Chlorine, a greenish yellow gas, is a major industrial chemical which is prepared commercially by electrolysis of sodium chloride. It has two stable isotopes. In natural chlorine approximately 75% of the atoms have the mass number 35 and 25% have the mass number 37. It is created by heating hydrochloric acid with manganese dioxide:
Geber (c.760 - c.815), wrote instructions for making hydrochloric acid. Chlorine was discovered by the Swedish chemist and pharmacist Karl Wilhelm Scheele (1742 - 1786) in 1774, although he thought that it contained oxygen. The discovery of liquid chlorine by Michael Faraday in 1823 led to a lengthy study by Faraday into the liquefaction of gases. Scheele noted the suffocating odour of the gas, and also that in solution it bleached cotton cloth. Before 1810, it was known as oxymuriatic acid. The British chemist Humphry Davy (1778 -1829) proved that it was an element in that year, and as the name oxymuriatic acid was no longer appropriate, gave it the name chlorine after the Greek word Chloros meaning "greenish yellow".