example, is prone to combine with other atoms in definite varied ways, and
the high degree of complexity which the protein molecule possesses may
depend in greater part upon the combining power of its carbon elements.
The nitrogen atom makes the protein an extremely volatile compound, so
that the latter burns readily in the tissue cells; and the hydrogen and
oxygen bring their specific characteristics to the total molecule. And
furthermore, it is evident that the great complexity of this constituent,
protein, gives to protoplasm its power of doing work, or, in a word, its
power of living. In constructing it, much energy has been absorbed and
stored up as potential energy, and so, like the stored-up energy in a
watch spring or in gunpowder, this may be converted, under proper
conditions, into the kinetic energy and the work of actual operation. On
account of its peculiar and complex nature, it possesses great capacity
for burning or oxidization, thus serving as a source of vital power. It
burns in the living tissue just as coal oxidizes in the boiler of an
engine; its atoms fly apart and unite with oxygen so as to satisfy their
chemical affinities for this substance. If we could only see what happens
to the protein molecule when it undergoes oxidization, we would witness a
violent explosion, like that of a mass of gunpowder. And the astonishing
fact is that this process is actually the same for the living molecule,
for exploding gunpowder, and for the fuel which burns in the locomotive
boiler. Does this mean that the essential process of what we call life is
a chemical one? So it would seem on the basis of this fact alone, but a
conclusion must be deferred until we reach a later point.

The second kind of substance which we find in protoplasm is the
carbohydrate. A typical member of this group is common sugar,
C_{6}H_{12}O_{6}; another sugar has the formula C_{12}H_{22}O_{11}. Starch
is again a typical carbohydrate, and its formula is C_{6}H_{10}O_{5}, or
some multiple of this. One sees at a glance that these substances agree in