The human body consists primarily of elements, but we ordinarily regard it as made up of compounds. These are groups of elements united in such fixed and constant proportions that they have as uniform properties, under given conditions, as the elements themselves. In discussing the composition and uses of human foods and their relation to the structure, composition, and activities of man as an organism, we refer chiefly to the compounds of carbon rather than to carbon itself. To be sure, the investigator of the problems of nutrition often conducts his researches and formulates his conclusions with reference to the elements, but when the information he secures reaches the language of practice, we speak of proteins, carbohydrates, and fats. Commerce recognizes these compounds, also. It is necessary, therefore, for the student of human nutrition, whether as a scientist or as one who would thoroughly understand dietetics, to become well informed about those substances that in various proportions form the organized structure of plants, and that furnish not only the energies that are manifested by animal life, but all the materials out of which animal tissues are built.

A. Classes Of Matter

Before passing to a consideration in detail of the proximate constitutions of plants and animals, it is desirable to reach a clear understanding of certain broad divisions into which we classify all matter, either living or dead, which has been organized by the vital forces of the various forms of life.

20. Combustible And Non-Combustible

One of the most common and familiar phenomena of the physical world is the destruction of vegetable or animal matter by combustion, with the result that only a small portion of the original material is left behind in visible and solid forms. Fuel, such as wood or coal, is largely consumed when ignited, and we have as a residue the ashes. If we incinerate hay, corn, or wheat, we get the same result. The gradual decomposition of exposed dead vegetable or animal matter that occurs in warm weather is a process essentially similar to the combustion of fuel, only more prolonged. In view of these facts, it is customary to classify all the tissues of plants and animals into the combustible and incombustible portions, the former being that part of the ignited or decayed substance which disappears in the air as gases, and the latter the residue or ash. It should be well understood that combustion does not involve a loss of matter; only a change into other forms. If we were to collect the gases which pass off from a stick of wood that is burned, consisting mostly of carbon dioxid, vapor of water, ammonia, and, perhaps, certain other compounds of nitrogen, we would find that their total weight, plus that of the ash residue, is even greater than that of the dry wood, because the carbon and the hydrogen of the wood have taken to themselves from the air, during the combustion, an increased amount of oxygen. The carbon, oxygen, hydrogen, and nitrogen of the plant or animal tissue belong mainly to the combustible portion, although two of these elements are found in the ash compounds. The remainder of the fifteen elements previously named are supposed to appear wholly in the ash. The relation in quantity of the combustible and incombustible parts of vegetable and animal dry matter and the wide variations in the proportions are illustrated below: -

The significance of these facts is, that the chemical change which we call combustion is one of the phenomena of human nutrition. Substances which may suffer either slow or rapid oxidation outside the human organism may undergo complete or partial combustion within this organism; or, stated in another way, the part of the plant which "burns up" in the fireplace or crucible is the part which in general undergoes the same change within the human organism in so far as the food is digested.

21. Organic And Inorganic

The terms combustible and incombustible are less used, perhaps, than two others, which represent practically the same divisions of plant or animal substance; viz., organic and inorganic. In chemical literature, the portion of a plant or animal which suffers combustion is called the organic, and the ash is known as the inorganic or mineral part. These terms are evidently based upon the erroneous assumption that the compounds which burn and break up into simpler ones are peculiarly those which sustain necessary and vital relations to life, and are formed only through the functions of living organisms. As a matter of fact, many of these so-called organic compounds have been synthesized (built up) artificially, and while the dry substance of the plant is organized chiefly by building up compounds of carbon, oxygen, hydrogen, and nitrogen, which suffer combustion, compounds of sulfur, phosphorus, chlorine, potassium, sodium, and calcium are also constant and essential constituents of the juices and tissues of the plant and animal; and, although the latter elements may finally wholly appear in the incombustible part or ash, they have, nevertheless, sustained in other combinations important relations to nutrition and growth. It is true, however, that the portion of a food material which is commonly spoken of as organic embraces those compounds that furnish practically all the energy which is utilized by animal life and much the larger part of the building material.