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.