I have already explained how carbon dioxid may enter the air. Thus we see that the carbon dioxid which is withdrawn from the air, by the growth of plants, is constantly replaced by combustion, and in this way the "carbon cycle" is completed. This is one of the most beautiful adaptations in nature. If the plant did not remove the carbon dioxid from the air, it would soon accumulate in such quantities as to become detrimental to life, and, on the other hand, if this gas were not returned to the air by combustion, by the breathing of animals, and by the decay of plants, the vegetable world would soon be without carbon dioxid, which is as essential to plant life as is the oxygen of the air to animal life.
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.
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.
The carbon compounds thus far considered have been mentioned to illustrate a few of the simpler or inorganic forms of carbon. We will now begin the study of _organic chemistry_ or the compounds of carbon which are commonly found only in plant and animal substances.
Carbon has wonderful powers of combination with other chemical elements, and may combine with the same elements in thousands of different proportions. This property of carbon to form so many different compounds is considered one of the fundamental facts of chemistry upon which life depends. For example:
[Sidenote: Carbon and hydrogen compounds]
Oxygen can combine with hydrogen in but two proportions--peroxid of hydrogen (H₂O₂) and water (H₂O)--while carbon and hydrogen can combine in more than a hundred different compounds. The simpler of these are acetylene (C₂H₂) and marsh gas or methane (CH₄), which is the fire-damp in mines.
The compounds containing carbon, hydrogen, and oxygen number into the thousands. A great many substances formed in plants contain these three elements, such as fruit-acids, alcohol, sugar, and fats.