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