may take place slowly or rapidly, the two rates being
designated as _slow_ oxidation and _rapid_ oxidation. Examples of slow
oxidation are found in certain kinds of decay and in the rusting of iron.
Combustion is an example of rapid oxidation. Slow and rapid oxidation,
while differing widely in their effects upon surrounding objects, are
alike in that both produce heat and form compounds of oxygen. In slow
oxidation, however, the heat may come off so gradually that it is not
observed.

*Movement of Oxygen through the Body.*--Oxygen has been shown in the
preceding chapters to pass from the lungs into the blood and later to
leave the blood and, passing through the lymph, to enter the cells. That
oxygen does not become a permanent constituent of the cells is shown by
the constancy of the body weight. Nearly two pounds of oxygen per day are
known to enter the cells of the average-sized person. If this became a
permanent part of the cells, the body would increase in weight from day to
day. Since the body weight remains constant, or nearly so, we must
conclude that oxygen leaves the body about as fast as it enters. Oxygen
enters the body as a _free_ element. The form in which it leaves the body
will be understood when we realize the purpose which it serves and the
method by which it serves this purpose.

*Purpose of Oxygen in the Body.*--The question may be raised: Is it
possible for oxygen to serve a purpose in the body without remaining in
it? This, of course, depends upon what the purpose is. That it is possible
for oxygen to serve a purpose and at the same time pass on through the
place where it serves that purpose, is seen by studying the combustion in
an ordinary stove (Fig. 54). Oxygen enters at the draft and for the most
part passes out at the flue, but in passing through the stove it unites
with, or oxidizes, the fuel, causing the combustion which produces the
heat.

[Fig. 54]

Fig. 54--*Coal stove* illustrating rapid oxidation.

Now it is found that certain chemical processes, mainly oxidations, are
taking place in the body. These produce the heat for keeping it warm and
also supply other forms of energy,(45) including motion. It is the purpose
of oxygen to keep up these oxidations and, by so doing, to aid in
supplying the body with energy. It serves this purpose in much the same
way that it supports combustion, _i.e._, by uniting with, or oxidizing,
materials de