CARBON. - Chemical element with atomic number 6 and atomic weight 12, one of the most important in nature as an essential component of living organisms, although it makes up only 0.1% of the Earth's crust. In the crystalline state it constitutes diamond and graphite; in the amorphous state it is found in the various kinds of coal, both fossil and vegetable; in the combined state it is found bound to oxygen to form carbon monoxide CO, a toxic gas, and carbon dioxide CO₂, which together with the carbonates, derivatives of the latter, constitute the inorganic components of this element; finally, it is a constituent of all organic combinations, so that the chemistry of carbon is called organic chemistry.
The cycle of carbon in nature is most suggestive. The green plant fixes and reduces the carbon dioxide contained in the air (algae that dissolved in water), using the energy of solar radiation in a reaction in which water also participates: oxygen is released and the first organic product of synthesis is formed, from which all organized matter takes its origin. All living organisms, plants and animals, in the process of respiration burn organic matter and return to the environment part of the carbon in the form of carbon dioxide. The sea acts as a stabilizer of the concentration of carbon dioxide in the atmosphere, in that it releases or absorbs it according to whether its concentration in the atmosphere tends to decrease or to increase. Making the carbon cycle more complex are the slow combustion of plant and animal residues, the emanations of carbon dioxide, and the combustions carried out by man (of wood, coal and petroleum) and fermentations: all causes that contribute to returning to the atmosphere the carbon dioxide indispensable for the nutrition of the crops from which come the foods for the continuously increasing human population.