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Entered according to Act of Congress, A. D. 1903, by A. N. BELL, in the office

of the Librarian of Congress, at Washington.

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Knowledge of the air we breathe, and of the boundary lines between that which is wholesome and that which is not; knowledge of air so impure as to be wholly unfit for respiration, and knowledge of the difference between it and that which is poisonous is everywhere essential to the maintenance of health.

It is even more important to know the quality of the air we breathe than of the water we drink. More necessary to be mindful of the medium in which we are constantly immersed and which is continually being introduced into our blood and passing through our bodies, than it is of that which is only occasionally and by some persons rarely introduced into the stomach, which contains a fluid possessing antiseptic and destructive power over substances injurious to health. Moreover, the consequences of exposure to impure air are more insidious and obscure, and more persistent than those of impure water.

The train of evils set in motion by breathing impure air is generally so insidiously laid as to frequently escape the observation of experienced physicians. Drowsiness, headache and nausea are the more common effects observed under occasional conditions. But these are insignificant as compared with the results common to habitual breathing of air fouled by the products of respiration alone-wholly apart from external sources of pollution. Malnutrition, dyspepsia, anæmia, disordered biliary functions and many indefinite chronic complaints from alleged unaccountable causes are common consequences of breathing impure air, which, besides all these, is predisposing to the most prevalent and the most fatal of all diseases, tubercular consumption. For, attribut

able though tuberculosis is to a specific organism, the effect of habitually breathing air surcharged with putrescible effluvia by rerespiration is, of all conditions and circumstances, the most potent, predisposing cause to it; and not to tuberculosis only, but to acute lung affections, bronchitis and pneumonia in their most fatal forms.

Air is the first condition of life and the first element of our bodily tissues, but to be healthful and life-sustaining it must be devoid of impurities. By respiration alone air supplies threequarters of our nourishment; and the other quarter, which is taken in form of liquid and solid aliment, consists of substances chiefly composed of the same elements as air.

Pure air principally consists of two ingredients, oxygen and nitrogen in the proportion by volume, of twenty-one per cent. of the former to (including argon) seventy-nine per cent. of the latter, and these proportions have been found to exist whencesoever taken, whether from the summits of the highest mountains, from sandy plains, or from over the sea. But more explicitly and besides these two gases atmospheric air always contains varying quantities of water-vapor, small quantities of carbon dioxide (3 to 4 volumes per 10,000), and traces of ammonia.

As the sum of numerous analyses the atmosphere is composed of: Oxygen, 209.6 per 1,000 volumes; nitrogen (including argon, about i per cent.) 790.0 per 1,000 volumes; carbon dioxide, average, 0.4 per 1,000 volumes; water-vapor, variable with temperature, mean, 0.84 per 1,000. Traces of ammonia; ozone, occasionally ; peroxide of hydrogen; nitrous and nitric acids; salts of sodium and other mineral substances; organic matter, dead and living, and “dust” of substances variable and innumerable.

The gases of which the air is composed exist in the form of a mechanical mixture not chemically combined. Nevertheless the chief elements are so intimately mixed and yet so fixedly proportioned in the free atmosphere as to be invariable.

The atmosphere is subject to a law which characterizes all elastic fluids, namely: It presses equally on all sides, and when any portion becomes lighter than the others the denser portions rise into its place and force it to seek a rarer medium, always creating a current from the point of greatest to that of the least pressure. When the disturbing cause is local, transient and irregular, partial derangement ensues on account of the action necessary to cause a speedy adjustment. But as the local disturbances are always as much wanting on one side as they are in excess on the other, they are equivalent to undulations of the same medium. Their balance will still maintain an equality of pressure; and whatever may be the disturbing causes the restoration of the equilibrium is the object of all the motions excited. In the free atmosphere its own weight is a compensating force, hence both its weight and elasticity diminish in ascending from the ocean level.

According to a natural law all gases tend to diffuse themselves through each other, irrespective of their specific or relative gravity. The action of this law is promoted by the wind, but it operates even in confined places; insomuch that in crowded rooms, especially if they are warm, carbon dioxide—the heaviest of the atmospheric gases—is frequently found more abundant at the ceiling than at the floor.

The atmosphere rotates with the earth upon its axis and consequently in its upper regions, and over the equator especially, it is hurled with incalculable swiftness and to the uttermost degree of attenuation. Penetrated by the sun's rays and in conjunction with the heat at the surface of the earth it is rarefied and acquires accelerated motion and gives rise to winds, varying in force from the gentlest breeze to the raging tornado. And so complete is the agitation by the continual disturbance going on in the lower regions by these forces that no difference has ever been discovered in the proportion of the essential components of the atmosphere at the various elevations at which it has ever been possible to collect it for analysis.

The winds are a fertile source of health because they serve to equalize the temperature, to scatter pernicious effluvia and condensed vapors which would otherwise accumulate in fortuitous places. But their immediate effects are favorable or unfavorable to health according to the region from which they set out or pass over. The northeast winds of the Atlantic States are damp and unhealthful because they sweep down the fog and mists of a northern seashore- more or less loaded with putrescible matter. The southerly and southwesterly winds are commonly clear, relatively dry and healthful because they are from warmer latitudes and from the pure surface of the ocean. Sea air is, indeed, the type of purity and promotive of health; pure water-vapor is its only isolation. The deposit of salt on the sails and on shipboard is chiefly from the spray and not from the air. However, the salt of the sea is borne to some extent by strong winds, and in seashore regions it is inhaled. But considering the small amount and

the perfect solubility of such infinitely small particles of pure sea-salt, in the fluids of the system, it is not incompatible with the most delicate organic function under any circumstances. The ill effects sometimes ascribed to sea air are usually owing to the want of ship ventilation and disregard to cleanliness.

The processes of respiration and combustion are perpetually tending to destroy those nicely adjusted proportions of the gases of the atmosphere. But their natural tendency to rapidly permeate each other and become equally diffused is so great that the polluted air is not allowed to accumulate, but rapidly diffuses itself; while the vital air rushes, by counter tendency, to supply whatever deficiencies take place. Hence the universal constancy of the atmospheric elements.

The life-sustaining element of true air is oxygen, a colorless gas without odor or taste. Oxygen is the most generally diffused element in nature. For besides constituting one-fifth of the bulk of the atmosphere and one-third of the bulk of water it is a component of almost all the earths and minerals found on the surface of the globe, and is a universal constituent of all animal and vegetable substances. At every moment of living existence the animal is taking oxygen into its system by respiration. Submitted to animals in its pure state, they breathe it at first with evident delight, but it greatly excites them, quickens the vital processes to such a degree as to throw them into a state of fever and finally kills them by excess of excitement. On the other hand, if an animal be placed into the residue of the air—that from which the oxygen has been abstracted—it suffocates instantly, without the power to take a single breath. The availability of oxygen for life and health depends upon its dilution with nitrogen in the proportion of not less than one part to four, per volume.

The quantity of oxygen consumed in respiration varies according to the condition of the system. Approximately a human adult consumes half a cubic inch of oxygen at every breath. The quantity may be increased by muscular exertion to fourfold what it is in a state of repose. If the exertion is excessive, however, so as to induce fatigue, the consumption of oxygen is diminished. The quantity consumed is also affected by temperature and diet. More is required at a low than at a high temperature, and by persons who chiefly live on animal food rather than those who subsist on vegetables.

The average capacity of the lungs of an adult person for air is about three hundred cubic inches; and when the lungs have been

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