Carbon dioxide (CO2) is a colourless, odourless and non-poisonous gas formed by combustion of carbon and in the respiration of living organisms. It is a linear molecule with two double bonds holding the oxygen atoms to the central carbon. These two double bonds are strong, making carbon dioxide a relatively stable compound.

An archaic term for carbon dioxide is "fixed air" from the fact that it is denser than regular air, and will settles to the bottom of a container without mixing with other gases. Joseph Black (1728 - 1799), showed that 'Magnesia alba' (white magnesium), on being heated lost weight owing to the escape of this fixed air - which was named carbonic acid by Antoine Lavoisier (1743 - 1794) in 1781 - and that the weight was regained when the calcined product was made to reabsorb the fixed air with which it had parted. These investigations, were described in the thesis De humore acido a cibis orto, et magnesia alba, which he presented for his doctor's degree in 1754; and a fuller account of them was read before the Medical Society of Edinburgh in June 1755, and published in the following year as Experiments upon magnesia, quicklime and some other alkaline substances..

John Dalton (1766-1844) represented carbon dioxide with the symbol a solid circle between two hollow circles

John Dalton (1766 - 1844) in A New System of Chemical Philosophy, published in 1808, revived the atomic theory of matter. He identified a formula for "carbonic acid" represented as a solid circle between two hollow circles.