_Water_ is found in all parts of the body, even in the very solid portions, as the bones and the enamel of the teeth; it also constitutes a large proportion of its semisolids and fluids, some of which are nearly all water, as the perspiration and the tears. Water usually is found combined with some of the salts, which seem to act as regulators of the amount which shall be incorporated into a tissue. Water is a necessary constituent of all tissues, giving them a proper consistency and elasticity. The power of resistance of the bones could not be maintained without it. It is also valuable as a food solvent, assisting in the liquefying of different substances, which are taken up by the various absorbent tubes, conveyed into the blood, and so circulated through the body. Most of the water of the body is taken into it from without, but it is also formed in the body by the union of hydrogen and oxygen.[29] _Sodium chlorid_, or common salt, is found in the blood and other fluids, and in the solids of the body, except the enamel of the teeth; it occurs in greatest proportion in the fluids. The part that this salt plays in nutrition is not altogether understood. "Common salt is intermediate in certain general processes, and does not participate by its elements in the formation of organs" (Liebig). Salt is intimately associated with water, which plays an intermediate part also in nutrition, being a bearer or carrier of nutritious matters through the body. Salt seems to regulate the absorption and use of nutrients. It is found in the greatest quantity in the blood and chyle. It doubtless facilitates digestion by rendering foods more savory, and thus causing the digestive juices to flow more freely. Sodium chlorid is contained in most if not all kinds of food, but not in sufficient quantity to supply the wants of the body; it therefore becomes a necessary part of a diet. _Potassium chlorid_ has similar uses to sodium chlorid, although not so generally distributed through the body. It is found in muscle, liver, milk, chyle, blood, mucus, saliva, bile, gastric juice, and one or two other fluids. _Calcium phosphate_ is found in all the fluids and solids of the body, held in solution in them by the presence of CO_{2}; both it and calcium carbonate enter largely into the structure of the bones. _Sodium carbonate_, _magnesium phosphate_, and other salts play important parts in nutrition. The various salts influence chemical change as well as act in rendering food soluble. For example, serum albumen, the chief proteid of the blood, is insoluble in pure water, but dissolves easily in water which has a little neutral salts in it.[30] Salts also help to give firmness to the teeth and bones.