Elements of surprise : oxygen's poisonous life force.

Oxygen, the element that keeps us alive, has surprising properties. The chemical revolution began in the 1780s with three chemists - Joseph Priestley, Antoine Lavoisier and Carl Scheele - who all investigated the chemistry of burning. Priestley, who believed in phlogiston, could not understand how mercury became heavier when burned, however Lavoisier realized something had been absorbed from the air. Lavoisier overthrew the Aristotelian view of matter by making a list of clear names which showed chemical compounds. Ozone - with three oxygen atoms - proved to be a good bactericide and so was used in water treatment and to fumigate cinemas. Twenty years ago the Ozone Layer, 50 miles above the earth, was discovered. Too little oxygen can be lethal - if levels drop from 20% to 15% we die. Too much - 25% - and a spark can make the air explode.