Oxygen
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Oxygen Oxygen is one of the most vital elements. People cannot exist without it. This element does not only give us life but also kills bacteria in our bodies without affecting the ones that we need. No other medicine is able to do it exactly like oxygen does. For almost five centuries the chemists all over the world have been arguing about who was the one to discover this element. There were three main candidates for this title. The first one was a pharmacist from Sweden Carl Wilhelm Scheele, the second was an English churchman Joseph Priestley, and the third was a French chemist Antoine Lavoisier. Scheele was the first person who noticed that during the heating of such elements as mercuric oxide, carbonate mercury and silver carbonate they give the same gas, which he called “fire air”. The pharmacist described this phenomenon in his book Chemical Observations and Experiments on Air and Fire, which he wrote in 1775 but it wasn’t published until 1777. Nevertheless, in reality, the secret of the “fire air” remained a mystery to him. The reason for this was a popular in those days theory, on which it was assumed that any substance can be burned only if it has a lot of special combustible matter – phlogiston. Karl Scheele was also a proponent of this theory, so he explained that "fire air" has a high affinity (attraction) to the phlogiston and therefore it burns so quickly. This was quite plausible, but it was really a big mistake in the chemistry doctrine (Farber, 1961). However, in the XVIII century, another great chemist Antoine Lavoisier disproved the myth about phlogiston. After it was done, the strange disappearance of "fire air" and many other obscure phenomena immediately lost their entire mystique. Despite this factor, the second official contender for the discovery of oxygen was the English clergyman and chemist Joseph Priestley. He was also a supporter of the phlogiston theory; nevertheless Priestley made some brilliant discoveries. One of them was proof that plants produce oxygen. He noticed that a mouse in the hermetic jar survives much longer if there is a plant inside it. Having discovered this phenomenon he wrote a letter to his good friend Benjamin Franklin, saying that people need to cut fewer trees because they are very important for the humanity. Nevertheless, his most famous experiment was heating the mercuric oxide with the sun’s rays. As a result, he received so-called "dephlogisticated air" that he tested on himself by puffing it. After that Priestley said that this air was much better than the regular (Anderson & Lawrence, 1987). The third official candidate of the oxygen discovery, Antoine Lavoisier knew that combustible material binds only one part from the five of the air, but the feature of this thesis was unclear. In 1774, when Priestley told him about the discovery of "dephlogisticated air", he immediately realized that this is precisely the part of the air, which is connected to the combustion of combustible materials. Repeating the experiments of Priestley, Lavoisier concluded that the air is a mixture of "life" (oxygen) and "choking" (nitrogen) air, and explained the process of burning a compound substance with oxygen. By 1777, Lavoisier came to the conclusion that it was a special kind of gas, one of the main components of the atmosphere (Holmes & Lawrence, 1985). The studies of Antoine Lavoisier played a prominent role in the development of the chemistry in the XVIII century. It is, above all, the creation for his scientific theory of combustion that marked the rejection of the phlogiston theory, which fundamentally distinguishes his work from the experiments of Scheele and Priestley. Since oxygen is such an important element of life, it does not really matter which one of those scientists has discovered it. Approximately 90% of the body energy is produced with its help. Furthermore, all functions of the body are regulated by it. Without it our brain would not be able to produce millions of items of information, since oxygen promotes the brain function; who knows what would happen without it. Also, it is very helpful for the nervous system, while it helps to overcome fatigue and mental stress. Furthermore, the human body uses it to convert proteins, fats and carbohydrates into energy and life, without it people would not get any fuel for living. The most important use of oxygen is probably respiration. In medicine it is used to treat the respiratory diseases. Also, it is mixed with other gases and used to provide respiration in the aircrafts, spacecrafts and submarines. Liquid oxygen is used as an oxidizer to fuel the big rockets. Formerly, it was the standard atomic weight for elements. It is widely used in the chemical industry to smelt iron ore into steel (Emsley, 2001). Furthermore, oxygen is used to produce a lot of products as polyester and antifreeze. However, despite all the advantages of this element, there is one minus which people usually forget. Oxygen gas can be toxic. Its toxicity usually occurs at elevating partial pressures, which leads to different health problems, for example convulsions. This is not a big problem for everyone, except for the patients on mechanical ventilators. Despite the fact that oxygen has been the part of the humanity from the beginning of its existence, it was discovered only in XVIII century. Nevertheless, from that point people started to use this element almost in every spectrum of their lives, and till nowadays oxygen remains the most demanded component for the human being.

References
Farber, E. (1961). Great chemists. New York: Interscience Publishers. pp. 255–261.
Anderson, R., Lawrence C. (1987). Science, medicine and dissent: Joseph Priestley (1733– 1804). London: Wellcome Trust.
Holmes, Lawrence F. (1985). Lavoisier and the chemistry of life. Madison, Wisconsin: University of Wisconsin Press.
Emsley, J. (2001). Oxygen. Nature's building blocks: An A-Z guide to the elements. Oxford, England, UK: Oxford University Press.