Definition: Food may be defined to be any natural substance, vegetable or animal, recognized as such, that has undergone neither the process of fermentation nor that of putrefaction. Food may be considered in its relation to two purposes--the nutrition of the tissues, and the production of heat. Under the first of these heads will be included many other allied functions, as for example, secretion and generation; and under the second, not the production of heat only as such, but of all other forces correlated with it, which are manifested by the living body. Foods derived from the animal kingdom are called nitrogenous substances, or azotized. They are also known by the name of proteids. These are mainly derived from meat, milk, eggs, etc. Of several we will examine the chemical composition. It will be well to state in general terms that all food substances contain in their composition from two-thirds to three-fourths, or even more, of water--some more, some less. Proteids. Albumen. Caseine. Syntonin. Gluten. Gelatine. Carbon, 72 Hydrogen, 112 Oxygen, 23 Nitrogen, 18 Sulphur, 1 Phosphorus, R. 2 Non-Nitrogenous Substances. Carbon. Hydrogen. Oxygen. 1. Starch (amyloids), 18 30 15 Sugar cane, 12 22 11 2. Oils and fats composed of stearic acid 18 36 2 of mutton or beef, 3. Mineral--Saline matters, as chloride of sodium, phosphate of lime. Animals cannot subsist on any but organic substances, and these must contain the elements which are naturally combined with them--in other words, not even organic compounds are nutritive unless they are supplied in their natural state. Pure fibrine, pure gelatine, and other principles purified from the substances naturally mingled with them, are incapable of supporting life for more than a brief time. Moreover, health cannot be maintained by any number of substances derived exclusively from one only of the two chief groups of elementary principles mentioned above. A mixture of nitrogenous and non-nitrogenous organic substances, together with the inorganic principles which are severally contained in them, is essential to the well-being, and generally even to the existence, of an animal. The truth of this is demonstrated by experiments performed for the purpose; and is also well illustrated by the composition of the food prepared by nature as the exclusive source of nourishment to the young mammals, namely milk. The composition of milk is: Human. Cow's.