When any form of carbon or carbonaceous matter burns, it forms a gas called carbon dioxide. If there is insufficient air or oxygen and considerable heat, a lower form of the oxide, called carbon monoxide, is the result of the chemical change. Carbon dioxide has a slight taste, but no odor and will not burn. Hence it is used as a fire extinguisher. Carbon monoxide is a very poisonous gas. It is a constituent of illuminating gas and burns with a blue flame.