About the years 350 B.C, the Greek philosopher Aristotle thought that all things were made of four elements -earth, air, fire, and water. And this was believed for nearly 2000 years. In 1774 the English scientist Joseph Priestley discovered the gas oxygen, and three years later the French scientist Antoine Lavoisier proved that air was a mixture of gasses and that about one-fifth of it was oxygen. The other four-fifth, which was mostly nitrogen, was found to be of no use in burning or breathing.
Chemists at that time also knew that air contained a small amount of carbon dioxide and water vapor. Scientists have since discovered five other gasses, which are in the air in small amounts- argon, neon, helium, xenon and krypton. All have some practical use. Argon and neon are used in electric strip lighting, krypton in special bulbs for miners’ lamps and with xenon in electronic flash tubes for photography and in lasers. Helium is used to fill large balloons sent up into the higher atmosphere for weather-recording purposes. Dust and bacteria are also found in air, and this means that special ways of ventilation and purification are sometimes needed.
Man has added to the air various substances that do not occur naturally. Car exhausts and gasses such as carbon monoxide and sulfur dioxide, and other chemical compounds. This occurs mostly near large town with factories.
Although nitrogen is not needed for burning or breathing, it is still very important. Certain bacteria can fix nitrogen, that is, combine it with other elements to form compounds that promote plant growth. Nitrogen is used is making fertilizers, explosives and various chemicals.
Oxygen & carbon dioxide:
Oxygen from the air diffuses, or spreads, into the blood flowing through the lungs of an air-breathing animal and is carried to all the cells of the animal’s body. Living cells use oxygen to release the energy stored in food in a process called respiration. Carbon dioxide is a waste product of respiration and is carried in the blood back to the lungs. From there it separates from the blood and is breathed out into the air, along with a little water vapor.
The mixture of gasses making up the air consists of molecules that are continually in motion. They surround the Earth and are prevented from escaping from it by the force of gravity. The higher the air is above sea level, the thinner it is because the effects of gravity are not as great. The closer it is to the Earth’s surface, the greater the pressure, or downward force, it exerts. At sea level, the pressure of the air is about one kilogram per centimeter square (about 15lb/inch). At high altitude, the air is thinner and mountaineers find it difficult to breathe.
Oxygen is slightly soluble in water, and fish “breathe” dissolved oxygen through their gills. It is also used when substances decay. Oxygen is absorbed by germinating seeds and growing plants. Without oxygen in the air, there could be no combustion, or burning, of fuel in fires, furnaces or engines.
So much oxygen is used in combustion and respiration that one might expect the amount of oxygen in the air to become less and less. But this does not happen because during the day the green leaves of plants take CO2 from the air. And make starch and give back oxygen. However, more carbon dioxide is being produced than can be absorbed by the plants, and building up in the atmosphere increasing the temperature.
Winds and air currents keep the gasses in the air well mixed. In a confined space crowded with people, the amounts of the carbon dioxide water vapor increase. This excess of water vapor makes the air feel free very damp and unpleasant.
Air can be changed to a pale blue liquid by compressing it. And cooling it to a temperature of about-200°C (-328°F). Liquid air is manufactured in great quantities and used in many ways. When the temperature of the liquid air is allowed to rise slowly it is possible to obtain in their pure state each of the gasses present in the air. This method, which is called fractional distillation, is used to obtain pure oxygen, for industry and breathing apparatus, for instance. Liquid air is also a source of argon, helium, krypton or xenon. And it can be used for cooling other gasses to liquefy them.