Chemistry has satisfactorily demonstrated the nature of those proximate principles of organic matter, upon the presence of which the nutritive qualities depend, viz. fibrin, albumen, gelatin, oil and fat, gluten, fecula, or starch, mucilage, sugar, acids, etc. Although a combination of two or more of these principles appears essential, yet assuming that the variety, observable in the nutritive value of different substances, arises from the predominance of some one of them, we may conveniently distribute the Nutrientia into the following nine classes: -

CL. I. Fibrinous Aliments

Comprehending the flesh and blood of various animals, especially such as have arrived at puberty: venison, beef, mutton, hare.

CI. II. Albuminous

Eggs; certain animal matter.

CL. III. Gelatinous Aliments

The flesh of young animals: veal, chickens, calf's foot, certain fishes.

CL. IV. Fatty And Oily Aliments

Animal fats, oils, and butter; cocoa, etc.; ducks, pork, geese, eels, etc.

CL. V. Caseous Aliments

The different kinds of milk, cheese, etc.

CL. VI. Farinaceous Aliments

Wheat, barley, oats, rice, rye, potato; sago, arrow-root, etc.

CL. VII. Mucilaginous Aliments

Carrots, turnips, asparagus, cabbage, etc.

CL. VIII. Sweet Aliments

The different kinds of sugar, figs, dates, etc.; carrots.

CL. IX. Acidulous Aliments

Oranges, apples, and other acescent fruits.

To these we may add Condiments; such as salt, the varieties of pepper, mustard, horse-radish, vinegar, etc.

168. In classing the different species of potations, we may, in like manner, be governed by the chemical composition which distinguishes them. They may be arranged under four divisions, viz.

CL. I. Water

Spring, river, well water, etc.

CL. II. The Juices And Infusions Of Vegetables And Animals

Whey, tea, coffee, etc.

CL. III. Fermented Liquors

Wine, beer, etc.

CL. IV. The Alcoholic Liquors, Or Spirits

Alcohol, brandy, rum, etc.

169. Before we attempt to appreciate the value of the different substances arranged under the foregoing classes, it will be necessary to caution the reader against the popular error of regarding the terms digestible and nutritive as synonymous and convertible. A substance may be highly nutritive, and yet be digested with difficulty; that is to say, it may require all the powers of the digestive organs to convert it into chyle, and yet, when so converted, it may afford a principle of highly-restorative energy: this is the case with some of the fatty and oily aliments1. On the contrary, there are substances which apparently pass out of the stomach with sufficient readiness, but afford but little comparative support to the body.

170. Writers on dietetics have descanted very learnedly upon what they please to term the perspirability and alkalescence/ of certain aliments. To the former I am quite unable to attach any precise meaning; with respect to the latter, I apprehend that it is intended to express a highly-nutritive quality, with a certain degree of indigestibility. Heavy and light, as applied to food, are terms equally vague and indefinite, and ought never to be introduced into writings which aspire to the character of philosophical precision. The observation may be extended to the epithet bilious.

171. It is only necessary to reflect upon the chemical and mechanical processes by which chymification is performed in the stomach to perceive, that the digestibility of a substance may depend upon other circumstances than that of its chemical composition. Its mechanical state, with regard to texture and consistence, is of the highest importance2; and if we attempt to deduce any law upon this subject, from the known solubility of a substance out of the body, we shall fall into several fatal errors. It will be necessary to investigate this question with some attention; for it not only explains the relative digestibility of aliments, but furnishes the only true basis for a system of skilful cookery.

1 It has been calculated, that an ounce of fat affords nutriment equal to four ounces of lean.

2 Some physiologists have gone so far as to believe that the mechanical condition of food is even more important than its composition; that the gastric juice is alike capable of acting upon all, provided the aliment be sufficiently comminuted; and that all the anatomical varieties in the structure of the stomach in different animals may be resolved into their mechanical effects upon the aliment.

172. The healthy stomach disposes most readily and effectually of solid food, of a certain specific degree of density, which may be termed its digestive texture; if it exceed this, it will require a greater length of time, and more active powers, to complete its chymification; and if it approaches too nearly to a gelatinous condition, the stomach will be equally impeded in its operations. It is, perhaps, not possible to appreciate or express the exact degree of firmness which will confer the highest order of digestibility upon food 1; indeed, this point may vary in different individuals; but we are taught by experience, that no meat is so digestible as tender mutton: when well-conditioned, it appears to possess that degree of consistence which is most congenial to the stomach; and in this country it is perhaps more universally used than any other animal food. Wedder mutton or the flesh of the castrated animal, is in perfection at five years, and is by far the sweetest and most digestible. Ewe mutton is best at two years old.

Beef appears to be not so easy of digestion; its texture is firmer, but it is equally nutritive - much, however, will depend upon the period which has elapsed since the death of the animal, and more upon the method of cookery; in short, it would be worse than useless to attempt the construction of any scale to represent the nutritive and digestive qualities of the different species of food: the observations here introduced are merely noticed for the sake of illustrating those general principles whose application can alone afford us any rational theory of diet.

1 Some experiments were instituted for this purpose by Gosse, of Geneva; but the conclusions deduced from them are by no means satisfactory. He confounds solubility with digestibility, which in itself is sufficient to vitiate his reasoning.

173. It will not be difficult to understand why a certain texture and coherence of the aliment should confer upon it digestibility, or otherwise. Its conversion into chyme is effected by the solvent power of the gastric juice, aided by the churning which it undergoes by the motions of the stomach; and unless the substance introduced possesses a suitable degree of firmness, it will not yield to such motions: this is the case with soups, and other liquid aliments; in such cases, therefore, nature removes the watery part before digestion can be carried forward. It is on this account that oils are digested with so much difficulty; and it is probable that jellies, and other glutinous matters, although containing the elements of nourishment in the highest state of concentration, are not digested without considerable difficulty; in the first place, on account of their evading the grappling powers of the stomach, and, in 10 the next, in consequence of their tenacity opposing the absorption of their more fluid parts.

For these reasons I maintain, that the addition of isinglass, and other glutinous matter, to animal broths, with a view to render them more nutritive to invalids, is a questionable custom.

174. The texture of animal food is greatly influenced by the age, sex, habits, condition, diet, and description of death of the animal which furnishes it. In proportion, generally, to the age, its flesh is coarser and more firm in texture, as every one must have noticed in eating birds. If the flesh of mutton and lamb, beef and veal, are compared, they will be found of a different texture; the two young meats are of a more stringy, indivisible nature than the others, which makes them harder of digestion. It has been also justly observed, that young animals differ from old ones in the distribution of the fat, which in the latter is chiefly collected in masses or layers, external to the muscles; whereas, in the former, it is more interspersed among the muscular fibres, giving the flesh a marbled appearance, which is always a desirable property of butchers' meat. The texture of food will also vary according to the wild or domesticated state of the animal; that of the former is more dense, although highly nutritive.

The sex also modifies the quality of the flesh, that of the female being always more delicate and finer-grained than that of the entire male, whose fibres are denser: the influence of the genital organs upon this occasion is very extraordinary; it is generally believed, that the flavour of the female is even improved by removing the ovaries, or spaying them, as it is called. Every day the testes are permitted to remain, even though totally inactive with regard to their proper functions, injures the delicacy of the veal of the bull-calf; and an animal which is not castrated until after puberty always retains much of the coarseness of the entire male. The mode of killing an animal has been considered, from the remotest ages, as capable of affecting the quality of its meat. The flesh of hunted animals is characterised by peculiar tenderness; the same effect is produced by any lingering death. This fact probably explains the policy of those old municipal laws, which ordained that no butcher should offer or expose any bull-beef for sale, unless it had previously been baited; and it is upon the same principle only, that the quality of pigs' flesh could be improved by the horrid cruelty of whipping them to death, as said to be practised by the Germans. The action of vinegar, administered to an animal some hours before killing it, is also known to be capable of rendering its flesh less tough.

It is a common practice in the country to give a spoonful of this acid to poultry, when they are intended for the immediate service of the table.

175. Nothing, however, tends more effectually to ameliorate the rigidity of the animal fibre, than incipient putrefaction. The length of time that meat ought to be kept after it is killed will necessarily depend upon its tendency to undergo the putrid fermentation, and the prevalence of those circumstances which are inclined to favour it.

176. The circumstances which have been just enumerated, as being capable of influencing the texture of our food, and consequently its degree of digestibility are, however, unimportant when compared with the modifying powers of cookery, which I shall now proceed to examine.

177. By cookery, alimentary substances undergo a twofold change; their principles are chemically modified, and their textures mechanically changed. The extent and nature, however, of these changes will greatly depend upon the manner in which heat has been applied to them; and if we inquire into the culinary history of different countries, we shall trace its connexion with the fuel most accessible to them1. This fact readily explains the prevalence of the peculiar species of cookery which distinguishes the French table, and which has no reference, as some have imagined, to the dietetic theory, or superior refinement, of the inhabitants.