Carbon dioxide (CO₂ ) is a compound consisting of one carbon atom and two oxygen atoms. It was the first gas to be distinguished from ordinary air, perhaps because it is so intimately connected with the cycles of plant and animal life. When we breathe air or when we burn wood and other fuels, carbon dioxide is released; when plants store energy in the form of food, they use up carbon dioxide. Early scientists were able to observe the effects of carbon dioxide long before they knew exactly what it was.

Around 1630, Flemish scientist Jan van Helmont discovered that certain vapors differed from air, which was then thought to be a single substance or element. Van Helmont coined the term "gas" to describe these vapors and collected the gas given off by burning wood, calling it gas sylvestre. Today we know this gas to be carbon dioxide...