Another form of fuel is even more necessary than coal and wood. In the winter we warm our rooms so that we may not suffer from the cold; but the stove does not warm us when out of doors. Then we put on our heavy winter wraps, but these give us no warmth: they merely keep in the heat of the body or keep out the cold blasts of the wind. We all know that the body is warm of itself; that there is something within us that produces heat, like a fire. When our fingers become chilled by the frosty air we may warm them with our breath. The temperature of a room may be seventy degrees or less, but if we place the bulb of a thermometer beneath the tongue we shall find that the mercury rises to ninety-eight degrees.
The fire in the body and the fire in the stove act very much alike. If the draughts of the stove are closed tight and no air is admitted, the fire dies down and goes out. If the air which enters the body is foul, the fire feels the effect and our health is injured. If the lungs are filled with water or anything else which keeps out the air, the fire goes out and life is lost.
The fuel which we call food is just as necessary for the fire in our body as is wood or coal for the fire in the stove. Three times a day or oftener we take this food-fuel into our bodies; thus we keep the fire steadily burning which makes us warm and keeps us alive.
On the other hand, fuel for the body must be very different from fuel for a stove. In the stove heat alone is wanted; therefore one form of fuel is enough. In the body bones must be enlarged and strengthened, muscles must be developed, fat must be provided in sufficient quantities, and brain-matter must be produced. Therefore the food-fuel must provide not only heat but also the different materials of which the body is made. One kind of food is necessary for the bones, another for the blood, another for the flesh, and another for the nerves. Thus while in studying common fuel we have only to learn about wood, either in the form of trees or pressed into the form of coal, in studying food-fuel we find that the kinds are almost numberless. Meat and vegetables, fish and fruit, roots and nuts, in their infinite varieties, are all included in the word food.