17. THE DISCOVERY OF OXYGEN

by Maurice Goldsmith

For centuries it was recognised that there was some active part in the air. The Chinese called it yin; the Italian artist, Leonardo da Vinci (1452-1519) stated that the air was not completely consumed in respiration or combustion and said there were two gases in the air. George Ernst Stahl (16601734), Professor of Chemistry at Halle University, in 1697 stated that bodies that burned possessed a property of combustibility that he called phlogiston (from the Greek, phlox or flame): bodies that did not burn did not contain this property, and were therefore "dephlogisticated".

In 1731, Stephen Hales (1677-1761), heated saltpetre (potassium nitrate), and made oxygen, but did not realise that this gas he had prepared was also found in the air. The original discovery is attributed to the English clergyman Joseph Priestley (1733-1804). In 1775, he informed the Royal Society of how he filled a test-tube with mercuric oxide and inverted it over a pan of mercury. He heated the red powder with the sun's rays by focusing them on the test-tube with a burning glass 12 inches in diameter. A gas was given off, and a heavy silver-white metallic liquid was left. A burning candle placed in the gas shone brilliantly: a mouse placed in a jar of the gas rushed about excitedly. But the Swedish chemist, Karl Wilhelm Scheele (1742-1786), had also prepared oxygen about the same time. His account was not published until 1777, so Priestley obtained first credit for the discovery. Scheele called his gas empyreal or fire air; Priestley called it dephlogisticated air, because it did not contain the property of combustibility but absorbed it from

around. He found that in burning and in breathing it was the dephlogisticated air that was used up. Both Scheele and Priestley understood that air was a mixture of two gases, one of which was "combustible" and the other "useless". But they did not understand the real nature of combustion. The limiting factor for them was the phlogiston theory of combustion. This is a most interesting example of the way in which the accepted view seems to put the mind in blinkers. But the phlogiston theory was an important advance on man's previous level of understanding in its day; and neither Scheele nor Priestley, brilliant investigators though they were, could step out of their day, although their researches led to the abandonment of the phlogiston theory and laid the foundation of modern chemistry.

It was left to the brilliant Frenchman, Antoine Laurent Lavoisier (1743-1794), to effect the great revolution that was to lead chemistry away finally from alchemy and make it a qualitative science. When he heard of Priestley's experiments, he realised that the theory of phlogiston did not correspond to the experimental facts. The new gas alone was responsible for combustion. He called it oxygen or acid-producer (from the Greek, oxys, sharp or acid; gennao, I produce), a word he coined himself because he thought it to be an essential component of acids. What a great change now occurred in chemistry, as revolutionary as the industrial and political changes sweeping Europe!

(from The Young Scientist's Companion, Souvenir Press, London, 1961)