Before proceeding to inquire into the interior composition of the egg, we will consider the exterior covering, or the shell—the physical and chemical structure of which is exceedingly interesting and wonderful. The white, fragile cortex called the shell, composed of mineral matter, is not the tight, compact covering which it appears to be; for it is everywhere perforated with a multitude of holes too small to be discerned by the naked eye, but which, with the aid of a microscope, are distinctly revealed. Under the microscope the shell appears like a sieve, or it more closely resembles the white perforated paper sold by stationers. Through these holes there is constant evaporation going on, so that an egg, from the day that it is dropped by the hen, to the moment when it is consumed, is losing weight and diminishing in volume. This process goes on much more rapidly in hot weather than in cold, and consequently perfect eggs are not so readily procured in summer as in winter. If, by any means, we stop this evaporating process, the egg remains sound and good for a great length of time. Covering the shell with an impervious coat of varnish, or with mutton suet or lard, aids greatly in their preservation. The substance used to stop transpiration must not be soluble in watery fluids, or liable to be removed. By chemical agencies, that is, by actually filling up the little holes in the shell by lime placed in solution (the solution holding the proper chemical substances to form an impervious coating of carbonate of lime over the entire surface), we have preserved eggs for months, and even years, in a sweet condition. Not long ago, eggs broken in a laboratory in Boston were found to be quite fresh, which, according to the memorandum made upon the vessel, were placed in the solution four years ago.
The shell of the egg is lined upon its interior everywhere with a very thin but pretty tough membrane, which, dividing at or very near the obtuse end, forms a small bag which is filled with air. In new-laid eggs this follicle appears very little, but it becomes larger when the egg is kept. In breaking an egg, this membrane is removed with the shell to which it adheres, and therefore is regarded a part of it, which it is not.
The shell proper is made up mostly of earthy materials, of which ninety-seven per cent is carbonate of lime. The remainder is composed of two per cent of animal matter and one of phosphate of lime and magnesia. Carbonate of lime is the same material of which our marble quarries and chalk beds are composed: it is lime, or oxide of calcium, combined with carbonic acid, and is a hard, insoluble mineral substance, which does not appear to form any portion of the food of fowls. Now, where does the hen procure this substance with which to form the shell? If we confine fowls in a room and feed them with any of the cereal grains, excluding all sand, dust or earthy matter, they will go on for a time and lay eggs, each one having a perfect shell made up of the same calcareous elements. Vanvuelin, the distinguished chemist, shut up a hen ten days and fed her exclusively upon oats, of which she consumed 7,474 grains in weight. During this time four eggs were laid, the shells of which weighed nearly 409 grains: of this amount 276 grains was carbonate of lime, 17-1/2 phosphate of lime, and 10 gluten. But there is only a little carbonate of lime in oats, and from whence could this 409 grains of the rocky material have been derived? The answer to this question opens up some of the most curious and wonderful facts connected with animal chemistry, and affords glimpses of many of the operations of organic life, which, to the common mind, seem in the highest degree paradoxical and perplexing. The body of a bird, like that of a man, is but a piece of chemical apparatus made capable of transforming hard and fixed substances into others of a very unlike nature. In oats there is contained phosphate of lime, with an abundance of silica; and the stomach and assimilating organs of the bird are made capable of decomposing or rending asunder the lime salts and forming with the silica a silicate of lime.
This new body is itself made to undergo decomposition, and the base is combined with carbonic acid, forming carbonate of lime. The carbonic acid is probably derived from the atmosphere, or more directly, perhaps, from the blood. These chemical changes among hard, inorganic bodies are certainly wonderful when we reflect that they are brought about in the delicate organs of a comparatively feeble bird, under the influence of animal heat and the vital forces. They embrace a series of decomposing and recomposing operations which it is difficult to imitate in the laboratory.
In the experiment to which allusion has been made, the amount of earthy material found in the eggs and the excrement of the hen exceeded that contained in the food she consumed. This seems paradoxical, and can only be explained upon the ground that birds as well as animals have the power, in times of exigency, of drawing upon their own bodies for material which is required to perform necessary functions.
The shell of an ordinary hen's egg weighs about one hundred and six grains, that is, the inorganic portion of it; and if a bird lays one hundred eggs in a year, she produces about twenty-two ounces of nearly pure carbonate of lime in that period of time, which would afford chalk enough to meet the wants of a farmer, or perhaps even of a house carpenter of moderate business, for a twelvemonth.
If a farmer has a flock of one hundred hens, they produce in egg shells, about one hundred and thirty-seven pounds of chalk annually; and yet not a pound of the substance, or perhaps not even an ounce, exists around the farm-house within the circuit of their feeding grounds. This is a source of lime production not usually recognized by farmers or hen fanciers, and it is by no means insignificant. The materials of the manufacture are found in the food consumed, and in the sand, pebble stones, brick-dust, bits of bones, etc., which hens and other birds are continually picking from the earth.
The instinct is keen for these apparently innutritious and refractory substances, and they are devoured with as eager a relish as the cereal grains or insects. If hens are confined to barns or outbuildings, it is obvious that the egg-producing machinery cannot be kept long in action, unless the materials for the shell are produced in ample abundance.
Within the shell the animal portion of the egg is found; which consists of a viscous, colorless liquid called albumen, or the white, and a yellow globular mass called the vitellus, or yolk. The white of the egg consists of two parts, each of which is enveloped in distinct membranes. The outer bag of albumen, next the shell, is quite a thin, watery body, while the next which invests the yolk, is heavy and thick. But few housekeepers who break eggs ever distinguish between the two whites, or know of their existence even.