*Digestion not a Simple Process.*—Digestion is by no means a simple process, such, for instance, as the dissolving of salt or sugar in water. These, being soluble in water, dissolve at once on being mixed with a sufficient amount of this liquid. The majority of the nutrients, however, are insoluble in water and are unaffected by it when acting alone. Fats, starch, and most of the proteids do not dissolve in water. Before these can be dissolved they have to be changed chemically and converted into substances that are _soluble in water_. This complicates the process and _prevents the use of water alone_ as the digestive solvent. *A Similar Case.*—If a piece of limestone be placed in water, it does not dissolve, because it is insoluble in water. If hydrochloric acid is now added to the water, the limestone is soon dissolved (Fig. 62). (See Practical Work.) It seems at first thought that the acid dissolves the limestone, but this is not the case. The acid produces a chemical change in the limestone (calcium carbonate) 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.