This section is from the book "Philadelphia Cook Book: A Manual Of Home Economies", by Sarah Tyson Heston Rorer. Also available from Amazon: Philadelphia Cook Book.
The first and most important point in making good soup is to have the best of materials. To make our soup a perfect food we must change the solid meat into a liquid form; to do this, we must first soften the fibrin, so as to draw out the juices and blood, consisting of albumen and fat; the gelatine, which exists in the bone, cartilages, membranes and skin, which is nitrogenous matter, but not nutritious; and the osmazome, that substance which gives odor and flavor. As a low degree of heat changes the albumen (which is exactly similar to the white of an egg) into a solid form, we at once see the necessity of using cold soft water. Soft water, because it makes its way into the tissues more readily than hard water, thereby softening the texture of the meat and allowing the juices to escape more easily; and we also see the importance of not boiling the soup, as the albumen on the surface of the meat immediately coagulates and prevents the gelatine, fat and osmazome from dissolving and being drawn out into the water. Salt should never be added until the soup is done, as it hardens the water; and we have found that soft water is the best. As the water begins to heat a small portion of albumen coagulates, forming a fine fibrous net throughout the liquid entangling any substance that may be floating in it, bringing it first to the surface and then settling to the bottom, showing that we must watch and skim at this time to have a clear soup.
You will notice that in the recipes for Consomme and Bouillon, to clarify, we boil after adding the whites of the eggs, thereby making a perfectly clear soup, but a stimulant rather than nutrient, as we rob it of its albumen and fibrin by boiling and straining; two things which cannot exist at the same time, a very clear and a very nutritious soup.
Another very important point is to have a porcelain-lined or better still a granite iron soup kettle with a close cover. Why? Because the juices of the meat are always acid and will act upon a metallic kettle thereby giving the soup an inky, bitter taste. A close cover to keep in the steam and prevent evaporation and also to keep the dust and smoke out.