Man's physical universe

xanabras

l.U THE EARTH AS MAN'S ABODE

of the atmosphere and man breathes oxygen to burn these foods in his

body to furnish energy.

The weight of the atmosphere is about 5,000,000,000,000,000 tons,

or 30,000,000 tons per square mile.

In temperate regions on the average of at least 100,000 tons of water

\apor are present over each square mile of the earth's surface.

In the

tropics several times this amount of water vapor may be present,

because warm air can hold much more water vapor than cool air.

At high altitudes mountains such as Mount Everest are always very

cold. At thirteen miles above sea level the temperature is 80° F. below

zero. At this height there is very little matter to absorb heat. At this

altitude the atmosphere is so rarefied that no breathing organisms could

survive unless they took a supply of compressed air

them.

or oxygen with

Ninety-five per cent of the atmosphere is contained in the layer

within thirteen miles of sea level.

The Atmosphere May Once Have Had a Different Composition.

It is quite possible that all of the water on the earth was once contained

in the atmosphere at a much higher temperature than that

which now exists on the earth's surface. Perhaps the atmosphere likewise

contained much more carbon dioxide and less oxygen than it does

today. Plants build their body structures largely out of the carbon

dioxide that they take from the air and the water and its dissolved salts

taken from the soil. By this process oxygen is added to the atmosphere.

On the other hand, animals use oxygen from the air to burn the plants

which they eat to produce heat and other forms of energy. Carbon

dioxide is added to the air by the process of respiration, as well as by

the decay of plant and animal tissues and the burning of fuels. It is

removed from the air, on the other hand, by its reaction with substances

in water solution to form vast deposits of carbonates such as

limestone and marble. Today these processes seem to have reached a

condition of equilibrium. At least so we trust, for a very small decrease

in the carbon dioxide content of the air would result in the extinction of

all

plant life.

Whole mountain ranges of carbonates exist ; and tremendous deposits

represent huge amounts (at least 30,000 times as

of peat, coal, and oil

much as in the air today) of carbon which must have been present at

one time or another in the earth's atmosphere. The very luxuriant

plant growth which was responsible for these coal deposits may be

attributed to the higher temperatures and higher carbon dioxide content

of the atmosphere at that time. Undoubtedly plant growth added

a great deal of oxygen to the air during these periods.

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