This is the only nutritive fluid with which nature has presented us; but if we examine its chemical composition, we shall soon discover that it possesses an ingredient which is instantly coagulated in the stomach; so that, in fact, it must be regarded as a mixture of solid and liquid aliment; the latter, however, considerably exceeding the former in quantity, and thereby demonstrating the necessity of a greater portion of fluid than of solid matter, for the reparation of that habitual waste, upon which the necessity of alimentary supplies is founded.

194. Although recent milk appears as a homogeneous liquid, it may be resolved, partly by standing, and partly by agents that do not essentially alter the nature of its components, into three proximate ingredients, the cream, curd, and whey.

1. The Cream

The Cream rises to the surface of the milk, after it has stood for some hours, and may be skimmed off, and thus separated from it. It appears to possess many of the properties of oil; is smooth and unctuous to the touch, and stains cloth in the same manner as other unctuous substances. By standing for some days it becomes gradually thicker, and at length forms a soft solid, in which the flavour of cream is no longer perceived, and that of cheese is substituted in its place. According to the experiments of Berzelius, cream is a compound body, consisting of butter, 4.5; cheese, 3.5; and whey 92 parts: but since the whey holds certain salts in solution, we may consider the whole of the solid matter contained in cream as amounting to 12.5 per cent. When cream is agitated, as is done by the common process of churning, it is separated into two parts; a thick animal oil, well known by the name of butter, and a fluid which possesses exactly the same properties as milk that has been deprived of its cream. This change has been supposed to be owing to the combination of the cream with the oxygen of the atmosphere; but it takes place, though perhaps not equally well, in vessels from which the air has been excluded. 2. Curd. When milk, either deprived or not of its cream, is mixed with certain substances, or allowed to stand till it becomes sour, it undergoes a change which is called coagulation, consisting in its separation into a solid substance termed curd, and a fluid called whey.

This change may be effected by several agents, such as alcohol, gelatine, and all astringent vegetables; by acids, and many neutral salts; by gum, sugar, and more particularly by the gastric juice. The effect is supposed to arise from the affinity of the coagulating substance for water, the curd being principally albumen, having very little affinity for the same; but this theory can hardly explain the operation of the gastric juice: the infusion of a piece of calfs stomach, not larger than a half-crown, will coagulate a quantity of milk sufficient for making a cheese of sixty pounds weight, although the quantity of coagulating matter cannot in this case exceed a few grains. 3. Whey, or the liquid which remains after the separation of the curd, is a thin and almost transparent fluid, of a yellowish-green colour, and a pleasant sweetish taste. It still contains, generally, a portion both of curd and of butter; the former of which may be separated by a boiling heat, in the form of coagulum. The buttery matter also separates by heat, especially if the whey be previously allowed to become sour. Whey contains, indeed, in its recent state, some acetic acid.

When whey, which has been deprived as much as possible of the butter and curd, is slowly evaporated, it yields the peculiar substance termed "sugar of milk" which may be obtained, by clarification with whites of eggs, in the form of crystals. The presence of this saccharine matter held in solution in whey enables that fluid to undergo the vinous fermentation; and it is accordingly employed by the Tartars for making a sort of wine, which is called Koumiss. For this purpose mare's milk is selected, as containing a larger portion of sugar than that of the cow. Whey also contains several saline bodies, viz. muriate of potass, phosphates of lime and of iron, and sulphate of potass; and a peculiar animal matter; which gives a precipitate with infusion of galls, and affords carbonate of ammonia by distillation.

From these investigations, the constituents of skimmed milk from the cow appear to be as follow: -

195. Although nature has presented us with this compound fluid for the purposes of nourishment, and although it is evident that its several ingredients are wholesome, and designed for the various objects of aliment, yet, when separated by art, they are frequently unwelcome to the stomach; that viscus would appear to dislike the interference of the cook, in the performance of an analysis which its own powers are so well calculated to perform. We are well assured, that the first process which takes place in the stomach for the chymification of milk, is its separation into curd and whey; and yet the former of these substances, when obtained by art, frequently proves highly oppressive to the stomach, and sometimes occasions obstructions in the bowels. Cheese, again, which is nothing more than the coagulum of milk, pressed, salted, and partly dried, with a portion of butter, which, having been enveloped in the curd, is not afterwards separable, is one of the least digestible of our aliments, and is only adapted to strong stomachs, and to such persons as use great and constant exercise. When toasted, it is still more injurious, from acquiring a tenacity of texture highly hostile to the digestive function of the stomach.

Butter, from its oily nature, is apt to disagree with delicate stomachs, and when rendered empyreumatic by heat, produces heartburn, and other distressing symptoms: the use of hot buttered toast or muffins should, therefore, never be allowed to dyspeptic invalids. Whey differs considerably in its dietetic value, according to the method employed for its separation. When this is effected by rennet, it always holds a portion of cream and curd suspended in it, besides its quantity of sugar. It is, therefore, considerably nutritive; but it is, at the same time, more acescent than milk, and hence it is liable to produce flatulence in those whose stomachs are disposed to encourage fermentation. Whey that has been produced by spontaneous coagulation always contains less nutritive matter, is more acid, and consequently more objectionable, unless, perhaps, as a drink in certain states of fever.

196. From this account of the composition of milk, several properties of the entire fluid may be understood. By boiling it, its albuminous part is not coagulated into a mass like the white of an egg, on account of the large quantity of water through which it is diffused; but a thin pellicle forms on the surface, which, if removed, is immediately replaced by another; and thus the whole of the albumen may be separated in successive portions. The effect of this process is therefore to diminish the nutritive quality of the milk; but it may at the same time render it more easily digestible. I have known many invalids who could take boiled milk, but were unable to bear that fluid in its natural state.

197. Milk, in its dietetic relations, may be considered as intermediate between animal and vegetable food; it is easily assimilated, and therefore affords a quick supply of aliment to the system, while it does not excite that degree of vascular action which is produced by other animal matters. Its nutritive powers may be increased by various additions, which have also, on some occasions, the effect of correcting its natural tendency to acidity, and on others, that of obviating the costiveness which it is liable to occasion; such objects are sometimes fulfilled by adding oatmeal gruel to it. In certain states of organic disease, I have found that milk, impregnated with the fatty matter of mutton suet, is a valuable article of diet: such a repast is best prepared by inserting the suet in a muslin bag, and then simmering it with the milk. In common cases of dyspepsia it would prove injurious, for the reasons so often alluded to in the course of this work; but where it is an object to introduce much nutritive matter in a small space, I am not acquainted with a better form of aliment. With some practitioners it is a custom to recommend an admixture of lime-water with milk, to prevent the acidity which it is apt to create in feeble stomachs.

I have occasionally experienced the benefit of such a practice, especially in cases of tabes mesenterica.

198. Before quitting this subject, it is necessary to observe, that there exists some difference in the composition of the milk of different animals. That of the human subject is much thinner than cow's milk; is of a bluer colour, and contains much more saccharine matter. It also yields a larger proportion of cream, but from which the butter cannot be separated by agitation. It deposits a part of its curd by mere repose. Asses' milk bears a stronger resemblance to human milk than to any other; it contains more sugar than that of the cow, and the proportion of curd is so considerable as even to separate on standing, before the milk becomes sour. Goats' milk yields a remarkably thick and unctuous cream, and abounds also in curd.