Tables of the composition of foods are usually made in the following terms: Refuse, Water, Protein, Fat, Carbohydrates, Ash. Added to these there will often be a division headed "Calories." The calorie nevertheless is not a food substance, it is the unit by which energy-giving heat is measured. Just as a ribbon is measured in yards and molasses in cupfuls, so heat is measured in calories. "Refuse" means that part of food which cannot be eaten or which could not be used by the body if it were eaten, as bones, fibres, seeds, parings, pods and shells. "Protein" is an inclusive word for the chief substances in food which the body can use in rebuilding itself as use wears it out. "Carbohydrates" are the fuel of the body. They are converted at once into heat and energy, or if there is a surplus they are often stored in the body in the form of fat to be used when nourishment is less abundant. "Fat" is also fuel, a more concentrated form of fuel than the carbohydrates. A certain quantity is stored in the body as a reserve heat supply. The word "Ash" in food tables stands for the mineral matters which are used in our bodies for building bones and teeth, and for a few other purposes; these minerals are for the most part building materials, but are not so important as protein and are needed in smaller quantities. Human bodies are constituted to withstand adversities and to bear the experiments and mistakes which we make; therefore it is that though these food substances usually serve the purposes attributed to them above, yet when need arises the body is able, for a time at least, to use one for the other. This is a provision, however, for special and adverse occasions. Ordinarily food should be supplied in the variety and proportion which will enable the body to use each class of nourishment for its own purpose. Roughly estimated, an average person's diet should be about one-fifth protein, one-fifth fat and three-fifths carbohydrates. That the carbohydrates exceed the others in quantity is easily accounted for. They are not such concentrated fuel as fat, therefore a greater quantity is needed; they are consumed to make heat instead of being built into the body as protein is; therefore, we need more carbohydrates, just as we need to renew the coal supply in a house more frequently than to renew the carpets. The foods from which we derive protein are chiefly meat, fish, milk, beans, peas, bread and other articles made of wheat, corn, oats, and like grains. Vegetables, with the exception of beans and peas, furnish chiefly carbohydrates. Fats are derived for the most part from the animal food which we eat. Butter, for instance, is chiefly fat, and the proportion of fat in bacon is more than half.