NATURE OF ATMOSPHERIC AIR.

are so adjusted and so blended, as to form an economy altogether worthy of the wisdom by which it was devised, and the power by which it was effected.

Atmospheric AIR is the next fluid I shall consider, and the next in density to that of water. It is matter under an invisible form; its general properties are gravity, elasticity, and transparency. It may be compressed into a space much less, or rarefied to occupy a space much larger, than it naturally fills. It invests this globe to a considerable height, and accompanies it every where along its orbit round the sun. It is not a simple but a compound body; and it is composed of three different

i

to be water proof. The water had made a powerful effort to force in the cork, twine, and canvas altogether, and had moved them about half an inch; but there they remained, and the bottle was empty.

If there were no atmosphere surrounding the earth, only that part of the sky would appear light in which the sun was placed; and if a person should turn his back to the sun, he would directly perceive it dark as night; for in that case there would be no substance to reflect the bright rays of the sun to his eyes. It is owing to refraction that the sun enlightens the earth some time before it rises, and some time after it sets. See Parkes. *There are indeed many other intermediate fluids, natural, others artificial, between water and air; such as blood, milk, oils, wines, spirits, &c.; but water is a principal constituent part of them all: some of them will be noticed in another part of this Essay. An examination of their properties would lead too far into chemical analysis, to be found generally interesting.

some

NICE ADJUSTMENT OF THE ATMOSPHERE.

gases or airs', combined together in various proportions, according to their different degrees of importance in the operations of nature. Twenty-two parts of oxygen, with seventy-eight parts of nitrogen, form atmospheric air, (with a very small portion of carbonic acid gas,) the great supporter of animal and vegetable life. But seventy-five parts of oxygen, with twenty-five of nitrogen, form nitric acid, or aquafortis, one of the most corrosive and destructive of all substances. None but a Being of infinite intelligence could have had a conception how such different substances could be produced from the same principles.

If the atmosphere contained one of them onlyoxygen gas, or vital air, for instance, though every thing for one moment would wear a most flourishing aspect, yet the next moment, all animals and vegetables would be in a state of putrefaction. If it were formed of the other only, (nitrogen and carbonic

'These three gases are nitrogen, oxygen, and carbonic acid gas, in the following proportions :---Suppose a portion of atmospheric air divided into a hundred parts; seventy-eight of those parts would be nitrogen or azotic gas; twenty-one would be oxygen gas, or vital air, and one would be carbonic acid gas. Sixteen cubic inches of atmospheric air weigh nearly five grains. A column of air the whole height of the atmosphere, one inch square, is equal to fifteen pounds weight.---N. B. It has been lately proved, that the carbonic acid gas of the atmosphere is only one part in a thousand.

m Parkes.

HOW THE PURITY OF THE ATMOSPHERE IS PRESERVED.

acid gas) though putrefaction would not take place, yet nothing possessed of animal life could exist in its vital state. Plunged into such an atmosphere, animals would instantly expire; the vegetable creation would sicken and die. On a proper mixture of these different airs, depends, in a great measure, the healthfulness of our atmosphere. Were any one of these component parts to prevail beyond the proportion in which it is generally found, in that exact proportion would the atmosphere be injured. What goodness is seen in this constitution of things! What wisdom appears in the nice adjustment of their proportions, and the combination of the whole!

The oxygen gas, which is secreted by vegetables, is detained in the lower regions of the air, that it may combine with the nitrogen gas produced by animal respiration, to preserve the purity of the atmosphere. The carbonic acid gas, which is still heavier, and which is formed also by animal respiration and combustion, is confined within the reach of vegetables, which absorb it with avidity, and separate it into its original elements."

The quantity of carbonic acid gas which is formed by the respiration of animals and combustion, and the hydrogen and carbonated hydrogen gases which arise from marshes and stagnant pools, are so prejudicial to the animal creation, that if some means were

Parkes.

NOXIOUS VAPOURS DESTROYED.

not appointed by the all-wise Creator, no animals could exist. This evil is prevented by the structure of vegetables which feed principally on these gases, and whose organs seize upon them while they emit oxygen gas to renovate the atmosphere by its union with nitrogen, which is rejected by animal respiration. The wisdom, the simplicity, and the beneficence of this arrangement, are so striking, and address us with so much effect, that the mind of the Reader may be left to its own reflections on the goodness of God.°

The usefulness of this arrangement will be further apparent, when the known fact is considered, that the carbonic acid gas has the property of combining with putrid exhalations, and rendering them innoxious. It is therefore probable that a portion of this gas is diffused through the atmosphere for the purpose of uniting with malignant vapours as soon as they are formed by putrefaction, and of preventing their dangerous effects. How great would be our exposure to perpetual infection, and constant diseases, without this benevolent contrivance!

The uses to which atmospheric air is destined, are most important. Plunged into any other element, as soon as we come into this world, the end of our creation would be lost in the immediate extinction of the vital flame. Air affords to all animals an

• Parkes.

USES OF ATMOSPHERIC AIR.

essential nutriment. Animals that live in water, require atmospheric air as well as others. Excluded from it, fishes soon die. Insects called hydrocanthari, have a curious apparatus for obtaining it. This is in their tails, which they raise above the water, and take in a small portion of air which they carry with them to the bottom, and keep it till it has lost its power of supporting life, when it is discharged, and the animal ascends for more. By being inhaled into the lungs of animals, the oxygen, or vital air, is immediately separated, and imparted to the circulating fluids, and is essential to animal life. With the food which they take into their stomachs, they receive a portion of air which aids the decomposition or the separation of its parts, and assists digestion. Air is also essential to the progress of vegetation; for when vegetables are excluded from air, they die. It enters their pores, and flows along with their juices through all their mysterious labyrinths. Rarefied by the external heat, it enlarges their vessels; again, condensed by cold, it occupies a smaller space: thus a sort of palpitation is produced by the alternate expansion and contraction of their organs, which may aid the progress of vegetation. It is by means of air that the important process of evaporation, and the consequent formation of clouds, is effected; for were air excluded, this process would not go on.

The quantity of water evaporated from the surface of the sea and the land, by means of the atmosphere,