The earth has four important atmospheres and others of less importance. The principal ones are oxygen, nitrogen, vapor of water, and carbon dioxide, each comporting itself as it would do if the others were not present. There is space between the molecules of each gas, and therefore it is easily compressed. A doubling of its pressure reduces its volume one half.
Composition of Atmospheric Air. It is difficult for the mind to form a picture of the infinitely small molecules of the air. Let us therefore use terms and comparisons that will the more directly appeal to the human senses. First let us imagine each molecule enlarged to the size of a small grain of sand. Then with the molecules from one cubic inch of air transformed into grains of sand we could build a roadway ten feet deep and one hundred feet wide extending from New York to San Francisco. May one still further grasp the idea of the atom, many of which are required to make up the molecules? If so, the imagination has been stretched to its limits to enable the human mind to comprehend some of the simplest facts with regard to the wonderful fluid in which we live.
Sir William Thomson, afterwards Lord Kelvin, in endeavoring to give relative values that would appeal to the imagination, said that if a drop of water were enlarged to the size of the earth, the molecules of which it is composed would be no larger than cricket balls, and the smallest about the size of small peas.
More than a thousand years before the birth of Christ a great Phœnician philosopher believed that all matter—solids, liquids, and gases—was built up from infinitely small aggregations of atoms. The learned men of Greece enlarged upon his views but this philosophy passed into oblivion with the destruction of Rome and the coming of the Dark Ages, and it was not revived until about one hundred and fifty years ago. The ancients could not prove their theory, while we to-day can count the atoms and determine their size and motions; and, exceedingly small though they be, we no longer believe them to be indivisible in structure. On the contrary, we know that each atom consists of particles of positive and negative electricity. The negative electrons arrange themselves about a positive electron for a nucleus and, rotating about it as if it were a central sun with planets, constitute an atom. All matter reduced to the ultimate electron is precisely alike. The difference in matter is determined by the number of negative electrons that are attracted and held in place by the positive nucleus that is at the center of each atom of which a particular kind of matter is composed. Each of the ninety-two elements which we believe constitute the ninety-two different forms of simple matter has an atom with its own peculiar type of nucleus, which nucleus differs from those of the others only in the amount of positive electricity it contains. Thus hydrogen, the lightest of all gases, whose weight is taken as unity in measuring the magnitude of other gases, has a nucleus whose positive charge of electricity is only sufficient to attract one negative electron. The next element, helium, has a nucleus with a double positive charge and consequently holds two electrons or planets to pay it homage. In like manner the carbon atom contains six electrons; oxygen, eight; aluminum, thirteen; nitrogen, fourteen; sulphur, sixteen; iron, twenty-six; copper, twenty-nine; silver, forty-seven; gold, seventy-nine; mercury, eighty; lead, eighty-two; bismuth, eighty-three; radium, eighty-eight; thorium, ninety; and uranium, ninety-two. The chemical union of these elementary forms of matter creates other forms. For instance, the union of two atoms of hydrogen and one of oxygen constitutes a molecule of water. But the gases of the atmosphere are not in chemical union; they exist in the form of a mechanical mixture, each acting as though the others were not present.
It is important that this mixture of gases that constitutes our air be maintained in the right proportion. Only a slight difference in relative amounts might be disastrous to life. An increase in the oxygen would stimulate mental and physical activities and hold the human faculties at a higher tension. Man would accomplish more in a given time, but his span of life would be shortened; and too great an increase in the proportion of this stimulating element would quickly terminate life. Conversely an increase in the nitrogen would render all life more lethargic and man would be slower to act and to think; and too great an increase would smother every living thing.
In addition to the gases named, the air contains small amounts of many other substances,—argon, nitric acid, ammonia, ozone, xenon, krypton, and neon; as well as organic matter, germs, and dust in suspension. Over the land it contains sulphates in minute quantities, and over the sea and near the seashore salt left from the evaporated spray.
| By Volume | By Weight | |
|---|---|---|
| Nitrogen | 78.04 | 75.46 |
| Oxygen | 20.99 | 23.19 |
| Argon | 0.94 | 1.30 |
| Carbon dioxide | 0.03 | 0.05 |
| 100.00 | 100.00 | |
Nitrogen. Its principal functions are to dilute the oxygen and to furnish food to vegetation. It is inert and does not manifest many marked chemical affinities. Its lack of activity is shown by the fact that it will neither support combustion nor burn.
Oxygen. Oxygen, unlike nitrogen, is an active element that readily enters into chemical combination with many other elements, and it is second in quantity to nitrogen. With hydrogen it constitutes eight ninths, by weight, of water; combined with other elements it constitutes forty to fifty per cent. of the crust of the earth. It burns so readily that were it not greatly diluted by an inert gas like nitrogen it would be difficult if not impossible to stop a conflagration when once started. It is the vitalizing principle in all forms of life. By its chemical union with carbon in the tissues of plants and animals it develops the energy manifested in their movements.
In the free air up to about seven miles high there is no variation in the proportion of oxygen. But variations of marked importance to health and life occur in places where ventilation is restricted, and especially where living creatures exist in closed rooms, and where combustion occurs in confined places. The following variations in percentages by volume were found in careful analyses by Robert Angus Smith: On the seashore of Scotland, 20.99; open places in London, 20.95; in a small room where a petroleum lamp had been burning six hours, 20.83; pit of a theater at 11:30 P.M., 20.74; in a court room, 20.65; in mine pits, 20.14. He took samples from one mine that showed 18.27, the candles going out when the amount had decreased to 18.50.
The absorption of oxygen by putrid matter and by living beings in the process of breathing, and the giving out of carbon dioxide by both explain the deficiency of oxygen that is found over large cities, which is more marked when the air is moving but little and where the city is located in a depression or near swampy lands.
Both animals and plants inhale oxygen and exhale carbon dioxide with the unchanged nitrogen. The process automatically proceeds both night and day. It should not be confused with the opposite action of plants under the influence of sunlight in taking in and decomposing carbon dioxide and expelling pure oxygen.
Carbon Dioxide. It forms the chief food supply of all green-leaved plants. It is as necessary to the life of vegetation as is oxygen in the supporting of animal life. In the ratio of seventy-seven to one hundred there is less of this gas present in the atmosphere in the winter than in the summer; there also is a diurnal maximum and minimum. In the open country the amount averages about 0.035 per cent. by volume. In cities the amount is considerably greater, frequently rising to 0.07, and at times to 0.10 when the wind velocity is too low to scatter the excess amount that accumulates near the ground. Any quantity in excess of 0.06 per cent., especially if combined with the organic matter exhaled from the lungs and from the pores of the skin by animals and man, is injurious to health. Angus Smith found as much as 0.32 per cent. in crowded theaters, and 2.50 in mines. The latter amount soon would destroy animal life.
Vegetation, in addition to the inhalation of oxygen and the expiration of carbon dioxide at all hours, absorbs the latter during the day, and under the influence of sunlight the green granular matter that constitutes the chlorophyll of the cells of the leaves decomposes it, the plant retaining the carbon and giving out the oxygen. Because of the absence of sunshine the chemical activities of the plant are altered at night and the absorption of carbon dioxide ceases; therefore over the land the maximum amount occurs during the nighttime. This gas is dissolved in sea water and given off with a rise in temperature, which causes the maximum amount over oceans to occur at midday.
Carbon dioxide is 1.50 times as dense as an equal volume of atmospheric air. Its greater density causes it to collect in mines, sewers, cellars, and other low places, unless there is forceful ventilation.
The American cold wave should be welcomed as the mighty scavenger of the air. Its high velocity and great density cause it to search into cracks, crevices, sewers, and cellars and expel foul accumulations. How sweet and clean the air smells and how vigorous physically and buoyant mentally one feels after a rain and high winds! All nature smiles and every form of life adds its pæan of joy. Rain washes out the carbonic acid gas (carbon dioxide) from the air, with dust and other particles in suspension; and the cold wave enters our places of habitation and drives out the thieving accumulations of poisonous gases that would rob us of health and maintain conditions of morbidity.
It cannot be too forcefully stated that oxygen, the life-sustaining principle of the air, decreases, and carbon dioxide, a poison, increases in air that is breathed, or in air in which lamps or gas jets are burning; and that all places of habitation, especially sleeping rooms, should have a continuous supply of fresh air.
Water Vapor. It is only a little over one half as dense as atmospheric air. In the arid regions of the west it may form only a fraction of one per cent. of the air by weight; while in the humid regions in the eastern part of the United States it may constitute as much as five per cent. The temperature being the same, the same amount is required to saturate a given space, whether it be a vacuum or whether it be filled with air. Air doubles its capacity for water vapor with each increase of eighteen to twenty degrees. On a hot day in summer, near large bodies of water, it may constitute as much as one twentieth by weight of the lower air, while on a cold day in winter it may form no more than one thousandth part. When the air contains all the water vapor it can hold, it is said to be saturated; no more can be added to it until its temperature is raised, and but a slight lowering of its temperature will precipitate a part of its water vapor in the form of dew, frost, rain, hail, or snow. This is the reason it is usually called water vapor instead of a gas. Under the influence of heat that is below the freezing point, ice and snow may be changed from the solid to the gaseous form, and water vapor may be precipitated as frost or snow without passing through the liquid state.
The Dew Point is the temperature of saturation,—the temperature to which a body of air must be reduced before condensation can occur and some of its water vapor return to the liquid or solid state.
The Relative Humidity is expressed in percentages of the amount necessary to saturate. At a temperature of 32° air may continue to increase its vapor of water until it contains 2.11 grains per cubic foot, when it will be saturated and its relative humidity be one hundred per cent. If this same air be suddenly raised in temperature to 51° its capacity per cubic foot will be increased to twice what it was at 32°, the 2.11 grains will be equal to only one half the number necessary to saturate, and the relative humidity be expressed by fifty per cent. instead of one hundred per cent. In this way does the capacity of air for water vapor increase. Thus it is seen that the relative humidity of the air may increase during the cooling of nighttime without the addition of any vapor of water, and, in fact, with a decrease. The increase of relative humidity after nightfall is greater in the country than in the city, where the presence of pavements and brick buildings retards the loss of heat.
The Absolute Humidity is expressed in grains the cubic foot. The hygrometer is employed to measure the amount of water vapor.
Hydrogen is the lightest of all known gases. Its density in comparison with ordinary air is only .0692. It is combustible, and when five volumes of atmospheric air are mixed with two volumes of hydrogen the mixture explodes when ignited. It is supplied to the air by active volcanoes and in other ways, but the speed of its molecules is such that it readily escapes from the earth’s attraction and passes outward into space.
Ozone (Greek, ozo, I smell) is highly electrified oxygen, in which the molecules are broken up and reformed so as to contain additional atoms. It is formed by the disruptive discharge of lightning and by the great amount of electricity present in the high levels of the atmosphere, and possibly in minute quantities by the evaporation of fog and water near the earth. It is always found in the presence of waterfalls and spraying fountains. It is a powerful sanitary agent, readily entering into union with decaying matter. This fact accounts for the total absence of ozone from the air of large cities.
Ozone, in the minute quantities found in nature, is healthful, but when breathed in a condensed form it has a highly irritating effect on the mucous surfaces of the respiratory passages, and the quantity is not large that will cause death. The healthfulness of mountain air may be due largely to the increase with elevation in the quantity of ozone and electricity in the air, as well as to the less number of disease germs and dust motes. The invigorating effects of the crisp air of the frosty morning and of the cold wave in winter may be increased by the activities of ozone.
Ozone has two daily maxima, the principal one occurring between 4 and 9 A.M. The minima occur between 10 A.M. and 1 P.M., and between 10 P.M. and midnight. The winter furnishes an amount greatly in excess of the summer, due not only to the less amount of decaying matter to take up the ozone in winter, but to the higher and more persistent winds mixing the lower and upper air. The amount is greater over the sea than over the land, probably due to the absence of oxidizable matter, which allows the ozone to accumulate over the water. It is more abundant with westerly than with easterly winds, due to the fact that westerly winds have a downward component of motion; but if the westerly winds be weak and the easterly winds come from over a large body of water the conditions may be reversed.
Microbes of the Air. The air transports vast armies of unseen workers. Some are enemies; others are benefactors of the human family. The useful varieties are energetic in clearing away the refuse of animal and vegetable life, in fixing fertilizing gases in the soil, in giving flavor to fruits and proper growth to leguminous crops, in transforming the crudest must into the best claret, and the poorest tobacco leaf into the fragrant Havana; in curing cheese and butter and fermenting beer, and in a multitude of other useful employments. The malevolent varieties, if they gain lodgment in suitable human tissues before sunlight weakens their virility, disseminate certain forms of disease.
In picking a permanent place of abode, remember that there are many less disease microbes in the air of the open country than in that of the city, and that few are found in the air of mountains, or in that of the ocean. The average number of bacteria in a cubic meter of air in the city of Paris has been found to be 4790, while ten miles away in the country the number was only 345.
Accurate analyses of the air of crowded tenements always have shown large numbers of bacteria, but the number was found to be small in well-ventilated city houses that let in an abundance of sunshine to their interiors. It is better to have color in the cheeks of the occupants than in the furnishings of a house. Curtains and heavy drapery not only furnish a refuge for the microbes of disease, but they may be so hung as to exclude the purifying sunshine. The amount of sunshine is nearly as important as the quantity of air, for most of the microbes of disease quickly die, or are rendered less virulent, under its influence.
Bacteria exist in small numbers, if at all, at altitudes where snow forms, but snow gathers them as it falls through the lower air. Ice contains bacteria, but not in any such quantity as the water from which it freezes. Ice forms in the open at the surface of the water, or about numerous small particles of matter in suspension, which rise at once to the top as soon as the ice congeals about them in the form of a buoyant covering; meanwhile sediment is continually settling to the bottom, carrying bacteria with it. Ice forms more readily in quiet water, where sedimentation has been most rapid, and where, therefore, there are the fewest bacteria in position to be included. More disease germs exist in river water in winter than in summer, which may be due to the greater disinfecting power of the sun’s rays during summer.
Dust Motes of the Air. As the earth pursues its course about the sun, dust rains into its atmosphere from outer space. Meteors that are burned through the heat generated by striking into our air contribute to the supply, as do volcanoes, combustion, spray from the ocean, and matter lifted up by the action of the wind.
Dust from the eruption of Krakatoa was wafted entirely around the earth, falling upon the decks of ships in all the seas of the world. It affected the colors of the sky for two or three years after the explosion.
As in the case of microbes, the number of dust particles is far greater in cities than in the country, being least on high mountain tops and over the oceans. The air in large cities invariably shows hundreds of thousands of dust motes to the cubic centimeter, that of the village thousands, and that of the open country some hundreds. Dust-free air is also germ-free. Many experiments have shown that air freed of dust motes has at the same time been cleared of the microörganisms that cause disease, putrefaction, and fermentation; and that germ-free flesh or liquids may be indefinitely exposed in such air without fermentation or decay.
How Dust Motes Are Counted. Many of the particles are too small to be seen by the highest powers of the microscope, yet Aitken, by a most ingenious method of making them centers of condensation—that is, making them the nuclei of small raindrops—was able to count the number in a given volume of air. When ordinary air is saturated and then cooled the cloud formed is so dense that it is impossible to count the tiny droplets that form the cloud. But we can make the number of dust particles (and therefore the number of visible points of condensation) in a given volume of air as small as we wish by mixing a little dusty air with a large amount of dustless air, and we can allow the particles to fall on a bright surface and can count them by means of a lens or microscope. By simply allowing for the proportion of the dustless to the dusty air, and making a corresponding allowance for the dilution, we calculate the number of particles.
Dust Motes and Illumination of the Atmosphere. One of the most important functions of dust motes is the diffusion or scattering of sunlight. What a different world this would be without these tiny inanimate friends of man! If there were no dust in suspension in the air, nothing would be visible except what received direct light, or light reflected from some illuminated surface, and the air occupying space between illuminated objects would be practically dark. If the observer be in a room with a powerful electric light he would see the walls and the objects in the room, but if the air were free of dust motes, he would find that the space between him and the walls and between the various objects would be as inky black as is the space between the twinkling stars on a clear night.
Figure 2 is a cubical box, with a glass front. If a glutinous substance be spread over the bottom and the box allowed to remain quiescent for from five to seven days the dust motes will slowly settle down and attach themselves to the bottom. The air then will be what is called “optically pure.” Now, if it be taken into a dark room and an inclosed lamp at a be allowed to send a beam of light into the window at b and out at c, it will be found that the interior remains dark no matter how powerful the light from the lamp. The light is seen to enter and to leave but where it encounters the dust-free air there is nothing to scatter the light rays and they remain invisible to the eye.
Dust Motes Prolong Twilight. The bending or refraction of light as the sun’s rays pass obliquely through the air at sunrise and at sunset displaces the apparent position of the sun, elevating it by an amount about equal to its own apparent diameter, so that one may see it and receive its light when geometrically it is entirely below the horizon. A little later in the evening and its rays fall upon the upper air too obliquely to be bent down to the earth by refraction; but darkness does not yet ensue, for the rays are scattered by the dust motes and possibly by the molecules of the gases and sent downward from particle to particle, resulting in a soft shimmering light that almost imperceptibly fades away, and which in higher latitudes may last for hours.