CHAPTER XIX.

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.