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inflated, by the first respiration of the babe, they are never again destitute of air to the end of life. The constant residuary amount of air in the lungs is about one-third of their cubic capacity. By respiration this residuary air is alternately augmented and diminished, and, by the diffusion of gases, the whole quantity is renewed by about every ten respirations of pure air. The half a cubic inch of air that disappears every respiration is wholly oxygen. It is replaced by a relative proportion of carbon dioxide, exhaled.

The average amount of oxygen thus consumed by a healthy adult person being half a cubic inch every respiration, amounts in a day to about fifty thousand cubic inches, or more than twentyfive cubic feet. Now, as oxygen constitutes but one-fifth of the volume of the air, a single adult person renders not less than one hundred and twenty-five cubic feet of air unfit for respiration every twenty-four hours. What becomes of the enormous quantity of oxygen thus introduced into the system? It is in part returned to the atmosphere in the form of carbon dioxide and the vapor of water. And since no part of the oxygen taken into the system is otherwise given off, and as the carbon and hydrogen are supplied in the food, it is evident that the amount of nourishment required by the animal body to maintain it in health must be in direct ratio to the quantity of oxygen inhaled.

Ozone is intensified oxygen; oxygen in allotropic form; a variation in physical properties without change of substance. Ozone, though a normal constituent of the atmosphere, is far from being always present; and is never present in the air of populous places, of occupied rooms, or over foul soil. It has a remarkably strong affinity for putrescible matter; it is a deadly germicide and a most powerful oxidizing agent; hence its absence from air that is charged with putrescible matter of one kind or another is explicable by this property. Moreover, this property has been rendered available for the sterilization of water and the purification of sewage.

Ozone is most frequently present in the atmosphere of the ocean, in or near forests and in mountainous regions distant from dead and decomposing matter. It is generally formed during thunder storms, and is sometimes so abundant in favoring regions as to be irritating to the respiratory mucous membranes, causing sore throat and predisposing to bronchitis and pneumonia. Its presence in the air of any place indicates the absence of putrescible matter; it is thus indirectly a proof of purity. But it is very doubtful whether ozone itself is beneficial to animal life.

Nitrogen is distinguished by its negative qualities. It is frequently called azote, from two Greek words, which signify in ability to support life. A burning taper is instantly extinguished in this gas, and an animal soon dies in it, not because the gas is injurious, but from the deprivation of oxygen. Its function in the atmosphere appears to be only that of a diluent of the oxygen, reducing its strength and regulating its activity.

Argon and its congeners, recently discovered, are allied with nitrogen; and so far as yet known, like it, inert.

Carbon dioxide is like oxygen and nitrogen in being devoid of color, but unlike them in that it possesses a slightly pungent odor and a perceptibly sour taste. Besides, it is a compound substance of two elements, carbon and oxygen (CO2); and although universally present in the atmosphere in the average proportion of 0.4 per 1,000 volumes, it is liable to exist in variable and dangerous proportions in circumscribed places. It is constantly given off in the process of animal respiration and is always produced when carbon in any form or any compound of carbon is burnt with a free supply of air. It is non-inflammable and irrespirable. An animal immersed in it dies instantly. Even when greatly diluted with air it cannot be respired for any length of time without producing insensibility. Air containing more than its average normal proportion is always detrimental to health. It acts upon the system as a narcotic poison, hence the danger of crowded and illventilated quarters.

The weight of carbon dioxide as compared with the other gases of the atmosphere is about one-half greater. This fact suggests at first thought that carbon dioxide ought always to be found in larger proportion on the surface of the earth, and nearer the floors of ill-ventilated buildings than the ceilings. But the truth is that although carbon dioxide is the heaviest gas of the atmosphere it is quite as abundant in the air at the greatest elevation at which the atmosphere has ever been examined, as it is at the surface of the earth; and in crowded and illy ventilated buildings, particularly if they are warmed, it is frequently more abundant at the ceilings than at the floors. This is due to the natural law of gaseous diffusion before referred to.

The quantity of carbon dioxide thrown off by respiration, like the consumption of oxygen, varies according to circumstances, It is doubled in the same person in passing from a temperature of ninety-five degrees to sixty-five; and in the same ratio as temperature diminishes. During active exercise and after full meals the quantity of carbon dioxide exhaled is much increased. On the other hand the quantity is diminished by repose and sleep. The quantity also differs somewhat according to peculiar conditions of age and sex.

The amount of vapor retained in the atmosphere varies with the temperature, and more or less with relation to surface conditionssea and land, arid and wooded regions, altitude and sea levels, geographical positions, the seasons and prevailing winds. But at the same temperature and under the same pressure the maximum quantity is invariable. It is rarely less than one two-hundreth, or more than one-sixtieth of the volume of the atmosphere anywhere, and gives a mean ratio of 0.84 of the volume. Excessively dry air, as it frequently is over arid regions and high altitudes, where there is but little rainfall, and in schoolhouses and other inhabited buildings heated by radiators or hot air furnaces, without adequate provision for fresh air and moisture, devours moisture of the nostrils and eyes and is a frequent cause of catarrh, bronchitis and ophthalmia.

Excessive moisture of the atmosphere apart from unclean local conditions, as it is over the ocean and commonly in pine forests, as compared with that of high altitudes and arid regions, is not by itself insalubrious. But such air has a powerful affinity for organic matter in process of decomposition, which process it retards and thus promotes conditions congenial to the cultivation and propagation of microbic organisms. Hence in estimating the relation of atmospheric humidity to health, the condition of the locality with relation to putrescible matter in the soil, or roundabout, should be always considered. And with special reference to assemblies, in schoolhouses, churches and halls, it always should be borne in mind that the exhalation of vapor by respiration adds to the danger of crowds in close quarters. Expired air is loaded with vapor—that is to say, it is completely saturated. The absolute amount added to the air by respiration with the temperature and the quantity of vapor already existing in the air before it is respired; but under ordinary conditions at a temperature of from 60° to 70° F., and the relative humidity of the air being not above 75 per cent. of saturation, an adult gives off by the lungs about half an ounce, and by the skin an ounce of water-vapor hourly. By analysis one thousand parts of vapor thus exhaled by the

lungs contain pure water, 907 parts; carbon dioxide, 90 parts, and animal matter, 3 parts. And such air in thoughtless crowds is sometimes respired again and again!

The amount of air respired by an adult individual daily, under ordinary conditions, is about 400 cubic feet. It contains four or five per cent. less oxygen; four or five per cent., or one hundred times more carbon dioxide than it contained before it was respired; it is saturated with water-vapor-no matter how dry it may have been when taken into the lungs—and a quantity of putrescible animal matter estimated at about three parts in every 1,000 parts of the water-vapor exhaled. Brown-Sequard and d'Arsonval reported in 1888, as the result of several experiments, that the condensed liquid from expired air contains a volatile poison resembling a ptomaine; and if a cubic centimetre of the liquid be injected into a rabbit it quickly dies. Earlier observers have recorded similar results by enclosing animals in glass cases, absorbing the carbon dioxide produced, and supplying oxygen ; yet death ensued. Nevertheless, more recent investigations upon this point by Haldane and Smith, at Oxford, indicate that the results of the experiments of Brown-Sequard and d’Arsonval must be capable of some other interpretation than that expired air contains organic matter which is of the nature of a volatile poison. Their chief conclusions are to the effect that (1) the immediate danger from breathing air highly vitiated by respiration arises from the excess of carbon dioxide and deficiency of oxygen, and not from any special poison; (2) that any hyperpnoea which ensues is due to excess of carbon dioxide and not to the corresponding deficiency of oxygen; the hyperpnæa usually appears when the carbon dioxide is present to the extent of from three to four per cent.; (3) that the frontal headache so commonly produced by vitiated air is due to the excess of carbon dioxide; (4) the hyperpnæa from defect of oxygen begins to be appreciable when the oxygen in the air breathed has fallen to a point which appears to differ in different individuals. Similar conclusions have been formulated by Bergery, Weir Mitchell and Billings, as the result of their inquiry into "the composition of expired air and its effects upon animal life.” They believe that the discomfort produced by crowded, ill-ventilated rooms, in persons not accustomed to them, is not due to the excess of carbon dioxide, nor to bacteria, nor, in most cases, to dusts of any kind. The two great causes of such discomfort, though not the only ones, are excessive temperature and unpleasant odors. These odors which are perceptible to most persons on passing from the outer air into a crowded, ill-ventilated room may be due in part to volatile products of decomposition contained in the expired air of persons having decayed teeth, foul mouths, or certain disorders of the digestive apparatus, and in part to volatile fatty acids produced from the excretions of the skin, and from clothing soiled with such excretions. The direct and indirect effects of odors of various kinds upon the comfort, and perhaps also upon the health of men are probably more considerable than are indicated by any tests now known for determining the nature and quantity of the matters which give rise to them.

Altogether the weight of evidence is that the quickly fatal effects of close and unventilated quarters is due to the excess of carbon dioxide and the diminition of oxygen, and not to the animal matter, which, though doubtless poisonous, is less rapidly so than the deprivation of oxygen and excess of carbon dioxide.

In ordinary respiration in the open air the force with which we expire tends to keep the respired air for a time in suspension. But for this force the carbon dioxide would fall from its greater specific gravity than the other gases. The water-vapor suspends the animal matter, pure air commingles with the whole, the vapor condenses and falls or disperses according to the atmospheric temperature, and the animal matters continue to float, fall, oxidize and decompose according to the conditions and plenitude of the surrounding medium.

In crowded and illy ventilated or artificially lighted quarters, and in dense outdoor assemblies this disposition of respired air is retarded or varied. Carbon dioxide emitted by burning gas lamps or candles is not more than half as heavy as the air, and it readily ascends and is diffused, if not impeded; but otherwise, in close quarters, it quickly cools and becomes heavier and then occupies the lower stratum. In close and warmed and artificially lighted quarters there are frequently present, besides the exhalations from the lungs and skin, sulfuretted and carburetted hydrogen—the common results of imperfect modes of warming and lighting—and ammonia gas, the result of decomposed animal matter.

The danger of the repeated passage of the same air through the lungs is cogently expressed by the following incidents: The half-civilized nabob of Calcutta, in 1756, enclosed 146 persons

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