Carbon is a very common element, which exists widely in the atmosphere, crust and living things in various forms. The understanding and utilization of simple carbon has a long history, and a series of compounds of carbon-organic matter are the basis of life. Carbon is one of the components of pig iron, wrought iron and steel. Carbon can chemically combine with itself to form a large number of compounds, which is an important molecule in biology and commerce. Most molecules in organisms contain carbon. Carbon was discovered in prehistoric times, and carbon black and coal were the earliest forms of human use of carbon. About 2500 BC, China knew that diamonds and coal, as a form of carbon, were used in the Roman era in the same chemical way as in modern times: wood covered with clay was heated in spine buildings to remove air. 1722, René Antoine Ferchault de Réaumur proved that iron can be transformed into steel by absorbing some substances, which are known as carbon. In 1772, Anthony lavoisier proved that diamond is a form of carbon. When he burned some samples of diamonds and coal, he found that they did not produce water, and the amount of carbon dioxide produced by each gram of diamonds and coal was equal. In 1779, karl william scheele showed that graphite, once considered as the existing form of lead, was actually a mixture of carbon and a small amount of iron, and he gave the name of the product when it was oxidized with nitric acid ("air acid"), namely carbon dioxide. 1786, French chemists Claude Louis Berthollet, gaspard monge and C.A. Vandermont treated diamond graphite oxide with lavoisier, which proved that graphite was almost entirely composed of carbon. 1789, lavoisier listed carbon in the list of elements in his textbook. It is rare for elements to move from one place to another under ground conditions. So the carbon content on the earth is an effective constant.
The flow of carbon in nature constitutes the carbon cycle. For example, plants absorb carbon dioxide from the environment to store biomass energy, such as carbon respiration and calvin cycle (a process of carbon fixation). Some biomass can be transferred by predation, while some carbon is exhaled by animals in the form of carbon dioxide. The structure of carbon cycle is much more complicated than the pattern diagram on the right. For example, some carbon dioxide will dissolve in the ocean, and the remains of dead plants or animals may form coal, oil and natural gas, which can release carbon through combustion, but bacteria cannot use it.