Carbon dioxid is an important factor in the life activity of the earth. The leaves of plants absorb carbon dioxid from the air, and by means of the chemical activity of the green coloring-matter or chlorophyl, the plant has the power of combining the carbon dioxid with water, and with the mineral salts which have been absorbed from the earth by the roots of the plant. Sunlight is necessary to this action, especially in the manufacture of starch. This formation of food material in plants by the combination of simple chemical substances, such as carbon dioxid and water, is one of the fundamental life-processes. Animals do not possess this power of utilizing simple or inorganic chemical compounds, therefore they must take their food substances in the more complex forms which have been created by the power of sunlight acting upon the plant. [Sidenote: The wonderful carbon cycle] 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 Thus we see that the carbon dioxid which is withdrawn from the air, by the growth of plants, is constantly replaced by combustion, and in this way the "carbon cycle" is completed.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. CARBON MONOXID (CO) 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. [Sidenote: Properties of carbon monoxid] 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. ORGANIC CARBON COMPOUNDS 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. [Sidenote: Combining power of carbon] 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]