THE PURIFICATION OF AIR.
In addition to the artificial measures which will be discussed in the
next chapter, various natural agencies are constantly at work for
the removal of the impurities discussed in preceding chapters. Of
these, the most important are the action of plants, the fall of rain,
natural methods of ventilation, and certain natural constituents of the
atmosphere.
1. =Plants=, by virtue of the chlorophyll contained in their green
parts, absorb carbonic acid from the atmosphere, liberating oxygen in
an active condition. In addition, ammonia and nitrous and nitric acids
are dissolved from the air by rain-water, and assimilated by plants.
During the night plants only give off carbonic acid.
2. =The Fall of Rain= clears the atmosphere of any solid particles
contained in it, the impurities being transferred to rain-water which
generally contains an appreciable amount of ammonia as well as other
impurities. Rain not only washes and purifies the air, but by washing
the ground, diminishes dust, and prevents its escape into the air. It
is the great natural scavenger.
3. =Ventilation=—that is, the interchange of pure and impure air,
is constantly being effected. Before entering on the details of
ventilation, we must consider the _physical causes_ at work which
tend to purify the air, apart from all artificial contrivances. These
are three in number—namely, diffusion, winds, and differences of
temperature of masses of air.
(1) =Diffusion= causes the rapid mixture of gases placed together.
Every gas diffuses at a certain rate—namely, inversely as the square
root of its density. In any room which is not air-tight, diffusion
is constantly occurring, air passing in and out at every possible
point. Through chinks and openings in the carpentry-work of a room,
the air diffuses rapidly. Bricks and stone commonly allow air to pass
through them; diffusion occurs to a slight extent even if the wall is
plastered, but very little through paper. Diffusion alone is quite
insufficient to purify a room under ordinary circumstances; and solid
particles including the organic matter evolved from the skin and lungs,
not being gaseous, are unaffected by it. To remove these, the room must
be periodically flushed with air, and washing of all dirty surfaces
must be carried out.
Diffusion sometimes produces evil results, when the sanitary
arrangements of a house are bad. If there is a leakage of sewage under
the kitchen floor, the foul gases from it diffuse upwards; occasionally
foul air diffuses from the dust-bin through the wall into the rooms
of a house. These results are helped by the fact that the internal
temperature of a house is commonly higher than the external.
(2) =Differences of Temperature= cause active movements of air. In fact
winds are caused by movements between large masses of air of unequal
temperature and consequently of unequal density. Light gases ascend, as
familiarly illustrated by the smell of dinner perceived in bedrooms,
or the smell of a cigar lit in the hall perceived in the attic. In
rooms differences of temperature of the air are caused by the heat of
fire, gas, and our own bodies. Currents of air result; the warmer and
lighter air ascends up the chimney or towards the ceiling, while colder
and denser air rushes in under the door or through the floor, etc.
The lighter gases carry with them solid particles in suspension and
thus tend to remove the most important impurities. Assuming that the
external air is colder, if admitted into the lower part of a room, it
produces a draught; if admitted at the top of a room, being heavier, it
falls by its own weight on the heads of those in the room. The problem
of ventilation is to secure a sufficient interchange of air without the
production of perceptible currents.
Movements of air are constantly occurring, so long as the temperature
of the air is subject to changes. This cause alone will suffice to
ventilate all rooms in which the air is hotter than the external air.
It may thus happen that a room with windows and doors closed in winter,
may possess purer air than the same room in summer with these thrown
widely open. The value of diffusion of air through the walls, and the
influence of temperature on this diffusion are well illustrated by some
experiments of Pettenkofer.
When the difference between the outside and inside temperatures was
34° Fahr. (66° inside and 32° outside), and the doors and windows were
shut, an ordinary room in his house, of the capacity of 2,650 cubic
feet, which was built of brick, and furnished with a German stove
instead of an open fire-place, had its entire atmosphere changed once
in an hour. With the same difference of temperature, but with the
addition of a good fire in the stove, the change of air rose to 3,320
cubic feet per hour. On lessening the difference between the external
and internal temperature to 7° Fahr. (64° and 71°), the change of air
was reduced to only 780 cubic feet per hour. In these experiments, all
crevices and openings in doors and windows were pasted up.
It is instructive to note the greater amount of ventilation effected
through the walls, etc., than by the draught of the stove.
The amount of ventilation through walls varies with the material of
which they are built. Mortar is exceedingly porous when dry; sandstones
and bricks are easily permeated by both water and air. Limestone is
almost impervious to air, but requires much mortar in building, which
effects a partial compensation (see page 206).
The rise of temperature caused by the bodily heat and by the combustion
of illuminating agents, is well shown by some figures of Dr. Angus
Smith. He found that the rise of temperature of 170 cubic feet of air
in one hour, produced by the bodily heat of one man was 5°·6 Fahr.; by
the combustion of a candle 3°·8 Fahr. Thus, in a room 8 feet high, 4
feet broad, and 6 feet long, a man burning a candle would in an hour
raise the temperature from 60° to 70° Fahr. This rise in temperature
would not only cause currents of hot air towards the upper part of the
room, but would probably make the room uncomfortable, and so lead to
the opening of a door, etc.
(3) =Winds= are of great value in flushing rooms with fresh air. They
ought to be utilised as often as possible, by throwing windows widely
open; without, however, taking the place of constant ventilation in
the intervals. They are especially valuable in getting rid of organic
matters which are unaffected by diffusion.
The wind will pass through wood, and even brick and stone walls. When
it is allowed to pass directly through a room, as from window to door,
it produces a more powerful effect than can be produced in any other
way. The average rate of movement of winds in this country is 10 feet
per second, or about 7 miles an hour. If the surface which a man
exposes to this average wind = 6’ × 1½’ = 9 square feet, then 90
cubic feet of air flows over him in one second, and 324,000 in an hour.
If 3,000 cubic feet were the allowance for each person indoors—a much
greater allowance than is usually given—he only receives 1∕108 of the
air with which he is supplied in the open.
Winds act as a ventilating agent in two ways—=directly by perflation=,
driving impure air before them, or freely mixing with it; and
=indirectly by aspiration=, drawing the impure air along with them. In
the last case, the wind causes a partial vacuum on each side of its
path, towards which all the air in its vicinity flows. Thus, the wind
blowing over the top of a chimney causes a current at right angles
to itself up the chimney. In a spray-producing apparatus we have a
familiar instance of the same principle, the current of air or steam
along the horizontal tube causing the fluid to rise in the vertical
tube till it is scattered in spray. In Sylvester’s plan of ventilation,
both these forces are used (see page 150).
4. =Certain Constituents of the Atmosphere= have an important purifying
effect. Of these oxygen is by far the most important. By its means
organic impurities become oxidised, and thus rendered harmless. It is
probable that much of this oxidation is effected by means of ozone—a
peculiarly active and concentrated form of oxygen. A large part of this
ozone is probably produced during thunderstorms and similar electrical
disturbances of the atmosphere. The ammonia and organic impurities in
air become changed into nitrites and nitrates—chiefly of ammonium—and
being washed down by rain, form an important part of the food of plants.
5. =For Chemical Measures= of purification of the atmosphere see page
324.