribed. By experiment, it is found that the access of atmospheric air is no less necessary for plants than it is for the continuation of animal life. Like animals, they are found to die when confined within a vacuum, or deprived of the vital air. The influence of the atmosphere is equally essential at every period of their existence, from the germination of their seeds to the full development of all their organs in the perfect plant. Their leaves, acting in some measure like the lungs of animals, absorb oxygen gas during the night, and carbonic-acid gas during the day; and this alternate process is found to be essential to their growth and nourishment. Even the green colour of plants, which is produced chiefly by the influence of light, is proved not to be perfected without the co-operation of oxygen gas.

It is found that pure air, or oxygen gas, may be procured by putting the leaves of plants into water, and exposing them to the sun.

In purifying contaminated air, Dr. Priestley discovered that vegetables answered this purpose most effectually. Having rendered a quantity of air very noxious, by mice breathing and dying in it, he divided it into two receivers, inverted in water, introducing a sprig of mint into one of them, and keeping the other receiver, with the contaminated air in it, alone. He found, in about eight or nine days after, that the air of the receiver into which he had introduced the sprig of mint had become respirable; for a mouse lived

very

well in this, but died immediately upon being introduced into the other receiver, containing the contaminated air alone. It is likewise proved by experiment, that the simple component principles which are essential to the formation of vegetable matter are but three in number, namely, carbon, oxygen, and hydrogen; and these form the bases of carbonic-acid gas, oxygen gas, and hydrogen gas. From the various proportions in which these ingredients are combined, results almost all the variety of vegetable matters which fall under our notice.

To the atmospheric influence, therefore, we

are indebted for all the productions of our fields and gardens, and for all that diversity of prospect and colouring which the vegetable tribes spread over the landscape of the world. It is true, indeed, that water is also necessary for the production of plants. But what is water ? It is nothing else than a composition of two kinds of air, oxygen and hydrogen, combined in certain proportions. Now, it is found that plants have the power of decomposing water into these two principles, throwing off a part of the one, and absorbing a part of the other. The elasticity of the air has likewise an important influence on the air-vessels of vegetables ; for the contained air, alternately expanding and contracting, according to the increase or diminution of the heat, alternately presses the vessels, and eases them again, thus keeping up a perpetual motion of their juices. It has likewise been ascertained, from recent experiments, that the pressure of the atmosphere has a powerful influence on vegetation, which suggests to us one of those causes which prevent trees from flourishing on the elevated sides of lofty mountains.

2. The pressure of the atmosphere has an influence in preserving water in the state in which we find it. Nothing is of more importance to the comfort of man and other creatures, and to almost all the processes of the arts, than water-without which our globe would be transformed into an immense desert. But, it there were no atmosphere, all the waters on the face of the earth would boil, and be evaporated with a very slight degree of heat. The ocean would be drained to its lowest caverns, the rivers would cease to flow, the springs would be dried up, and the whole surface of the land exhausted of that moisture so essential to the existence the animal and vegetable world. Indeed, it is not improbable, that all the substances on the earth, solid as well as fluid, would be dissipated into vapour. That such effects would actually take place, appears from a variety of experiments. If we fill a long-necked bottle with boiling water, and sork it close, so as to exclude the air, and place it in a bason of cold water, the water will sink in the neck of the bottle as it cools. This shrinking of the water will produce a vacuum in the upper part of the bottle, and the water within it will be seen to re-commence boiling with great violence, which can arise from nothing but the cork taking off the pressure of the atmosphere from the water. In like manner, if we place water that has been cooled several degrees below boiling, under the recciver of the air-pump, it will begin to boil as soon as the air is exhausted. It requires a heat of 212° of Fahrenheit's thermometer to make water boil under the common pressure of the atmosphere ; but in the exhausted receiver of an air-pump, it boils when heated to only about 67°. The phenomenon exhibited by what is called the pulse-glass, is also owing to the same

This glass, having two bulbs, is partly

cause.

filled with spirits of wine, the air is extracted, and the glass hermetically sealed; and when the hand is applied to one of the bulbs, it causes a heat which produces an ebullition in the spirits of wine. It is likewise owing to this pressure that porter, ale, and other fermented liquors are preserved in botties; without which they would either rush with violence out of their mouths, or burst them to pieces. It is owing to the same power that boiling water is preserved in our pots and kettles, when used in cooking, without the influence of which it would soon dilate itself, rush over the vessels, and be dissipated into vapour.

3. It is to the atmosphere we are indebted for the action of fire and flame. Fire is essentially necessary to human existence, even in the warmest climates of the globe.

By its means the inhabitant of the desert frightens from his dwelling the beasts of prey, and drives away the insects which thirst for his blood. By its means also, man, in every country, prepares his food, dissolves the metals, vitrifies rocks, hardens clay, softens iron, tempers steel, and gives to all the productions of the earth the form and combinations which his comfort and necessities require. But, without the vital air, no flame can be extricated, nor fire made to burn. This is proved by putting a burning taper within the receiver of an air-pump, and when the air is extracted it is instantly extinguished. The act of combustion effects an analysis of the air; it separates its component

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