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 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.