These substances, like starch and sugar, consist of carbon, hydrogen, and oxygen as their chemical elements, but the proportions in which the elements are combined are not analogous to those in the former group; and the fats have other distinctive characteristics also. They are both of animal and vegetable origin. They constitute the greater part of the adipose tissue or fat of animals, more than 25 per cent. of the yolk of eggs, the whole of the butter derived from cow's milk, 9 per cent. of Indian corn, 32 per cent. of olives, and in walnuts and filberts as much as 50 or even 60 per cent. Fat, in some one or more of these forms, is extremely useful and perhaps indispensable as an article of food. The fact that it constitutes over 3 1/2 per cent. of human milk, which is the first and exclusive food of the infant, shows this to be the case, at least for that age; and the general desire which is felt by the healthy appetite for a certain proportion of fat is a sufficient indication of its importance. 4. The last group of alimentary materials comprises albuminoid substances. (See Albumen.) They are distinguished from both the starchy and fatty substances by the fact that they all contain nitrogen; and they are sometimes designated as the nitrogenous elements, in distinction from the others, which are non-nitrogenous. The albumen of the white of egg is one of the most important and familiar.

A substance very similar in composition to albumen, namely, musculine, forms the principal mass of muscular flesh, and is the chief ingredient in lean meat used for food. Caseine is present in milk, and in a co-agulated form constitutes the principal part of cheese. Legumine is found in peas and beans, and gluten is the albuminoid ingredient of i wheat flour. Altogether, an adult man usually consumes rather more than a quarter of a pound of albuminoid matters (calculated in the dry state) during 24 hours. - No one of the groups of alimentary materials enumerated above, taken singly, is sufficient for the continued nourishment of the body. This is sufficiently evident for the inorganic substances, such as water and the mineral salts. Vegetables have the power of assimilating these matters, and converting them into the ingredients of the vegetable fabric; but animals require for their nourishment materials which are already animal or vegetable in their nature. But even these substances, combined with starch, sugar, or oil, are also insufficient. Dumas and Milne-Edwards found that bees fed upon pure sugar and water soon ceased to work, and afterward perished. They thrive only when supplied with waxy and other vegetable substances in addition.

Magendie found that dogs fed upon starch and sugar, or upon an exclusive diet of fat, became after a time debilitated, and died with symptoms of great disturbance of the nutritive functions. Boussingault fed a duck upon butter alone; but, although the quantity of this alimentary substance was abundant, namely, from 1,350 to 1,560 grains per day, the animal at the end of three weeks died of inanition. All the tissues of the body were infiltrated with oleaginous material, but this substance had proved incapable of supporting life. Lehmann put himself upon a regimen consisting solely of non-nitrogenous substances, such as starch, sugar, gum, and oil, but was only able to continue this course for two or at most for three days at a time, owing to the disturbance of the general health produced by it. The unfavorable symptoms, however, rapidly disappeared on his resuming an ordinary mixed diet. The substances just mentioned being deficient in so important an element as nitrogen, this was at one time regarded as a sufficient explanation of their inability to sustain life; and the albuminoid or nitrogenous materials were therefore supposed to be the only absolute and completely nutritious ingredients of the food.

Direct experiment, however, showed that these substances themselves, when taken alone, were also insufficient. Magendie fed dogs upon pure gelatine and pure fibrine, and found at the end of some days that the animals lost their relish for the food, became emaciated, and died with symptoms of inanition. To be completely nutritious, therefore, the food must contain not one but all of the groups of alimentary substances, and these substances must be present in their true proportion. This shows the futility of the attempts which have sometimes been made to fix the nutritive value of different kinds of food by ascertaining their ultimate chemical composition, and particularly by the amount of nitrogen which they contain. The nutritious qualities of an article of food depend upon the proportion of it3 different ingredients, not only as taken alone, but also as used in combination with other substances.

| Its digestibility and the extent to which it conforms to the appetite and natural taste are also important elements in the question. The nutritive value of an article can therefore only be determined by direct experiment and observation; that is, by employing it as food, alone and in combination. Thus all those substances which are found by universal experience to be the most useful are distinguished by a variety of composition. Milk, which for the young infant is during a certain period the only food employed, contains water, mineral salts, caseine or an albuminoid ingredient, butter or fat, and a peculiar variety of sugar. Eggs contain albumen, water, fat, and salts. Wheat flour, as well as the bread which is made of it, contains gluten, water, salts, starch, and a small quantity of sugar. In practice, at least for adults, a judicious variety in the diet is found to be indispensable for the maintenance of health. - Of all articles of food, bread is perhaps the most important. The best and most nutritious bread is that made from wheat flour. The flour contains, in 100 parts, on the average, 72 parts of starch, 7 3/10 parts of gluten, 5 4/10 parts of sugar, and 12 parts of water, together with gum, phosphates of lime and magnesia, alkaline sulphates, and a little chloride of sodium.

It is first kneaded into a paste with about one half its weight of water, a little yeast added and thoroughly mingled with the mass by continued kneading, and the dough then allowed to remain for some hours at a moderately warm temperature. During this time the yeast excites in the sugar of the flour a fermentation, by which it is converted, as in ordinary fermentation, into alcohol and carbonic acid. The alcohol penetrates the dough and escapes by evaporation. The carbonic acid, however, is developed throughout the dough in the form of minute gaseous bubbles, which are confined and entangled by the tenacious gluten of the flour; and the whole mass thus becomes inflated or puffed up by the gaseous expansion. This is the rising or fermentation of the dough. It is then transferred to an oven and kept there at a temperature of about 380° E. until the baking is complete. The effect of baking at this high temperature is as follows: First, the starch upon the outside of the loaf is converted into dextrine and hardened into a brownish, brittle layer, which is the crust; secondly, the gluten throughout is also solidified and at the same time acquires an agreeable and wholesome flavor; thirdly, the starch grains become swollen, fused, and hydrated, fixing permanently in their substance a certain proportion of the water with which the flour was mingled.

Thus, after baking, the bread always weighs more than the flour Of which it was made, owing to the necessary combination of water with the starch in the baking process. Usually one pound of flour is found to produce in this way one pound and a quarter of bread. When removed from the oven and cut open, the interior of the loaf is seen to present a spongy appearance, owing to a multitude of little cavities distributed through its substance. These are the cavities originally produced by the bubbles of carbonic acid developed in fermentation, and which retain their figure in consequence of the stiffening and coagulation of the gluten by the baking process. This is one of the main objects of fermentation; for the spongy texture which the bread thus receives enables it to be more easily masticated and mingled with the saliva and gastric fluids, and thus renders it more healthy and digestible. - Cheese is made by coagulating the caseine of milk with rennet, after which the coagulum is compressed, in order to free it from the watery, oleaginous, and saline ingredients of the milk; and when reduced to a sufficiently solid condition, it may be kept for an indefinite time.

In many kinds of cheese, however, more or less of the oily ingredients of the milk are retained entangled with the caseine, by which it acquires a richer and stronger flavor. Butter, on the other hand, is simply the oleaginous portion of the milk, separated from the remaining constituents. In the natural condition of the milk the butter is in the form of microscopic globules, or spherical masses, of a semi-solid consistency, suspended in a state of minute subdivision in the serous liquid. By the operation of churning, these little globules are made to cohere mechanically together, and gradually the whole of the oleaginous substance is separated in a distinct pasty mass. It is still further freed from the accompanying ingredients of the milk by pressure and kneading under water, and is finally obtained as butter in a nearly pure condition. - The effect of cooking upon food is twofold. In the first place, it softens and disintegrates the substances which are naturally too hard for digestion, and thus renders them amenable to the digestive operations. This is the effect produced upon many vegetable substances, such as starch grains wherever they may be found, and all substances having a resisting envelope or a tough and solid texture, such as peas, beans, potatoes, turnips, and the like.

In animal substances, on the other hand, the most useful effect of cooking appears to be the partial transformation of the albuminoid matters, as in roast meat, by which they acquire a peculiar and agreeable flavor. There is reason to believe that this flavor, besides being pleasant to the palate, is also the indication of a chemical change in the albuminoid matters, by which they are prepared for digestion and become better fitted to subserve the nutrition of the body.