The white of an egg is a pellucid viscous liquor, thinner towards each end, and thicker in the middle. That part which is more dense and close than the rest is called gallat.ura.
The industry of later physiologists ha3 discovered three different kinds of albumen in each egg, of different densities. The external is the most liquid; the second is less so, and the third still less fluid. It is to this third portion that the shape of the albumen is confined; the others surround the yolk: this consists of two segments of spheres of unequal diameters, applied to the sides of the yolk, and connected with it by a somewhat denser albuminous process, near each extremity; though not, as has been represented, at the poles. These albuminous processes are styled chalazae. Each of the portions of this internal albumen is penetrated by a convoluted cord: that, on one side, is membranous; the other, vascular. The former is contiguous to the pellicle of the yolk; the latter, analogous to the umbilical cord in the mammalia, forms the communication between the albumen and the yolk.
The albumen of the egg, in its early period, is less homogeneous than at a later; since, in boiling, it concretes into a curdly fluid. Some water escapes from it, and is collected in a pellicule, on the top; and it is probable, though by no means certain, that some oxygenous gas is absorbed. It is said to be destined for the nourishment of the chicken; but, in the process of incubation, it is not materially diminished in quantity, and we know that the yolk is the real nutritious substance, and is taken into the body of the chicken at the end of the period of incubation. A milder nourishment may probably be required in the early stages: nor is it very improbable that the three kinds of albumen maybe designed as nourishment for the chicken at its different ages, and the waste repaired by the absorption of humidity. If this is prevented, the progress of the embryo is checked, and the egg continues in its first state.
The albumen is peculiarly mild, resembling the serum of the blood, which is a watery fluid, with an admixture of the gluten; a portion of which appears to be chemically combined, and the larger part mechanically mixed. It is soluble in hot or cold water, coagulated by heat of 165° of Fahrenheit; by acids, and by alcohol. When diluted by ten times its weight of water, heat does not coagulate it; but acids and alcohol continue to produce this effect until it is more largely diluted. In the coagulation, the bulk is not increased; and the coagulum neither absorbs nor emits air. The cause of the coagulation is probably the addition of caloric; but to ascertain this idea, which is originally Scheele's, the capacity of the albumen in each state for heat should be ascertained.
Albumen naturally contains a proportion of soda and a little sulphur. In water of 80°, it soon becomes putrid, and exudes through the broader end of the shell. Alkalis and alkaline earths dissolve it; disengaging some ammonia, in consequence of its decomposition. A solution of train precipitates albumen in the form of a yellow precipitate, of the consistence of pitch; and, however minute the proportion of albumen combined with water may be, it is discoverable by means of tanin. When dissolved by alkalis, and precipitated by acids, its qualities are altered. When coagulated, water no longer dissolves it, but mineral acids have this power, and tanin only will precipitate it: alkalis have no effect. In its analysis, it resembles the gluten of the blood, but contains a less proportion of azote than the fibrin: the other ingredients arc carbone and hydrogen. See Blood and Tanin.
The yolk of the egg differs in appearance, rather than its nature, from the albumen. It has a portion of the gluten, and contains a resinous, or rather an oily fluid. By heat, the oil is entangled in the coagulable substance; but a portion may be expressed, and is employed as an antiphlogistic remedy against sun-burns. The colouring material is not known. Fourcroy supposes it to be iron; but it is more probably sulphur, as a putrid egg exhales a strong hepatic gas. The shell of the egg consists chiefly of carbonat, and phosphat of lime, with gelatinous matter. The membrane that lines it, though apparently dense, suffers some fluids to escape, and some gases probably to be absorbed, since acrid and deleterious vapours destroy the chick. This membrane is a part of the albumen, and, of course, a part of the embryo, since its vessels may be injected from those of the chick.
Eggs are chiefly employed as nourishing substances. In the arts, the albumen forms, with quick lime, a strong cement for china; and, diluted with water, it has been used to lessen the rigidity of the tendons; in pharmacy the yolk is employed as an intermede, to mix or suspend oils, balsams, and resins in water. The oil of the yolk we have already mentioned.
As a nutritious substance the yolk is the most distinguished. It is not certain that the albumen abounds in nourishment. It is very bland when fresh, and highly deleterious when in the slightest degree putrid. Even in its best state it disagrees with many stomachs; producing eructation, sickness, and sometimes erysipelatous eruptions. The yolk is very nourishing; but, when firmly coagulated by boiling, it is slow of digestion. It was some years since in high reputation as an alimerit for weak stomachs. A fashionable physician ordered it, in one instance, and every one took it. The consequence might be easily supposed; but the practice continued while the physician was fashionable. In jaundice and liver complaints, a fresh unboiled egg has been recommended every morning, and it is said with success: we know not for what reason, except that it is yellow. In general, it is useful in weak stomachs, as it contains much nutriment within a small compass; and, though the hard egg is slow of digestion, we have not found it increase hectic exacerbations. We shall see that slow and difficult of digestion are not synonymous. Nature seems anxious to retain the food in the stomach; and the best digestives are those which retard the process. Too rapid a digestion is, in many views, productive of inconvenience. See Digestion.
The eggs of different birds do not differ essentially: those of geese and ducks are said to be the most gross and alkalescent: those of pea-hens and gallinas the least so. The latter have certainly less flavour.