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]
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.