re
is not space in a work of this size to consider them all carefully, but
we may select a few of the vital functions as illustrations of the
method which is pursued. It will be assumed that the fundamental
processes of human physiology are understood by the reader, and we shall
try to interpret some of them in terms of chemical and physical force.

_Digestion._--The first step in this transformation of fuel is the
process of digestion. Now this process of digestion is nothing
mysterious, nor does it involve any peculiar or special forces.
Digestion of food is simply a chemical change therein. The food which is
taken into the body in the form of sugar, starch, fat or protein, is
acted upon by the digestive juices in such a way that its chemical
nature is slightly changed. But the changes that thus occur are not
peculiar to the living body, since they will take place equally well in
the chemist's laboratory. They are simply changes in the molecular
structure of the food material, and only such changes as are simple and
familiar to the chemist. The forces which effect the change are
undoubtedly those of chemical affinity. The only feature of the process
which is not perfectly intelligible in terms of chemical law is the
nature of the digestive juices. The digestive fluids of the mouth and
stomach contain certain substances which possess a somewhat remarkable
power, inasmuch as they are able to bring about the chemical changes
which occur in the digestion of food. An example will make this clearer.
One of the digestive processes is the conversion of starch into sugar.
The relation of these two bodies is a very simple one, starch being
readily converted into sugar by the addition to its molecule of a
molecule of water. The change can not be produced by simply adding
starch to water, but the water must be introduced into the starch
molecule. This change can be brought about in a variety of ways, and is
undoubtedly effected by the forces of chemical affinity. Chemists have
found simple methods of producing this chemical union, and the
manufacture of sugar out of starchy material has even become something
of a commercial industry. One of the methods by which this change can be
produced is by adding to the starch, along with some water, a little
saliva. The saliva has the power of causing the chemical change to occur
at once, and the molecule of water enters into the starch molecule and
forms sugar. Now we do not understand how