body. On the contrary, we have good reason to think that within the body the conversion of one group into another is common. In Chapter III the tissues of the animal body were shown to consist of chemical compounds, which have been classified into certain groups. It has also been stated that the tissues are constantly undergoing chemical changes inseparable from their life, and that for these changes a supply of nutritive material is necessary. The nutriment required for an animal is made up of substances which may be divided into the same chemical groups as the tissues of the body, viz., proteids, fats, carbohydrates, salts and water. So that each of the various substances which we make use of as food, contains in varying proportions several of the different kinds of nutrient material, either naturally or artificially mixed so as to form a complex mass, the important item water being the only one which is commonly used by itself. These substances may be considered to be the chemical bases of the food, as they are also the chemical bases of the animal body. The following classification shows the relationships between the chief constituents of food, from a chemical point of view, and their distribution in the various substances we eat : I. ORGANIC. I. Nitrogenous (A) Albuminous—abundant in eggs,. milk, meat, peas, wheaten flour, etc. (B) Albuminoid-in soups, jellies, etc. 2. Non-nitrogenous (A) Carbohydrates (sugar, starch)-abundant in II. INORGANIC. 1. Salts-mixed with all kinds of food. 2. Water-mixed with the foregoing or alone. The nutritive value of any kind of food depends upon a variety of circumstances, which may be thus summed up :— I. Chemical composition, of which the main points are— 1. The proportion of soluble and digestible matters (true food stuffs) to those which are insoluble and indigestible, such as cellulose, keratin, elastic tissue, etc. 2. The number of different kinds of nutrient stuffs present in it. II. Mechanical Construction.-The degree of subdivision in which the substance is introduced into the stomach materially influences its nutritive value, since the smaller the particles the greater the amount of surface exposed to the action of the digestive juices. The relation of the nutrient to the non-nutrient parts is also of importance, as is seen where the nutritious starch of various vegetables is enclosed in insoluble cases of cellulose, which, if not burst by boiling, prevent the digestive fluids from reaching the starch. III. Digestibility.-This depends partly upon how the substances affect the motions of the intestines, and partly upon their construction. Thus, some substances, such as cheese, though chemically showing evidence of great nutritive properties, by their impermeability resist the digestive juices, and are not very valuable as food. IV. Idiosyncrasy.-In different animals and in different individuals, and even in the same individuals under different circumstances, food may have a different nutritive value. FOOD REQUIREMENTS. Chemically, foods are composed of a limited number of elements similar to those found in the animal tissues, viz., carbon, oxygen, nitrogen and hydrogen, together with some salts. If nothing more were needed by the economy than a supply of these elements and salts in a proportion like that in which they exist in the tissues, such could be easily obtained from inorganic sources; but, as has already been stated, it is necessary that an animal obtain these elements associated in the form of organic materials of complex construction (namely, proteids, etc.). ́ Allowing the necessity of organic food, it might be supposed that since the elements exist in proper proportion in the proteids, an abundant supply of proteids would suffice for all nutritive purposes, and alone form an adequate diet. Theoretically, proteid alone ought to be sufficient for nutrition. It, however, has been frequently tested by experiment, and practically decided, Diagram showing the proportion of the principal food stuffs in a few typical comestibles. The numbers indicate percentages. Salts and indigestible materials omitted. that an animal will not thrive upon a free supply of pure proteid food alone; and in the human subject such exclusive diet would induce dangerous abnormal conditions in a short time. Since nitrogen is an important element in nearly all parts of the body, we could hardly expect that a diet composed of non-nitrogenous food stuffs alone could support the animal economy. In short, the results of numerous experiments show that no one group of the food stuffs enumerated can alone sustain the body, but rather prove that a certain proportion of each is absolutely necessary for life. SPECIAL FORMS OF FOOD. The articles of diet we make use of are animal or vegetable, according to the source from which they are derived. It will be seen that a varying quantity of all chemical classes of food stuffs is present in most kinds of food, whether animal or vegetable. The diagram on the preceding page shows the proportion of the more important food stuffs in some examples of the materials commonly used as food. Among animal foods are included milk, the flesh of various animals, and the eggs of birds. These may be more fully described as typical examples. Milk.-For a certain period of their lifetime the secretion of the mammary gland forms the only food of all mammals, and it is the one natural product which when taken alone affords adequate nutriment. It consists of a slightly alkaline watery fluid, containing— 1. Proteids, casein and albumin in solution. 2. Fats, finely divided to form perfect emulsion. 3. Carbohydrate, sugar in solution. 4. Salts, in solution. 5. Water. Owing to the action of certain organisms which readily propagate in milk, if exposed to the air at a warm temperature for some time, it loses its alkaline reaction, and becomes sour from the formation of lactic acid from the milk sugar, by a kind of fermentation, the probable equation for which may be written thus: CH,O. == 2CHO. If fresh good milk be allowed to stand, the fatty particles tend to float to the surface, thus forming a layer of cream. The milk of different animals is similar in all essential points, but differs slightly in the relative proportion of the ingredients, as may be seen in the following table: Milk varies both in the amount of solids in solution and fat, according to the age and general condition of the animal, period of lactation, time of day, etc. Since human milk is much poorer in proteid, fat and salts (see Table), and richer in sugar, than that of the cow and other domestic animals, it is necessary to dilute the latter with water, and add sugar, when it is substituted for human milk in feeding infants. The great value of milk as nutriment depends upon the fact that it contains every class of food stuff, viz., proteids, fat, carbohydrates, salts and water, in the proportion demanded by the economy; the salts in milk being those required for building up the bones of the infant, viz., phosphates and carbonates of lime, etc. The normal variations in these proportions are not very great, but as adulteration with water is common, a knowledge of the method of testing the purity of milk is necessary. Milk Tests.-The specific gravity of milk gives an easy measure of the solids in solution, but unfortunately it gives no accurate estimate of the amount of fat suspended in the emulsion. Therefore, to test milk adequately two methods must be employed: one to estimate the degree of density of solution, and the other the degree of opacity of the emulsion. I. To test the density, a specially graduated form of hydrometer is generally used. This is graduated so as to indicate specific gravities from 1014 to 1042. The latter being the |