Albumen (from Lat. albus, white, because the albumen of the fowl's egg, on being coagulated by cooking, turns white), an organic substance, more or less fluid in its natural condition, which is coagulated or solidified by the action of heat, alcohol, the mineral acids, and the metallic salts. The characters of albumen were first recognized in the transparent and colorless portion of the contents of the fowl's egg. When an egg is boiled and then opened, it is found to consist of two different portions; namely, an internal portion or yolk, which is yellow, and an external portion, which is white. The external portion before boiling is transparent, semi-fluid, and nearly colorless; and the increased consistency, opacity, and white color which it assumes on cooking, are due to its containing as its principal ingredient the substance in question, which is coagulated under the influence of heat. The composition of albumen of the white of an egg is stated by Dumas to be: carbon, 54.3; hydrogen, 7*1; nitrogen, 15.8; oxygen, 21.0; sulphur, 1.8; - of the serum, or thin part of the blood of man, 0.05 less of carbon, 0.19 more of hydrogen, 0.07 less of nitrogen, and 0.07 less of oxygen. The sulphur in the white of an egg, uniting with hydrogen, forms sulphuretted hydrogen, which tarnishes silver.

Albumen is found not only in the egg, but in the blood, in the chyle and lymph, in the interstitial fluid of the muscles, and in the moisture of the serous cavities, as the pericardium and the peritoneum. In the blood, where it is most abundant, it is in the proportion of about 75 parts per thousand; in the lymph and chyle, from 12 to 35 parts per thousand. It is coagulated by a temperature of 160° F., and when in tolerable abundance, as in the serum of the blood, the whole fluid, on boiling, becomes solidified or gelatinous in consistency. The presence of an alkali or an alkaline carbonate, however, in due proportion, will prevent this; and after the albumen is once coagulated, the coagulum may be redissolved by the action of the alkali. In the blood, the albumen is regarded as its most nutritious ingredient, being employed for the nourishment of the various tissues, by which it is absorbed and afterward converted into materials similar to their own. It is not discharged from the body under its own form with the excretions, except in cases of disease, but is retained and employed for the maintenance of the vital operations. - Albumen is regarded as the representative of a large class of organic substances, as well vegetable as animal, which are known as the albuminoid substances. (See Aliment.) They are distinguished by the facts that they all contain nitrogen, in addition to carbon, hydrogen, and oxygen; that all those which are fluid or semifluid are coagulable by various means; that they are very ready both to excite and to undergo indirect or catalytic transformations; and that they are all susceptible of putrefaction.

They enter very largely into the composition of the food, and constitute its most valuable and nutritious ingredients. - According to recent researches (1865) of Hoppe-Seyler, there is a marked difference between the albumen of eggs and that found in other parts of the animal economy. Vegetable albumen has never yet been prepared in a pure form, and we are unable to say whether it may not constitute a third modification. The two modifications known to chemists at the present time are the soluble and insoluble,