http://www.bbc.co.uk/radio4/history/inourtime/inourtime_20071115.shtml In 1772, the British chemist, Joseph Priestley, stood in front of the Royal Society and reported on his latest discovery: “this air is of exalted nature…A candle burned in this air with an amazing strength of flame; and a bit of red hot wood crackled and burned with a prodigious rapidity. But to complete the proof of the superior quality of this air, I introduced a mouse into it; and in a quantity in which, had it been common air, it would have died in about a quarter of an hour; it lived at two different times, a whole hour, and was taken out quite vigorous.” Priestley had discovered Oxygen, or had he? Soon a brilliant French chemist, Antoine Lavoisier, would claim the gas for himself. And so began a rancorous dispute between the British and French chemical establishments, undertaken as chemistry itself was in the process of being rediscovered, even revolutionised. http://www.bbc.co.uk/radio4/history/inourtime/research_20071115.shtml -- http://www.juliantrubin.com/bigten/oxygenexperiments.html Everyone needs oxygen to survive – man and animals alike. Furthermore, oxygen is the third most abundant element in the universe and makes up nearly 21% of the earth's atmosphere. Oxygen accounts for nearly half of the mass of the earth's crust, two thirds of the mass of the human body and nine tenths of the mass of water. In this page we will try to outline the path to the discovery of this important substance. Oxygen was discovered for the first time by a Swedish Chemist, Carl Wilhelm Scheele, in 1772. Joseph Priestly, an English chemist, independently, discovered oxygen in 1774 and published his findings the same year, three years before Scheele published. Antoine Lavoisier, a French chemist, also discovered oxygen in 1775, was the first to recognize it as an element, and coined its name "oxygen" - which comes from a Greek word that means “ acid-former”. There is a historic dispute about who discovered oxygen. Most credit Priestly alone or Both Priestly and Scheele. Experiments Oxygen from Minerals In 1772, Carl Wilhelm Scheele discovered that red-hot manganese oxide produces a gas. He called the gas "fire air" because of the brilliant sparks it produced when it came in contact with hot charcoal dust. He repeated this experiment by heating potassium nitrate, mercury oxide, and many other materials and produced the same gas. He collected the gas in pure form using a small bag. He explained the properties of “fire air” using the phlogiston theory, which was soon discredited by Lavoisier. He carefully recorded his experiments in his notes, but waited several years before publishing them. In 1774, Priestley repeated Scheele’s experiments using a 12-inch-wide glass "burning lens", he focused sunlight on a lump of reddish mercuric oxide in an inverted glass container. The gas emitted, he found, was "five or six times as good as common air." In succeeding tests, it caused a flame to burn intensely and kept a mouse alive about four times as long as a similar quantity of air. Priestley, a big supporter of the phlogiston theory, called his discovery "dephlogisticated air" on the theory that it supported combustion so well because it had no phlogiston in it, and hence could absorb the maximum amount during burning. Oxygen from Plants In August of 1771, Joseph Priestley, put a sprig of mint into a transparent closed space with a candle that burned out the air until it soon went out. After 27 days, he relit the extinguished candle again and it burned perfectly well in the air that previously would not support it. And how did Priestley light the candle if it was placed in a closed space? He focused sun light beams with a mirror onto the candle wick (Priestley had no bright source of light, and had to rely on the sun). Today, of course, we can use more sophisticated methods to light the candle like focusing light from a flood light through a converging lens, or by an electrical spark. So priestly proved that plants somehow change the composition of the air. In another celebrated Experiment from 1772, Priestley kept a mouse in a jar of air until it collapsed. He found that a mouse kept with a plant would survive. However, we do not recommend to repeat this experiment and hurt innocent animals. These kinds of observations led Priestley to offer an interesting hypothesis that plants restore to the air whatever breathing animals and burning candles remove - what was later coined by Lavoisier "oxygen". In these experiments, Priestly was the first to observe that plants release oxygen into the air - the process known to us as photosynthesis. --

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