having twice as many hydrogen atoms as there are oxygen atoms, the same
proportion that the hydrogen bears to the oxygen in the compound water,--a
characteristic which makes it easy to remember the general constitution of
carbohydrate as compared with the protein. The substances of this second
class are obviously much less complex, both as regards the different kinds
of atoms and in respect to the numbers of each kind that enter into the
formation of a single molecule. Therefore the carbohydrates do not possess
so much power or energy as the protein molecule; in short, they are not
such good fuels for the living mechanism.

Finally, we find almost always in protoplasm other substances composed of
carbon and hydrogen and oxygen which are called hydrocarbons,
distinguished from carbohydrates by the fact that the number of oxygen
atoms is less than half the number of hydrogen atoms. These substances are
the fats and oils of various kinds, less powerful sources of energy than
the proteins, but they contain more potential energy than the
carbohydrates because they are more oxidizable.

Besides the characteristic substances of these three classes, protoplasm
contains certain other chemical compounds, like the various salts of
sodium, chlorine, magnesium and potassium, and a few others, which bring
the list of chemical elements to the number twelve. We have already noted
how strikingly small and restricted is the list of elements composing
living matter as compared with the long array of eighty-odd different
kinds of chemical atoms existing in the world as a whole.

But an astonishing result is reached through the brief analysis we have
just made. It is this: we do not find _peculiar_ kinds of atoms which
occur exclusively in living matter; the materials are exactly the same as
those of the outer world. In short, the elements of both the organic and