This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Fresh eggs are usually considered better for poaching than eggs in which the physical quality has deteriorated, i.e., eggs with watery whites. Class results substantiate this popular opinion, for eggs with thick viscous whites can be poached satisfactorily with far less care than those with thin whites. In poaching eggs it is desirable to have the temperature of the water near the boiling point when the egg is added. Thus the outer portion of the egg is coagulated in a short period of time. If desired, the cooking can be completed at a lower temperature.
Since both the white and yolk tend to flatten on standing after being broken out of the shell, poached eggs of better appearance are usually obtained if the egg is broken just before it is added to the poaching water. St. John and Flor have reported that the thin portion of the white coagulates as satisfactorily as the thick portion if 3/4 teaspoon of salt to a pint of boiling water is used. The salt aids in coagulation. Eggs with about the same proportion of thick and thin parts of the white are firmer when poached in salted than in unsalted water. But for what food teachers call standard products, eggs poached in unsalted water often have "a better appearance than those poached in salted water. The ones cooked in salted water are usually not so shiny. Occasionally the reverse is true. The added salt aids in coagulation, so that the thin portion of the white is less readily detached from the thicker portion. But the appearance of the thin portion after cooking in the salted water is often more puckered, wrinkled, and ruffled, or is in voluminous folds around the thicker coagulated part. This detracts from the appearance. Either a very hard water or a softened water is used for poaching eggs in class work. It is possible that the natural salt content of the water may effect coagulation, so that thin watery whites cooked in Ames's water do not give as good appearing products as those obtained by St. John and Flor.