Each living thing has its food, without which it dies. This food may vary within certain limits; beyond them disease sets in, even if life continues.
Every child who has kept chickens or rabbits knows how carefully and regularly they need to be fed. Every child who has grown house plants or cultivated a garden plot knows how necessary air, water, and soil are for their life and growth.
It is only needful to transfer this knowledge to ourselves to see that we, as living beings, need our food in the same way ; and that air and water, as well as meat and milk, sugar and eggs, are our foods, without which we cannot live.
The baby's food is milk, which contains all the substances needed except oxygen of the air. This must be breathed in through the lungs. To milk is added for the child of two years starch in various forms, rice, potatoes,, wheat bread, corn mush, etc. For the six years old there are added a few fruits and vegetables, eggs and a very little meat.
Animals, man included, are made up of millions of small cells, which do their work in a like manner. They live upon the ready prepared substances which the blood stream carries to them and change this material to forms which the green plants can use again, giving off CO 2 , water, ammonia, etc.
These cells, like the molds and other destructive plants, can live on either vegetable or animal matter, that is, we can eat either lettuce, peas, corn, apples, or meat, and derive health and strength from them all, but, like the green plants, we must have water in abundance and air, only we take oxygen from the air and give back to it the carbon dioxide which the green plants need.
In the course of this transformation in the living cells, some of the energy which the sunlight gave to the plant that was eaten is released, and our bodies make use of this energy to keep warm, to work, think, and feel.
The source of bodily heat, of human energy and power to work, is in the chemical changes which the food, whether animal or vegetable, undergoes in the cells and tissues of the body.
Everything is food for something else, each after its kind, and matter, carbon, oxygen, hydrogen, and nitrogen, for instance, is kept circulating like gold and silver, which is now made into amulets and images, now lining drinking cups, now buried in the earth, now stamped as coin and passing from hand to hand until melted and worked into rings again. It is gold or silver all the while.
The food of mankind is to be here considered, and we begin with the earliest and simplest, the food of the infant, milk. This is chiefly composed of five great classes of food stuffs, classes we shall find in all natural food materials.
Water; eighty-seven per cent, more than three-fourths of the whole, and we shall find that our food must always contain a large amount of water, or if it does not, that we must drink liquids to make up the amount.
The curd of the milk ; which contains not only casein but albumen and other proteids. 1 This class of substance is also found in all living things, whether animal or vegetable.
Fat; which we know best as cream or butter. This is also common to both vegetable and animal substances, as olive oil and fat of meat.
Sugar; dissolved in the water of the milk so that we do not recognize it at once. Except this sugar of milk, most sugars are derived from the juices of plants and their fruits, but so abundant and universal are these that all races of mankind have sugar in their diet.
Mineral salts ; also dissolved in the water of milk, so that we do not think about them in that form. Salt, as we put it on our food, and the fertilizer we water plants with are instances of mineral food. The mineral salts are as needful for the life of the cell as the other classes. As has been proved by experiment, it is not difficult to learn the composition of milk and the amount of the various substances which the child requires.
