As air may be said to be the very pabulum of life, it is highly essential that it should be pure, -- inasmuch as any deterioration of it never fails to render the blood impure, and thus ultimately to affect both mind and body.

Air covers the entire globe, pressing alike upon land and water, having a depth of about forty-five miles. This vast ocean of air we call an atmosphere, from two Greek words, signifying vapor and space, -- it being an immense fluid sphere or globe. This atmosphere presses upon man, and upon every object on the surface of the earth, with a force equal to fifteen pounds to every square inch. A man of average size has a surface of two thousand five hundred square inches; accordingly, the air in which he lives presses upon him with a weight of eighteen tons. This would of course crush every bone in his body, but for the fluids within him, which establish an equilibrium, and leave him unoppressed.

Pure air contains seventy-nine parts of nitrogen and twenty-one parts of oxygen. If we add a single part more of oxygen to the air, it would no longer be atmospheric air, but aqua fortis, an element capable of destroying everything coming beneath its terrible power.

The quantity of air consumed by a man of average size at each inspiration, is from fifteen to forty cubic inches, according to the capacity of the lungs. Thus, in about an hour, a person consumes about six thousand and sixty-six pints, or two hogsheads of air. This air meets in the lungs in one hour, about one half of that amount of blood, or twenty-four in twenty-four hours. In other words, the quantity of blood which circulates through the system is estimated to be about one-eighth of the weight of the body. So that a man weighing one hundred and fifty pounds will have in his circulation about eighteen and three-quarter pounds of blood. The whole of this large quantity of blood has been proved, by careful experiment, to circulate through the blood-vessels in the almost incredible brief period of sixty-five and seventy-six one-hundredths seconds of time and that is very little over one minute!  This indeed seems wonderful, when we consider the vast extent of vessels it has to travel through; the arteries, the veins, and the minute capillaries through which it must be urged with no little force.

The physiology of the respiratory functions explains the relation of an abundant supply of air to the maintenance of health and the attainment of longevity. Fresh air in the lungs is so immediately essential to life, that most animals in less than one minute, when deprived of it, suffocate, become unconscious, and appear to be dead, -- real death occurring in a few minutes if air is not supplied.

There are at least three objects to be accomplished by breathing, namely: the renewal of the blood and the taking of impurities out of it; the warming of the body; and the finishing up of the process of digestion, and the change of chyle into nutritive blood. That carbonic acid and water are borne out of the lungs with every breath may be easily proved. If we breathe into lime-water, it will become white. This is owing to the carbonic acid in the breath uniting with the lime, and producing carbonate of lime. Then if we breathe upon a piece of glass, it becomes wet, showing that there is watery vapor in the breath. That the blood receives oxygen from the air we breathe, is proved by the fact that the in-going breath has one-fourth more oxygen in it than the outgoing. The lungs, then, take out of all the air we breathe one-fourth of its oxygen. If we breathe it over a second , a third, or a fourth time, it not only has less oxygen each time, and is less useful for the purposes of respiration, but it becomes positively more hurtful by reason of the poisonous carbonic acid which, at every outgoing breath, it carries with it from the lungs.

Equal in importance with the quantity of air we breathe is its purity. The supply of air for an ordinary man to breathe each minute, is from seven to ten cubic feet. Now, suppose a hundred persons to be confined in a room thirty feet in length, breadth, and height, the room containing nearly thirty thousand cubic feet, it follows that the whole air of the room would be rendered unfit for respiration on account of the vast volume of carbonic acid thrown out of the lungs and skin of the one hundred persons thus crowded together. This proves the importance of always having an abundant supply of pure atmospheric air always kept in circulation in crowded assemblies, churches, school-rooms, theatres, factories, workshops, and dwellings.

Consider the effect of sleeping in a small room, seven feet by nine, not furnished with the means of ventilation. If a person sleeps eight hours in such a room, he will spoil during the time one thousand nine hundred and twenty cubic feet of air, rendering the air of the room positively dangerous to breathe. Every disease is aggravated by the breathing of bad air!  Yet it is common to close all the doors and windows where sick persons are confined, lest the patients should take cold. This is a bad practice. The sick should have plenty of fresh air. Their comfort is promoted by it, and their recovery hastened. It is utterly impossible for the lungs to be expanded in an impure atmosphere, because the air-passages, irritated by the extraneous particles, spasmodically contract to keep them out. The consequence of this is, those persons who reside permanently in an atmosphere charged with foreign ingredients or miasms, find their lungs continually contracting.

All sedentary habits weaken the abdominal muscles, and thereby lessen the activity of the breathing process. Intense, mental application, if long continued, powerfully diminishes the respiratory functions. Persons habitually in deep thought, with the brain laboring at its utmost capacity, do not breathe deep and free, and are consequently short-lived. All crooked or constrained bodily positions affect respiration injuriously. Reading, writing, sitting, standing, speaking, or laboring, with the trunk of the body bent forward, is extremely hurtful. In all mechanical or manual labor, the body should be bent or lean on the hip joints. The trunk should always be kept straight. Dispense with bed-curtains, if you can. In sleep the head should never be raised very high, as that position oppresses the lungs; nor should the sleeper incline toward the face with the shoulders thrown forward.

Grates and fire-places secure much better ventilation than stoves. No stove, especially furnaces, should be used without the means of the free admission of external air into the room. Lamps, candles, gas-burners, etc., are so many methods of consuming oxygen and rendering the air irrespirable. Smoking lamps are a very common source of vitiated air. The bad air of steamboats, railroad cars, stages, omnibuses, etc., are a source of constant suffering to many. I may here remark that the general misapprehension of the theory of catching cold frequently produces the evil sought to be avoided. More colds are taken in over-heated than in too cold places, and still more are owing to vitiated or foul air. In sleeping and other apartments, where thorough ventilation is impossible, the air may be rapidly changed and materially freshened, by opening all the doors and windows, and then swinging one door violently forward and backward. The rules of ventilation apply to all rooms and apartments alike, whether in dwelling-houses or travelling vehicles. There is not necessity for breathing air which has lost a part of its oxygen and acquired a portion of carbonic acid. The supply of good air is ample.

In connection with a full supply of atmospheric air to every human being, the importance of plenty of sunshine is not to be overlooked. Pure air for the lungs and bright sunlight for the eyes, is a physiological maxim which should never be forgotten. The nutritive process is materially checked in all vegetable and animal life when deprived of light for a considerable time. In the case of vegetables, they become etiolated or blanched. Almost the entire population of our large cities who occupy back rooms and rear buildings where the sun never shines, and cellars and vaults below the level of the ground, on the shaded side of narrow streets, is more or less diseased. Of those who do not die of acute diseases a majority exhibit unmistakable marks of imperfect development and deficient vitality. During the prevalence of epidemics, as the cholera, the shaded side of a narrow street invariably exhibits the greatet ratio of fatal cases. A certain amount of shade is essential to comfort, but when it reaches the point of excluding sunshine to a large degree, it becomes a positive evil. Let us always welcome the visits of the healthful air and glowing sunshine, and look out continually for the essential conditions of vigor and cheerfulness.