There is another important point in the history of our food, namely, its ultimate composition. We have spoken of starch, sugar, gum, albumen, and other substan ces as the proximate principles upon which we live ; but what is the ultimate consti tution of these secondary products, what are their true elements t It is curious that four elements only are principally con cerned in the production of our food. These are, carbon, hydrogen, oxygen, and nitrogen. Among vegetable sub stances gluten (including vegetable albu men) is the only one which abounds in nitrogen ; gum, sugar, starch, and the rest are constituted of carbon, hydrogen, and oxygen only ; and what is very re markable is, that in all these important principles, and also in lignin, the oxygen and hydrogen bear to each other the same relative proportions as in water, so that they may be figuratively, perhaps truly, described as compounds of charcoal and water. Now there are two very curi ous points in reference to that part of the chemical history of our food which has been adverted to ; the one is, that no animal can subsist for any length of time upon food which is destitute of • and the other, that a certain mixture of different kinds of food is absolutely es sential. An animal fed exclusively on starch, or sugar, or albumen, or jelly, soon begins to suffer in health ; peculiar diseases make their appearance, and his existence is painful and brief; but mix these together and occasionally modify their proportions, and he then thrives and fattens. Alagenclie's experiments on this subject, together with those of Tiedemann and Gmelin, well illustrate this fact. Thus, geese fed upon gum died on the 16th day, those fed upon starch on the 24th, and those fed on the boiled white of egg on the 46th; in all these cases they dwin dled away and died as if from starva tion.
Habit, as is well known, will do much in accustoming the stomach to particular descriptions of food ; many persons live exclusively, or almost so, on vegetable, others on animal matters, and particular kinds of diet are forced on the inhabi tants of many regions of the globe ; but, as far as we are concerned, a due mixture of vegetable and animal matter is not only most palatable, but most conducive to health. The variety in our teeth and the structure of the alimentary canal; point out a mixed food as the most ap propriate. The shortness of the intesti nal canal shows that man was not intend ed to live solely on vegetable diet.
Nothing is fit for food which has not already undergone organization,. and wa ter, though an essential part of the food of all animals, is obviously not in itself nutritious, though it performs the ex tremely important function of dissolving nutritive matter, so as to render it veyable by the lacteals and other absorb ents into the blood. No compound then of nitrogen, hydrogen, carbon, and oxy gen, which can be formed artificially, can constitute food. Air, water, and char coal, though involving the elements of our nutriment, are themselves unfit for our support ,• and it is only by passing through the hidden processes which are carried on in the vessels of living things, that they are so recombined and modified as to be rendered capable of supporting animal life. It is the vegetable world which commences this wonderful operation. Plants absorb their nutriment from the air and from the soil ; they assimilate in inorganic as well as organic matter ; they become the food of the graminivorous tribes, and from these man derives the great bulk of his animal food. Animals cannot create food ; they cannot form any one of these proximate principles we have mentioned ; this is the office of the plant. The plant is a creating being, the animal only assimilates what the plant had formed.
In speaking of the composition of food, that of milk, the most important of all food, must not be forgotten ; in it nature has wonderfully provided a mixture which, though secreted by an animal, partakes also of the nature of vegetable food, and it presents a perfect analogy to that combination of vegetable and animal matter which has been mentioned as most congenial to the palate and stomach.
The albumen or curd of milk is a highly elaborated animal principle, abounding in nitrogen, yet, from its attenuated and soluble state, easy of digestion. A second principle of milk is what is termed augur of milk • in composition and properties It resembles a vegetable product, and is intermediate between gum and sugar. The third component of milk is Irutter, partaking of the nature of vegetable oil and animal fat ; there are certain saline and acid substances in small proportion; and all these matters are either dissolved or suspended in a large relative propor tion of water.
The above table represents the relative proportions of solid digestible matter contained in 1000 parts of the different articles of food which are enumerated. When blood, for instance, is evaporated to dryness, at a temperature not exceed ing 212°, the residue amounts t o 215 parts in 1000, and may be regarded as almost entirely composed of digestible matters ; it consists of albumen and coloring mat ter, with small proportions of saline sub stances. The different kinds of meat were dried in the same way. The loss of weight during their desiccation is al most wholly referable to water ; and the dry residue composed of albumen or fibrine, with some gelatine, and perhaps traces of fat and of saline matters, repre sents the true nutritive value. Upon an average, therefore, the nutritive matter in a pound of meat is not more than four ounces. This, however, only applies to raw meat ; for when dressed a consider able portion of its constituent water is often dissipated. The nutritive matter of wheat is chiefly starch andgluten, and in this species of grain the gluten is in much greater relative proportion to the starch than in barley, oats, or rye. IL rice there is little else than starch. There can be little doubt that the great value of wheat as an article of food depends upon this excess of gluten, which is a nitro genous substance, and has not inaptly been termed the vegeto-animal princi ple. In the esculent roots, such as car rots, &c., but especially turnips, sugar is the leading nutritive matter ; and the common fruits contain sugar, gum, albu minous matter, and acids, together with a highly attenuated form of woody fibre, or lignm, which, in that state, is probably digestible.
The following table shows the ultimate composition of those proximateprinci ples which have been above adverted to as constituting the nutritive part of food.
Tables like the above have been used to point out the nutritive valuea of food; . those which contain the greatest amount of nitrogen being esteemed the most nutri tious ; this view must not, however, be too hastily adopted. If the nitrogen exist in the form of albumen, gluten, fibrin, or casein, any of the forms of what Mulder has termed Protein, the composition of which is, carbon 54.37, hydrogen 7.12, nitrogen 15.98, and oxygen 22.58, in 100 parts, it is undoubtedly true. But many substances contain nitrogen in other' forms, as the mushroom tribe; in these it exists, as ammonia, which does not in any way contribute to the nutrition of an animal. Such substances therefore have not a nutritive value at all equal to what the percentage of nitrogen in them would indicate.
The uses of these proximate principles to the animal frame are different. The substances containing nitrogen go to form the solid parts of the blood and muscle of the animal ; the starch, gum, and sugar assist in respiration and in producing animal heat; the oils and fat serve the same end, and are occasionally stored up as fat in the animal frame to meet the wants of the system when it may require it.