This compound is formed when a substance containing carbon is burned in an insufficient supply of air, as for example when the draught is partly shut off in a stove.

The wonderful carbon cycle.

Carbon monoxid is a colorless gas. It burns with a blue flame, forming carbon dioxid. The blue flame seen playing over the embers of a coal fire is carbon monoxid burning. This gas is extremely poisonous.

Carbon dioxid, CO2, is not poisonous.

The poisonous properties of illuminating gas are due to the carbon monoxid which it contains.