CHEMISTRY OF A HEN'S EGG.
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.
Each has its appropriate office to fulfil during the process of incubation or hatching; and one acts in the mysterious process as important a part as the other. If we remove this glairy fluid from the shell and place it in a glass, and plunge into it a strip of reddened litmus paper, a blue tinge is immediately produced, which indicates the presence of an alkali. The alkali is soda in a free condition, and its presence is of the highest consequence, for without it the liquid would be insoluble. A portion of the white of an egg, when diluted with water, and a few drops of vinegar or acetic acid added to it, undergoes a rapid change. The liquid becomes cloudy and flocculent, and small bits of shreddy matter fall to the bottom of the vessel. This is pure albumen, made so by removing the soda held in combination by the use of the acid. A pinch of soda added to the solid precipitate redissolves it, and it is again liquid. There is another way by which the albumen is rendered solid: and that is by the application of heat. Eggs placed in boiling hot water pass from the soluble to the insoluble state quite rapidly, or, in other words, the albumen both of the white and yolk becomes "coagulated."
No contrast can be greater than that between a boiled and unboiled egg. Not only is it changed physically, but there is a change in chemical properties, and yet no chemist can tell in what the change consists. It is true, the water extracts a little alkali, and a trace of sulphide of sodium; but the abstraction of these bodies is hardly sufficient to account for the change in question.
The hardening of the albumen of egg by heat constitutes the cooking process, and this deserves a moment's consideration.
Great as is the physical difference between a fully cooked and an uncooked egg, it is no less remarkable in the degree of digestibility conferred upon it by the process. Uncooked, it passes by the most simple processes of assimilation from the digestive to the nutritive and circulatory organs, and is at once employed in nourishing or sustaining the bodily functions. Unduly cooked, the egg resists the action of the gastric juices for a long time, and becomes unsuited to the stomachs of the weak and dyspeptic. A raw or soft-boiled egg is of all varieties of food the most nourishing and concentrated; a hard-boiled egg is apt to trouble the digestion of the strong and healthful, and its nutrient properties are sensibly impaired. The yolk contains water and albumen, but associated with these is quite a large number of mineral and other substances which render it very complex in composition. The bright yellow color is due to a peculiar fat or oil, which is capable of reflecting the yellow rays of light, and this oil holds the sulphur and phosphorous which abound in the egg. If the yolk be removed and dried, and the yellow oil separated, it will be found to form two-thirds of the substance. The whole weight in its natural state is about three hundred grains, of which three-fifths is water; of the white more than three quarters is water.
The yolk and albumen of a fecundated egg remains as sweet and free from corruption during the whole time of incubation as it is in new-laid eggs, and there is but little loss of water; whereas an unfecundated egg passes rapidly into putrefactive decay and perishes.
Any one who eats three or four eggs at breakfast consumes that number of embryo chicks.
All the materials which enter into the legs, bones, feathers, bill, etc., of the new-born chick, exist in the egg, as nothing is derived from outside. The little creature which has just pecked its way out of its calcareous prison-house, has lime and phosphorus in his bones, sulphur in his feathers, iron, potash, soda and magnesia in his blood, all of which mineral constituents come from the egg, and are taken into the stomach when it is eaten as food.
The valuable or important salts are contained in the yolk, and hence this portion of the egg is the most useful in some forms of disease. A weakly person, in whom nerve force is deficient and the blood impoverished, may take the yolks of eggs with advantage. The iron phosphoric compounds are in a condition to be readily assimilated, and although homœopathic in quantity, nevertheless exert a marked influence upon the system. The yolks of eggs, containing as they do less albumen, are not so injuriously affected by heat as the white, and a hard-boiled yolk may be usually eaten by invalids without inconvenience. The composition of a fresh egg, exclusive of the shell, may be presented as follows:
| Water | 74.0 | parts. |
| Albumen | 14.0 | parts. |
| Oil or fat | 10.5 | parts. |
| Mineral Salts | 1.5 | parts. |
| ——— | ||
| 100 |
The whole usually weighs about a thousand grains, of which the shell makes a tenth part.
The chick-making materials, exclusive of water, form only one-quarter of the weight of the liquid contents, or only about two hundred grains.
This seems to be a small beginning upon which to rear a full-grown rooster. The bulk or quantity as found in hens' eggs, and indeed in the eggs of all birds, is wonderfully disproportionate to the size of the mother bird. The laying of eggs must be regarded as a particularly exhausting process, and yet fowls will keep it up for a long time and not lose much in flesh. We have known a hen of the game variety which has laid twenty-two eggs in twenty-two consecutive days, and they average in weight one thousand grains each. This gives in amount twenty-two thousand grains, or rather more than three pounds avoirdupois, of which about two and a quarter pounds is water. The dozen or more ounces of rich, nutritive material, parted with in twenty-two days, would seem to be a prodigious draught upon the small physical structure of the bird, but there were no indications of exhaustion.
Whilst it is true that the quickening of an egg which results in the birth of a chick, is no more marvellous a process or result than the embryotic development of any creature endowed with the mysterious principle of life, yet there are some circumstances connected with it which make it a matter of greater perplexity and wonder. Here is an oval white body consisting of a calcareous shell, within which there are some semi-fluid substances, consisting mainly of albumen and water, without any signs of life. In fact, there is no life; it is simply a mass of dead, inanimate matter. Talk as much as we will about the germinal principles involved in the structure of the egg, we are totally unable to recognize it, or form any conception of its nature.
There is no evidence of the presence of any germ or principle of life whatever. The egg left to itself decays like other organized substances, but with our assistance in simply transferring it to a place where the temperature is in a certain uniform condition, in a few weeks, the albumen, water, oil and mineral salts are transferred into a living chick, which thrusts its little beak through the shell, and in ten minutes is running about, almost able to take care of itself.
Here is the development of life apparently without the agency of the mother, and what a marvel! The chemist may place together in a body in a warm place, just such elements or substances: he may carefully weigh the water, the albumen, the phosphotic compounds, the sulphur, the iron, soda, etc., and construct a very accurate egg mixture, but out of it all there will never come a living chick. In this, we obtain some idea how little we actually know about life, how dark is the region where the life principle begins, or where the vital forces originate. The indefatigable man of science has pushed his inquiries close up to the boundary between the inanimate and the animate; but he has never been able to obtain the least glimpse of anything upon the life side of the line. However great maybe our curiosity, our skill or knowledge in this state of existence, there is not the least probability that we shall ever be able to endow matter with life, or know much more than we do at present of its origin or nature.
Auntie, to a little four-year-old who is resting his head on the table—"Ah, Louis, you are sleepy; you will have to go to bed." "Oh, no, auntie, I aren't sleepy; but my head is loose, so I laid it down here."