this saliva possesses this
power to induce the chemical change. But apparently the process is of
the simplest character and involves no greater mystery than chemical
affinity. We know that the saliva contains a certain material called a
ferment, which is the active agent in bringing about the change. This
ferment is not alive, nor does it need any living environment for its
action. It can be separated from the saliva in the form of a dry
amorphous powder, and in this form can be preserved almost
indefinitely, retaining its power to effect the change whenever put
under proper conditions. The change of starch into sugar is thus a
simple chemical change occurring under the influence of chemical
affinity under certain conditions. One of the conditions is the presence
of this saliva ferment. If we can not exactly understand how the ferment
produces this action, neither do we exactly understand how a spark
causes a bit of gunpowder to explode. But we can not doubt that the
latter is a purely natural result of the relation of chemical and
physical forces, and there is no more reason for doubting it in the
former case.

What is true of the digestion of starch by saliva is equally true of the
digestion of other foods in the stomach and intestine. Each of the
digestive juices contains a ferment which brings about a chemical change
in the food. The changes are always chemical changes and are the result
of chemical forces. Apart from the presence of these ferments there is
really little difference between laboratory chemistry and living
chemistry.

_Absorption of food_.--The next function of this machine to attract our
attention is the absorption of food from the intestine into the blood.
The digested food is carried down the alimentary canal in a purely
mechanical fashion by muscular action, and when it reaches the intestine
it begins to pass through its walls into the blood. In this absorption
we find engaged another set of forces, the chief of which appears to be
the physical force of _osmosis_. The force of osmosis has no special
connection with life. If a membrane separates two liquids of different
composition (Fig. i), a force is exerted on the liquids which cause them
to pass through the membrane, each passing through the membrane into
the other compartment. The force which drives these liquids through the
membrane is considerable, and may sometimes be exerted against
considerable pressure. A simple experiment will illust