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Page:Popular Science Monthly Volume 35.djvu/108

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THE POPULAR SCIENCE MONTHLY.

colored eggs, and those which, possess concealed or covered nests, white eggs.

But few of those who break the shells of the cooked eggs of our common domestic fowls at the breakfast-table ever think of the wonderful nature of the structure they crush, or of the complex chemical nature of the contents consumed as food. 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. Under the microscope the shell appears like a sieve, or it more closely resembles the white perforated paper sold by stationers. 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 small, 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 as a part of it, which it is not. The shell proper is made up mostly of earthy materials. The proportions vary according to the food of the bird, but ninety to ninety-seven per cent is carbonate of lime. The remainder is composed of two to five per cent of animal matter, and one to five of phosphate of lime and magnesia.

If a farmer has a flock of one hundred hens they produce in eggshells 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. 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 innutritions 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 out-buildings, it is obvious that the egg producing machinery can not be kept long in action unless the materials for the shell are supplied 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 fulfill during the process of incubation or hatching, and one acts, in the mysterious process, as important a part as the other.