ATMOSPHERE. From two Greek words, signifying vapor and a sphere, is the body of air which surrounds the globe, and besides being necessary to the life of all terrestrial animals, is of special importance to the agriculturist, since upon its condition are we dependent for rain, the motion of the air tempering the heat of sum mer, and various meteorological conditions, with out which, the labors of. the farmer would he void. The air is in fact the great reservoir of the fertility of the earth. Elements noxious to animal life are constantly being poured into it, from various sources of contaminations; from all decaying bodies, whether animal or vegetable; from the lungs of all animals in breathing; from the miasma of marshes; from volcanoes, and the various other sources of contamination. On the other hand, living vegetation is constantly giving forth by day, during the growing season, fresh supplies of oxygen to keep it up to its normal state, .else the oxygen consumed by animals in breathing would at length exhaust the air of this very necessary element. Upon some of the phenomena of the atmosphere, Appleton!s New American Encyclopwdia says: Air consists essen tially of two gases, oxygen and nitrogen, in a state of mechanical mixture. But with these are always present a small proportion of carbonic acid gas and aqueous vapor. In the vicinity of large cities, ammonia is found too, in Small quantity; and nitric acid is generated in thunder storms by the chemical combination of nitrogen -and oxygen induced by the electrical shock. These, which may be regarded as accidental im purities, are soon dissipated in the great bulk of the atmosphere, or they enter into new combi nations, and are precipitated upon the earth, or are washed down by the rain. The proportions of the two elements of the air hardly vary— whether this is taken from the summits of the highest mountains, from extensive plains, from thickly populated cities, or from crowded hos pitals—nor are they affected by season, climate, or weather. In closely confined places exposed to putrescent exhalations, the purity of the air is necessarily much affected; the proportion of oxygen diminishes, and nuphitie gases, as sul phuretted hydrogen and carbonic acid are in troduced. Prof. Nicol gives an analysis of air collected in a filthy lane in Paris, in which the oxygen constitutes 13.79 per cent. only, instead of 23 per cent., its usual proportion; nitrogen was present to the amount of 81.24 per Bent., carbonic acid 2.01; and sulphuretted hydrogen 2.90 per eent. Carbonic acid gas and aqueous vapor are more variable in their proportions, and the former, though found in the highest altitudes, has sometimes escaped detection in air collected at sea. Its composition is: Regnault calculates, from numerous analyses, that it is by,measure oxygen 20.90, and nitrogen 79.10; and the article Atmosphere, in the Ency clopedia Britannica, gives as the mean of ten careful trials a proportion by volume of 79.9735 parts of nitrogen, and 20.0265 of oxygen. The near approach of these two gases to the number 80 of the one and 20 of the other, cannot fail to strike the attention of those who study the .analyses made by Cavendish, Davy, Gay, Lussae, Humboldt and others. And as a volume of nitro gen is equivalent to one atom, the inclination is very strong to consider air as a compound of these gases in the equivalent proportions of two atoms of -nitrogen and one atom of oxygen. But the differences of specific gravity, tem perature, structure or form, which usually aceompany the change by chemical combina tion are here wanting; and, moreover, air is recomposed by simple mixture of its elements, with no evidence of any chemical change taking place. The phenomena of refraction are such as indicate a mixture; and a still more conclusive proof is that air held in solution in water does not consist of the same proportions of its ele hut from the greater solubility of oxygen, it contains of this about thirty-two per eent., and of nitrogen sixty-eight per cent. We are there fore not authorized in the conclusion that air can be otherwise than a mechanical mixture of its elements. Carbonic acid gas, increased to the proportion of five to six per cent., renders air unfit for sustaining animal life. A candle ceases to burn when it contains three per cent. of this gas. One may live, however, in an atmosphere containing thirty per cent. of it for a short time, but not without suffering. But if carbonic oxide, which has only one atom of oxygen, instead of two atoms to one of carbon, is present, even in the small proportion of one per eent., it may prove instantly fatal. This poisonous gas is generated by the combustion of charcoal in con fined places. Carbonic acid is generated by combustion of carbonaceous substances; with free access of air, and by the analogous process of the breathing of animals, an atom of carbon unites with two atoms of oxygen, and the, solid matter takes the form of this invisible gas. By several processes it may be restored to a fixed or tangible shape Man requires from 212 to 353' cubic feet of air per hour. In breathing, the oxygen in part unites with carbon in the system, and the air expired contains four and a half per cent. of carbonic acid gas. This is immediately dispursed through the atmosphere by the property of diffusibility, possessed in such a remarkable degree by the gases; hut if confined in close places it soon accumulates and contaminates the• air. Though this is the heaviest of the gases, and is generated near the surface„ it is found in larger proportion in the air of elevated_ places, than in that below. The reason ascribed is, that this is owing to the plants absorbing it in the lower strata, and is not satisfactory, as it is in these strata produced. Growing plants are the compensating agents, that counteract the noxious influences of combustion and the breath ing of animals; as in the ocean the coralline insects as quietly perform their great office of separating from the water the soluble eon taminati ig ingredients, poured in from the innumerable rivers that feed it. Plants as well as animals breathe the air, but the effect of this respiration is just the reverse of that of animals. The carbonic acid gas is decomposed in the laboratory of their vessels, the solid carbon is added to their structure, and the pure oxygen is expired. It is true the process is reversed in the night, but with much less effect. This change in the action of plants at night is the reason why they should not be kept in sleeping apartments. Oxygen thus appears to be the life-sustaining element of the air for animals, while nitrogen has the negative duty of restraining, by its bulky proportions, the too active influence of its fiery partner. Oxygen is diluted with it, as strong food is with water, to make it wholesome. Both the weaker elements, however, have some other uses, being found as constituents of veg etable and animal substances. Water, more over, in the form of vapor, manifests its pres ence by condensing in visible moisture and drops upon cold surfaces. When the air is warm, its capacity of holding water is great; as it becomes cool this capacity diminishes, and the water that is now in excess appears as dew, or mist, or rain. The atmosphere is said to be dry when it has not so much moisture in it as it is capable of holding at its temperature; evapora tion then takes place. But let the temperature fall, and the same air that was called dry is now damp. The absolute quantity of vapor has not changed, but the relative quantity of what the air is capable of holding, and that actually in it. As the air becomes cool, and reaches a degree at which it is saturated with the water it contains, and this begins to condense upon cold surfaces, this degree of temperature •is called the dew point. If it is high, the absolute quantity of vapor in the air was great; if low, there was little vapor in the air. The relative quantity was the same in both instances, as it always must be at the dew-point. As the hot airs of the tropics are swept over the Atlantic in the trade winds they suck up the moisture like a dry sponge. Saturated with it, as they pass over the snowy summits of the Cordilleras, and their particles are compressed together with the cold, they shed it, like the same sponge squeezed in the hand.
Thus does the atmosphere fill its office as a com pensating agent, carrying away the excess of waters of the ocean, that, though all the rivers flow into it, it shall never be full, feeding, too, the dry places of the earth, that its wells and springs shall never lack its supplies. Air being a material substance, though invisible; possesses many of the physical properties of the solid and liquid bodies, as weight, inertia, elasticity, im penetrability, capacity for heat, etc. A vessel exhausted of air is found to weigh less than when filled with it; and in this manner it has been 'ascertained that 100 cubic inches of pure and dry air, at a temperature of 60°, and under a pressure of thirty inches of the barometer, weigh 31.0117 grains. Other gases are referred to air at this temperature for the expression of their comparative weight. Water is 815 times heavier than air; but at the freezing point the difference is as 770 to 1. From its weight result its inertia and the pressure of the atmosphere. It can not be set in motion without exertion of force, nor in motion be retarded without opposi tion of force. Its momentum, as with other bodies, is its weight multiplied by its velocity. Air in motion is a mechanical force, applied to propelling ships and wind-mills. ' The pressure of the atmosphere is the :weight of the column of air. If this were alike dense throughout its height, the upper limit of the atmosphere would be, easily calculated from the weight of a cubic inch, and the pressure of 14.6 pounds upon the square inch. It would be about five and one fourth miles. But from the property possessed by the gaseous bodies of expanding in bulk or becoming more rare, in proportion as the force that confines them is removed, the weight of a colunin of air is not directly proportional to its height. This tendency of the particles of air to separate from each other, as the pressure that confines them is taken off, is called the elasticity of the air. Its effect is, that every successive layer of air of any given thickness is of less density arid weight than the layer of the same thickness beneath it. The rate of this decrease of weight may be thus expressed: When the height increases in an arithmetical ratio, the volume increases in a geometrical ratio and the weight diminishes in the seine. For example, at the level of the sea, calling the volume one, and the density or weight one; at the height of 2.705. miles, the volume is two and the density one-half; at twice the height, the volume is four and the density one-fourth; at three times the height, the volume is eight and the density one-eighth. But notwithstanding this tendency of expansion, the atmosphere is proved by calculations based on its refractive properties to find somewhere a limit, and this appears to be not far from forty five miles above the surface. The pressure of the atmosphere is made apparent by removing the air from any tube, the lower end of which is. immersed in water or any other fluid. This fluid will be pressed up the tube to a height corres ponding to the pressure upon its surface. If this. be at the level of the sea, where the pressure is 14.6 pounds on the square inch, water will rise thirty-three feet and mercury twenty-nine inches. At any greater elevation, the pressure being less, a less height of the fluid will balance it. Such an instrument as this tube is the barometer, by which the difference of elevation is determined by the different heights of the column of mercury, the calculation being made on the principal above described, and corrected for temperature and the latitude of the place. There is another instrument used for the same purpose, based on the property of water boil ing at less temperature, as the pressure of air upon its surface is taken off. For every 549+ feet increase of elevation, it is found that the boiling point is one degree less. Correction, is in this case also to be made for the tempera ture of the air. Well constructed instruments of this kind have been made to produce very fair results in experienced hands. Familiar illustra tions of the pressure of the air are afforded by the common pump, which is but such a tube as. has been already referred to, furnished merely with a proper valve for lifting out the air, and then the water which follows it. The power applied to lift the air is equal to its pressure at the place multiplied by the height it is raised, or to the weight of the column of water. There can, therefore, be no expedients that will .lessen the power required to work a pump, unless they can present some form more simple, and which involves less friction than the ordinary form of the pump, and this seems hardly possible. The pressure of the air is also• well illustrated by the common leather " sucker," which the boys make for a toy—a mere disk of soft leather with a. string knotted at one end passed through the centre. When moistened and applied to any smooth surface, care being taken to expel the in tervening air, it is attracted to it by the external pressure. By the same principle insects walk upon the ceiling., and the patella or limpet, and. some other shell fish, hold upon the smooth rock. So great is this pressure, that the force exerted upon the body of a moderately sized man is estimated at about fifteen tons, sufficient to crush him, as it inevitably would, if applied to only a portion of the body; but quite harmless when pressing with perfect elasticity everywhere alike—from the external parts inwardly, and from those within outward. , Let the pressure be taken off from any portion, as by the cupping instrument, and one is immediately sensible of the power that is exerted upon the parts around, painfully pressing them into the vacant space of the instrument. Elasticity is possessed in a re markable degree by gaseous bodies. If the pressure is removed from them their particles repel each other, and the tendency is to expand indefinitely. This force, as expressed by the law of Mariotte, its discoverer varies in exactly the same proportion -as the density of the air. But as air has been made to expand to more than 2,000 times its usual bulk, and been com ,pressed into less than one-thousandth—and at these extreme , degrees of rarefaction and con densation it is difficult to determine its elasticity with rigor. From the freezing point upward air expands 1.493 of its bulk for every degree of its temperature. This is easily exemplified by heat ing air confined in a bladder. Its expansion soon swells the bladder and causes it to burst. As the bulk increases, its density or weight dimin ishes. The colder and heavier air presses through it, and the more buoyant fluid is lifted up. On this principle were constructed the first balloons. It is this principle that gives rise to the currents of air or wind, the colder bodies flowing along the surface to fill the space left by the ascending columns. Thus the trade winds blow from the temperate regions toward the torrid equatorial belt. The •whirling tornado, and all the phe nomena of the winds, owe their origin to local heating and rarefaction of the atmosphere. The rays of the sun pass through the upper strata of the atmosphere, imparting to them no heat. This the air receives only near the sur face. As we ascend the temperature diminishes one degree for every 352 feet. Near the equator perpetual snow covers the mountains at a height of 15,207 feet; in latitude 60° it is found at 3,818 feet; and in 75° at 1,016 feet. Did the sun's rays impart no effect to the atmosphere, the great body of it would be seen as blank dark ness; but a partial absorption of a portion of the rays takes place, and reflection' of the blue rays. This gives the color to the sky, while that of the clouds and the rainbow comes from the effect of the light upon the particles of vapor floating in the atmosphere. These colors are too faint to be perceived in any small quantity of air. It is only by , looking into the great depths of the atmosphere that they may become visible, as the color of the ocean is only apparent when the waters are seen in a mass.