This section is from the book "Many Ways For Cooking Eggs", by Sarah Tyson Heston Rorer. Also available from Amazon: Many Ways For Cooking Eggs.
Any single food containing all the elements necessary to supply the requirements of the body is called a complete or typical food. Milk and eggs are frequently so called, because they sustain the young animals of their kind during a period of rapid growth. Nevertheless, neither of these foods forms a perfect diet for the human adult. Both are highly nutritious, but incomplete.
Served with bread or rice, they form an admirable meal and one that is nutritious and easily digested. The white of eggs, almost pure albumin, is nutritious, and, when cooked in water at 170' Fahrenheit, requires less time for perfect digestion than a raw egg. The white of a hard boiled egg is tough and quite insoluble. The yolk, however, if the boiling has been done carefully for twenty minutes, is mealy and easily digested. Fried eggs, no matter what fat is used, are hard, tough and insoluble. The yolk of an egg cooks at a lower temperature than the white, and for this reason an egg should not be boiled unless the yolk alone is to be used.
Ten eggs are supposed to weigh a pound, and, unless they are unusually large or small, this is quite correct.
Eggs contain from 72 to 84 per cent, of water, about 12 to 14 per cent, of albuminoids. The yolk is quite rich in fat; the white deficient. They also contain mineral matter and extractives.
To ascertain the freshness of an egg without breaking it, hold your hand around the egg toward a bright light or the sun and look through it. If the yolk appears quite round and the white clear, it is fresh. Or, if you put it in a bucket of water and it falls on its side, it is fresh. If it sort of topples in the water, standing on its end, it is fairly fresh, but, if it floats, beware of it. The shell of a fresh egg looks dull and porous. As it begins to age, the shell takes on a shiny appearance. If an egg is kept any length of time, a portion of its water evaporates, which leaves a space in the shell, and the egg will "rattle." An egg that rattles may be perfectly good, and still not absolutely fresh.