About 2,500 years ago, the ancient Greeks identified air, earth, water and fire as one of the four elemental components of the universe. This notion which seems charmingly primitive now made excellent sense at that time. This is the reason that this idea persisted until the late 18th century.
During this time, the concept of ‘element’ was still evolving and researchers had distinguished no more than two dozen or so elements (depending upon who was counting). It wasn’t clear how will air fit into this table. Nobody knew the composition of air and researchers kept finding some or the other way through which it could be converted into a variety of “form of airs”. The principle method which they used to achieve was to heat or burn some compound in it. This method helped in the discovery of nitrogen (Daniel Rutherford in 1722), carbon dioxide (by Joseph Black in 1754) and hydrogen (by Henry Cavendish in 1766).
But none of these revelations told us the whole story. Researchers were still convinced that air was an element and this thought might have persisted even longer had there not been for a maverick theologian and an English chemist named Joseph Priestley.
Priestley was widely popular was for two reasons: his extremely notorious religious thoughts and his hugely productive research. He is credited for the invention of carbonated water, rubber eraser and identification of a dozen key compounds. He also wrote a paper on the history of electricity.
However, the world recalls him best as the discoverer of oxygen – one of the active ingredients of our atmosphere. In this process, he dethroned an idea which dominated our entire scientific understanding for 23 uninterrupted centuries. Few concepts “have laid firmer hold upon the mind,” he wrote, than that air “is a simple elementary substance, indestructible and unalterable.”
He went on systematically analyse the properties of different “airs” using a unique apparatus: an inverted container over a raised platform. This was done to ensure that the container captured the gases produced by various experiments below it. The container was then placed in a pool of mercury to effectively seal it and test whether it would still sustain flame or support life.
In course of series of these experiments, Priestley made a ground-breaking observation. He observed that flame went out when placed in a jar with a mouse in it. And mouse too eventually died. However, when he placed a green plant inside the jar and kept the jar in sunlight, the mouse survived and flame continued to burn. This is how Priestley observed photosynthesis, the process by which plants release oxygen into the air. Perhaps, Priestley wrote, “the injury which is continually done by such a large number of animals is, in part at least, repaired by the vegetable creation.”
He conducted his most famous experiment on August 1, 1774. He focused sunlight on a lump of mercuric oxide placed in an inverted glass, using a 12-inch-wide glass “burning lens”. He placed this entire setup in a pool of mercury. The gas emitted was found to be five to six times as good as common air. In succeeding tests, he found that it caused the flame to burn intensely and was enough to keep the mouse alive for about four times as long as a similar quantity of air.
He called this gas as “dephlogisticated air” and gave the theory that it was absorbed maximum during burning because it supported combustion so well (A year before, Carl Wilhelm Scheele isolated the same gas and observed a similar reaction. He called this gas “fire air.” But his findings were not published until 1777.)
In the series of experiments ending in 1774, he concludes that “air is not an elementary substance, but a composition,” in other words, it is a mixture of gases. Among them, the highly reactive and colorless gas which he called “dephlogisticated air,” was renamed by Antoine Lavoisier as ‘oxygen’.
Proving his observations correct, scientist now recognizes 92 naturally occurring elements that compose air.