rbonate) and converts it into a compound
(calcium chloride) that is soluble in water. As fast as this is formed it
is dissolved by the water, which is the real solvent in the case. The acid
simply plays the part of a chemical converter.

[Fig. 62]

Fig. 62--The dissolving of limestone in water containing acid, suggesting
the double action in the digestion of most foods.

*The Digestive Fluids.*--Several fluids--saliva, gastric juice, pancreatic
juice, bile, and intestinal juice--are employed in the digestion of the
food. The composition of these fluids is in keeping with the nature of the
digestive process. While all of them have water for their most abundant
constituent, there are dissolved in the water small amounts of active
chemical agents. It is the work of these agents to convert the insoluble
nutrients into substances that are soluble in water. The digestive fluids
are thus able to act in a _double_ manner on the nutrients--to change them
chemically and to dissolve them. The chemical agents which bring about the
changes in the nutrients are called _enzymes_, or digestive ferments.

*Foods Classed with Reference to Digestive Changes.*--With reference to the
changes which they undergo during digestion, foods may be divided into
three classes as follows:

1. Substances already in the liquid state and requiring no digestive
action. Water and solutions of simple foods in water belong to this class.
Milk and liquid fats, or oils, do not belong to this class.

2. Solid foods soluble in water. This class includes common salt and
sugar. These require no digestive action other than dissolving in water.

3. Foods that are insoluble in water. These have first to be changed into
soluble substances, after which they are dissolved.

*Summary.*--Materials called foods are introduced into the body for
rebuilding the tissues, supplying energy, and aiding in its general work.
Only a few classes of substances, viz., proteids, carbohydrates, fats,
water, and some mineral compounds have all the qualities of foods and are
suitable for introduction into the body. Substances known as drugs, which
may be used as medicines in disease, should be avoided in health. Before
foods can be passed into the body proper, they must be converted into the
liquid form, or dissolved. In this process, known as digestion, water is
the solvent; and certain chemical agents, called enzymes, convert the
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