"Noi viviamo," wrote Evangelista Torricelli, the 17th century inventor of the mercury barometer,"sommersi nel fondo d'un pelago d'aria elementare", or "We live submerged at the bottom of an ocean of elementary air."
Long before Torricelli, however, there were diverse ideas as to what this ocean might comprise. The Greek philosopher Anaximenes, for example, who lived in the 5th century BC, thought that air could change into water if it was cooled, and into soil or rock if it was compressed. Some years later, his compatriot Aristotle suspected air might have weight. To prove this, he took a leather bag and weighed it when it was "empty" of air, then when it was pressed flat and again weighed the bag when it was "full". To his disappointment he found there was no difference, and concluded, incorrectly, that air is weightless.
Nowadays, we know that air is a cocktail. It consists mainly of oxygen and nitrogen with a little CO2 and hydrogen and contains tiny amounts of other substances like argon, helium, neon, krypton, and xenon.
Nitrogen accounts for about 78 per cent of dry air. It is generated mainly by the decay of agricultural debris and animal matter, and is removed from the atmosphere by other biological processes.
The production and destruction of nitrogen in these ways roughly balance, so that its proportion in the atmosphere remains unchanged with time.
Oxygen is an abundant gas and is also crucial to human and animal life. It accounts for slightly less than 21 per cent of dry air. It is produced by vegetation, which takes up carbon dioxide from the atmosphere and manufactures oxygen through the process of photosynthesis.
On the other side, oxygen is removed from the atmosphere by the respiratory systems of man and animals. These processes are in balance, so the amount of oxygen stays roughly constant.
These two gases together account for about 99 per cent by volume of the air we breath. The other constituents exist in only tiny quantities, although their tenuous presence sometimes belies their great importance in the scheme of things.
Carbon dioxide, for example, plays an important role in regulating the temperature of our planet, while ozone is our shield against the harmful wavelengths of the sun's radiant energy. The atmosphere absorbs water vapour which may comprise up to 5 per cent of its volume, which has profound implications for our planet's weather.