CHAPTER XIV.

COMPOSITION AND PROPERTIES OF AIR.

An abundant supply of fresh air is necessary at all times. And yet its importance is commonly ignored in practical life. Strenuous efforts are made to ensure a supply of food, and water is commonly filtered or otherwise purified before drinking; but many are content to live in an impure atmosphere, which hardly suffices for the preservation of life, and certainly not of health. Deprivation of food, or even of water, only kills after several days or weeks; deprivation of air kills in a few minutes. Only about three pints of water are required daily, while at least 1,500 gallons of air are necessary every day for carrying on the vital functions.

=Composition of Air.=—The air constitutes a gaseous ocean in which we live, as fishes live in water. In virtue of its weight, it exerts a pressure of about 15 lbs. on every square inch. This pressure is usually measured by the _barometer_, and is equivalent on an average to that of a column of 30 inches of quicksilver. (See page 331).

Chemically, air consists of a mixture of various gases and vapours. These are chiefly =Oxygen= and =Nitrogen=; but in addition, there are minute quantities of carbonic acid, argon, hydrogen, water vapour, ammonia, ozone, and suspended matters.

The oxygen and nitrogen exist, in the proportion by volume of 20·9 of oxygen to 79·1 of nitrogen, or of 23·16 grains of oxygen to 76·84 of nitrogen, by weight.

These two gases do not exist in chemical combination, but mechanically mixed. This is proved by the fact, that they do not exist in air in the proportion of their combining weights, or any multiple of these; that the proportion varies slightly at different parts; and that the air which is dissolved in water does not contain the nitrogen and oxygen in the proportion 4 to 1 (as in the atmosphere), but in the proportion 1·87 to 1. This means that oxygen, being more soluble in water than nitrogen, has dissolved in a larger proportion; as it certainly would not have done, had the oxygen and nitrogen been chemically combined. The oxygen dissolved in water supplies fishes with the necessary oxygen for their respiratory processes. Similarly the oxygen in the atmosphere is its most essential constituent, being required in all processes of oxidation (_i.e._, combustion), whether in living organisms or in the inanimate world. Nitrogen serves as a diluting agent. It is incapable of supporting life alone; and many of the fatal accidents which have occurred through men descending deep wells without first testing, by means of a lit candle held well below them, the quality of the air near the bottom, have been due to an accumulation of nitrogen in the well.

=Ozone= is a condensed form of oxygen, which is present in minute quantities in pure air, and especially during a thunder-storm or after a fall of snow, and in the air near the sea. In it three volumes of oxygen are condensed so as to occupy two volumes. In this condensed condition it has powerful chemical affinities; often oxidising substances which oxygen cannot attack. It is generally absent from the close air of towns and dwelling houses, having been used up to oxidise the organic matter present in these places. Air without it is said to be “devitalised”; and ozone has been described as the scavenger of the air.

Ozone can be produced by hanging a piece of moist phosphorus in a room; and it is stated by Dr. Daubeny, that part of the oxygen given out by plants, especially by scented flowering plants, is in the condition of ozone. A small quantity is produced when an electrical machine is worked; its presence is evidenced by a peculiar smell (the name ozone is derived from the Greek word for smell).

=Test of Ozone in Air.=—Traces of ozone in air are detected by exposing strips of blotting paper moistened with a mixture of a solution of potassic iodide and starch. If ozone is present, the paper assumes a blue tint, due to the liberation of iodine, and its combination with the starch. Other acid gases may, however, produce the same effect. A second test should, therefore, be tried. Soak red litmus paper with a very dilute solution of potassic iodide, and expose as before. Potassic oxide is produced if ozone is present, and this turns the litmus blue.

=Aqueous Vapour= is always present in air, though the amount varies greatly. It is invisible in the ordinary condition, but by condensation becomes cloud or fog, rain, snow, or hail. The quantity of moisture present varies with the temperature of the air; the higher the temperature, the more water can be vaporised, without the point of saturation being reached. An increase of 27° Fahr. doubles the capacity of air for moisture. The amount of moisture that would saturate air at 50° Fahr. only gives 71 per cent. of the saturation amount at 60° Fahr. The amount of moisture is estimated by the _hygrometer_ (page 240).

Air saturated with moisture at 32° Fahr., holds vapour equal to 1∕160 of its weight; at 59° it holds 1∕80, at 86° 1∕40, at 113° 1∕20, and at 140° 1∕10.

=Ammonia= in normal air does not exceed one part in a million of air; but it is always present—either as free ammonia or as sulphate, chloride, carbonate, or sulphide of ammonia. From this source, plants derive some of the nitrogen they require as food; some also from the free nitrogen, which is fixed by certain microbes, growing in the nodules connected with the roots of peas, lentils, and other plants (page 274).

Traces of nitrous and nitric acid are also present in the air, produced by the direct combination of nitrogen and oxygen occurring as the result of the electric spark during lightning.

=Carbonic Acid= or carbon dioxide is always present in air, in the proportion of 3·36 to 4 parts in 10,000; but in impure air may be present in much larger amount. It is a heavy gas, incapable of supporting combustion, and therefore of supporting animal life. Being a heavy gas, it tends to accumulate where it is produced, as about lime-kilns by the heating of chalk. Thus CaCO₃ (chalk) (heated) = CaO (lime) + CO₂ (carbonic acid). Tramps have occasionally died of carbonic acid poisoning through sleeping near lime-kilns.

It is produced by the oxidation of carbonaceous matters, hence in all ordinary combustion, in many cases of putrefaction and fermentation, and in the respiratory processes of all animals.

Plants diminish the amount of carbonic acid in the atmosphere. Two processes occur in most plants: a process of respiration, as in animals; and a process of assimilation, by which the leaves and all other green parts of a plant under the influence of sunlight decompose the carbonic acid of the atmosphere, fixing its carbon and liberating its oxygen. Plants such as fungi, which are destitute of green colouring matter, cannot decompose carbonic acid; nor can any plants during the night. During the day green plants are air purifiers; during the night all plants vitiate the air to a slight extent.

=The Air in Relation to Respiration.=—The oxygen of air is absolutely essential for the continuance of life. In every organised animal, _lungs_ or analogous organs are provided, in order to supply the necessary oxygen to the system, and to remove the impure air from it.

The act of breathing occurs in man about seventeen times per minute. While the inspired air is in contact with the interior of the lungs, it undergoes important alterations. It comes into contact with the five or six millions air-vesicles which form the minute dilated terminations of the windpipe, and have an aggregate area of ten to twenty square feet. Each of the air-vesicles has extremely thin walls; and outside these delicate walls lie capillary blood-vessels, full of impure blood. An active interchange now occurs between the air and the gases dissolved in the blood. Oxygen passes through the intervening membrane into the blood, while carbonic acid and other impurities of the blood pass into the air-vesicle. The consequence of this is that the impure dark-coloured blood becomes bright scarlet and pure. This purification is not confined to any one portion of the blood; for the heart contracting 60 or 70 times per minute, pours successive portions of blood into the capillaries surrounding the air-vesicles; while at the same time, pure air is brought into the air-vesicles seventeen times per minute, and so the interchange is constantly kept up.

In view of the incessant character of respiration and circulation, it is clear that all the blood will be purified if the external air is pure; and that if there is any detrimental matter in the air, it probably will come into contact with the blood in the lungs.

The amount of air taken in with each inspiration is about thirty cubic inches. This is called the _tidal air_, as it is constantly ebbing and flowing from and to the lungs. By means of a very forced inspiration, about 100 cubic inches of additional air can be inspired; and similarly after an ordinary inspiration, one can expire forcibly an additional 100 cubic inches, though there will still be left in the lungs another 100 cubic inches of air. Thus:—

_Tidal air_ 30 cub. in.

_Complemental air_ 100 „

_Supplemental air_ 100 „

_Residual air_ 100 „ ———— _Total capacity of lungs_ 330 „

Corresponding to the respiratory changes in the lungs, there are changes in the tissues throughout the body. The pure and oxygenated blood leaving the lungs, is carried to all parts of the system. Oxidation and allied processes are actively carried on, the result of which is the formation of urea, carbonic acid, and smaller quantities of other effete matters. These are then carried by the blood to the excretory organs, urea being chiefly eliminated by the kidneys, and carbonic acid by the lungs.

=Examination of Expired Air= shows that—1. It is _heated_; in its passage through the nose and deeper respiratory passages it has acquired a temperature approaching that of the blood.

2. Its _moisture_ is increased. By the skin and lungs from 25 to 40 ounces of water pass off in the twenty-four hours; the relative amount varies somewhat.

3. It contains 4 to 5 per cent. less oxygen, and _4 per cent. more carbonic acid_ than inspired air. The carbonic acid, instead of being 4 parts in 10,000 of air, becomes over 400 in 10,000, while the oxygen is diminished in a somewhat larger proportion. Thus:—

OXYGEN. NITROGEN. CARBONIC ACID.

Inspired air contains 20·81 79·15 ·04

Expired „ „ 16·033 79·557 4·38

The _amount_ of carbonic acid expired varies under different circumstances. It is increased by active work, by an increase of food, by a diminution of the external temperature; it is greater when the surrounding air is pure, and when it is moist; and it varies with the season, being greatest in spring, and least in autumn.

Children require more oxygen, and expire more carbonic acid than adults, weight for weight. A child six or seven years old requires nearly as much oxygen as one twice that age. Boys usually require more air than girls, as they are more active and exhale a larger amount of carbonic acid and other impurities.

The average amount of carbonic acid eliminated by a healthy adult is at least 0·6 cubic foot per hour, or 14·4 cubic feet per day. This reckoned as carbon is equivalent to 160 grains per hour, or half a pound of carbon in the twenty-four hours. Liebig gives the amount of carbonic acid expired as 0·79 cubic foot per hour, or 19 cubic feet per day.

4. It contains _organic impurities_. These are chiefly gaseous, solid particles only being expired during coughing, or possibly during conversation. The danger from the “breath” of patients in infectious diseases is really associated rather with the dried discharges on handkerchiefs, etc., than from the “breath” itself; unless droplets of saliva discharged during speaking, or mucus during coughing, are directly inhaled.