When we leave the sleeping room, we open the windows to admit air. We may with advantage treat our lungs to an air bath by standing at the open window or by going out of doors for a few minutes to take in five or ten deep breaths. Next, perhaps, we shall use drafts of air to help us make a fire in the range or in a fire place.

Air is a real substance. It can be weighed. The air in a room 15 feet by 20 feet by 10 feet high weighs 210 pounds, and would fill ten ordinary water pails if liquified. Air will expand and may be compressed like other gases and it has been liquefied by intense cold and pressure. It requires considerable force to move it. When a bottle is full of air, no more can be poured in. Our houses are full of air all the time. It pervades all things - the cells and tissues of our bodies are full of air.

Wood and some metals even contain a little. In breathing we take a little from the room, but it is immediately replaced by expired air, which is impure. Were there no exits for this air, no pure air could enter the house, and we should die of slow suffocation. The better built the house the quicker the suffocation. Fortunately no house is air tight. Air does pass out through the walls and cracks, and comes in around doors and windows, but unless there is a great difference in the temperature indoors and out, this fresh air is neither sufficient to replace the bad air nor to dilute it beyond harm. Therefore in ordinary weather, the air of all rooms must be often and completely changed either by special systems of ventilation or by intelligent action in the opening of doors and windows.

The atmosphere surrounds the earth to a depth of fifty miles or more. The effect of gravity of the earth on this mass is to produce a pressure or weight of air on all things. This pressure is about fifteen pounds on each square inch, but we do not notice it, for the pressure is the same on all sides of us and the internal pressure in the cells of our bodies balances the external pressure of the atmosphere.

If it were not for the pressure of the air, we could not drink lemonade through a straw or pump a pail of water. When we exhaust part of the air by suction, we remove part of the pressure over the liquid in the straw and the air pressure on the surface in the glass forces the liquid up the straw. The same principle applies in a pump - the air is partially taken off the top of the water in the pipe, and then the pressure outside forces the water up in the pipe and by a proper valve arrangement, it is made to run into the pail. See Fig. 9.

Air Pressure

The pressure of the atmosphere at the sea level is sufficient to force water up into a vacuum about 34 feet vertically; but owing to mechanical imperfections of pumps, the practical limit is 27 or 28 feet rise between the surface of the water and the valve of the pump. It is customary to use a force pump if water is to be raised to a height above this. Fig. 10.

Unlike water, air is not the result of a chemical union of two unlike simple gases. Nevertheless, air contains more than one substance. It is made up chiefly of two gases simply mixed together, and each exhibits its own characteristics to some extent.

Pure air consists of oxygen, which we have found constitutes one-third of water, and of nitrogen (and argon). The oxygen forms about a fifth and the nitrogen four-fifths of the air. Besides these, several other gases are found in small but varying quantities.

To the oxygen gas is due the power of air to support combustion (fire) and life. Oxygen unites chemically with most other substances, and were the air all oxygen, the combustible part of the. earth would soon be consumed by its own fires. Fortunately four-fifths of the air is a gas that has little power of combination and this nitrogen serves to dilute the oxygen and to weaken its force, much as water would dilute and weaken a strong and powerful chemical.

The most marked characteristic of nitrogen is its sluggishness or inertness. Nitrogen, like oxygen, is a tasteless, odorless, colorless gas. It is fourteen times as heavy as hydrogen. Though nitrogen from the air unites with other elements with difficulty, it is found in all living tissues, both animal and vegetable, and when these decompose the familiar substance, ammonia, is formed. This is a compound of hydrogen and nitrogen.

Carbon dioxide is always present in the atmosphere. This is one of the countless combinations of carbon, the element present in all animal and vegetable materials. Carbon is nearly pure in the form of charcoal. Soot, graphite or the black lead of lead pencils, and the diamond are other forms. Carbon unites very readily with oxygen and the gas formed by their chemical union is called carbon dioxide because it contains two parts of oxygen to one of carbon. Wood, coal, gas - almost everything that will burn in the air - and even our own bodies contain carbon, though we would not suspect its presence because it is combined with other substances and has merged its own character in those of the substances of which it forms a part. All our food contains carbon in its combinations.

When we breathe we take into our bodies the oxygen of the air. This oxygen is needed by the various organs and is carried in the blood from the lungs to all parts of the body. During the circulation the oxygen is taken up by the cells and replaced by carbon dioxide. This is brought back by the blood to the lungs and breathed out. If we remain long in a closed room, a portion of the oxygen of the air in the room and of the substance of our bodies is changed into carbon dioxide, which is unfit to breathe. This is the reason for the special need of ventilation in the sleeping room.

Water in the form of vapor is constantly passing off into the air from the surface of bodies of water, from vegetation, and from animal organisms, as invisible vapor. The amount of water vapor present in the air is very variable. Warm air will hold more vapor than cold air. Ordinarily on a pleasant day, the atmosphere holds between 60 per cent and 70 per cent of the possible amount of water vapor.

Carbon Dioxide

Water Vapor

When the air is saturated or at the dew point, a slight lowering of the temperature causes the vapor to condense. That air will absorb only a certain amount of moisture explains why a draft of air is necessary when drying clothes within doors and why the washing drys slowly on a damp day.

The presence of vapor in the air is shown by bringing a pitcher of ice water into a warm room. The air against the cold surface of the pitcher is cooled until the dew point is reached, when it deposits part of its moisture. Any person who wears glasses knows the effect of such condensation in going into a warm room from out of doors on a cold day. That the air exhaled contains water may be shown by breathing upon any bright, cold surface.

The discomfort we feel in a crowded room is largely due to the excess of moisture resulting from the breathing and perspiration of so many persons. The danger of going from .a crowded reception or "tea" into the open air is also due to it. Crowded rooms become very warm, the air soon becomes saturated with vapor and cannot take away the perspiration from our bodies. Our clothes thus become moist and the skin tender. When we go into the colder, drier air, clothes and skin suddenly give up their load of moisture. Evaporation absorbs heat; the heat is taken from our bodies and a chill results. There is much to learn concerning the ventilation of rooms for social purposes.

Dew Point

How a Chill is Produced

The air also contains a very small amount of a gas called argon. This was discovered in 1894. It resembles nitrogen so closely that it long escaped detection. Several other gases are present in minute quantities.

Fig. 9. Suction Pump

Fig. 10. Force Pump