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.