~Albumins.~--The best examples are found in egg albumin (white of egg), lactalbumin (milk), serum albumin (blood), leucosin (wheat), legumelin (peas). Albumins are all soluble in pure water, and are coagulable by heat. Coagulation, due to the action of the ferments in the body, takes place in milk, blood, and muscle plasma. Certain albumens are particularly adapted for the building and repairing of tissues. Among those that have been used in feeding experiments to determine whether or not they were capable, when used as the sole protein in the diet, of maintaining animals in normal nutrition, and of supporting normal growth in the young animal,--may be cited lactalbumin and egg albumin. These experiments provided diets adequate in other respects, the object being to determine the value of the various proteins. It was found that the albumin from milk was more efficient in this respect than the egg albumin.[5] In the invalid dietary the solubility of the albumins in water makes them of especial value as reinforcing agents, since they may be introduced into fluids without materially altering either their flavor or their bulk. ~Globulins.~--Simple proteins, insoluble in pure water, but soluble in neutral salt solutions; examples, muscle globulin, serum globulin (blood), edestin (wheat), physelin (beans), legumin (beans and peas), tuberin (potatoes), amandin (almonds), arachin, and conarachin (peanuts). ~Alcohol-Soluble Proteins.~--Simple proteins soluble in alcohol of from 70-80% strength. Insoluble in absolute alcohol, water and other neutral solvents. Examples of these proteins may be seen in the gliadin of wheat, zein of corn, and hordein of barley. ~Albuminoids.~--These substances represent one group of incomplete proteins, inasmuch as they cannot alone support protein metabolism. However, they are classed with the proteins and may be substituted for at least a part of these compounds in the daily dietary, since they are able to do much of the work of the pure proteins. The best example of this group is seen in gelatin. This substance contains many of the structural units of meat protein but in very different relative amounts. It has not, therefore, the chemical units necessary to repair the worn-out parts of cell machinery.[6] ~Conjugated Proteins:--Nucleoproteins, Phosphoproteins and Hemoglobin.~ (a) ~Nucleoproteins.~--This type of protein is characteristic of all cell nuclei, and is particularly abundant in the highly nucleated secreting cells of the glandular organs, such as the liver, pancreas, and the thymus gland. The nucleoproteins are composed of simple proteins and nuclein. Nucleic acid is rich in phosphorus and upon decomposition yields some of the purin bases (xanthin, adenin, guanin), a carbohydrate and phosphoric acid.[7] (b) ~Phosphoproteins.~--Compounds in which the phosphorus is in organic union with the protein molecule otherwise than a nucleic acid or lecithin. Examples: caseinogin (milk), ovovitellin (egg yolk).