y, 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
inorganic divisions of the universe prove to be the same. Carbon is
carbon, whether it is part of the substance of a living brain cell, or
black inert coal, or the glistening diamond, or an incandescent part of
the fiery sun. Hydrogen is the same, whether it be a constituent of the
ocean, of the air, or of the living muscle fiber. And so it is with all of
the other elements of the living mechanism. This starts us upon a line of
thought which leads to a significant conclusion, namely, that a living
thing which seems so distinct and permanent is after all only a temporary
aggregate of elements which come to it from the not-living world; existing
for a time in peculiar combinations which render life possible, they pass
incessantly away from the living thing and return to the inorganic world.
Every breath we draw sends out particles which were at one time living
portions of ourselves; every movement we make involves the destruction of
living muscle cells, whose protoplasm breaks down into the ash and gas and
fluid wastes which eventually return to the world of dead things. A tree
loses its living leaves with each recurring season, and the antlers of the
stag are lost annually, to be replaced anew. Indeed the maj