_Water_ constitutes nearly two-thirds of the weight of the body. As water is constantly being given up in the life process, health demands an abundant supply of liquids to replace the waste. The average daily loss has been found to be between two and three quarts. Of this amount the urine constitutes nearly two-thirds; and the remaining third is eliminated through the skin, the lungs, and the bowels. Although the deficiency thus created is met in part by the water in our solid food, the greater part of the loss is made up by the liquids we drink, and we are warned, in a measure, by the sensation of thirst that they are needed. _Mineral material_ is of the greatest importance as a constituent of our food. It contributes to the welfare of the body in at least three ways; (1) it gives rigidity to the bones, (2) it supplies an essential ingredient of the living substance in all the tissues, (3) it is present in the blood and in the other body fluids, where it is of service in such vital processes as the beating of the heart, the transportation of oxygen to every portion of the body, and the maintenance of an acid or alkaline condition of the digestive juices according as the one or the other is necessary for the assimilation of the food. An animal deprived of mineral food will die as surely as one deprived of water. In arranging our diets, however, we are not compelled to take the minerals into account, for, with the exception of table salt (sodium chlorid), the meat and vegetables that we eat provide the mineral material the body requires. Iron, for example, which imparts to the blood one of its most essential qualities, occurs in relatively large amounts in apples, spinach, lettuce, potatoes, peas, carrots, and meats. Only now and then does it become advisable to add iron deliberately to the diet. Similarly lime (calcium) the material that makes the bones hard, is present in quantities ample for the needs of the body in the bread, milk, eggs and vegetables that we eat. The remaining mineral constituents of the body, among which the most conspicuous are magnesium, potassium, sulphur, and phosphorus, occur in foods which we are naturally inclined to take, so that we secure an abundance of them unconsciously. _Protein_, the third food-stuff which we must eat to keep alive, contains the chemical element nitrogen in such form that it can be incorporated in our tissues. Although most persons derive their protein in part from meat, milk, and eggs, it is possible to satisfy the requirements of the body on a purely vegetarian diet. Experience has shown, however, that it is both natural and advantageous that we employ a mixed diet.