Most everything has changed in the world of cooking in the past 250 years, but not the method for determining if you’ve got a bad egg.
There is a small air pocket in the large end of the egg. When the egg is fresh, the pocket is only about 1/8th of an inch deep and as large around as a dime. As the egg ages, however, it loses both moisture and carbon dioxide – shrinking – so that the size of the air space increases. And the size of the air space determines the buoyancy of the egg.
So if you submerge a very fresh egg in water, it will lie on the bottom. An egg that is a week or so old will lie on the bottom but bob slightly. An egg that is three weeks or so old will balance on its small end, with the large end reaching for the sky. And a bad egg will float.
According to Harold McGee, author of On Food & Cooking (Canada, UK), Hannah Glass gave this practical advice to cooks around 1750, and it’s as valid today – a “way to know a good egg, is to put the egg into a pan of cold water; the fresher the egg, the sooner is will fall to the bottom; if rotten, it will swim at the top.”