Boltwood had conducted studies of radioactive materials as a consultant, and when Rutherford lectured at Yale in 1904, Boltwood was inspired to describe the relationships between elements in various decay series.
Boltwood havde studeret radioaktive materialer som konsulent, og da Rutherford gav forelæsninger på Yale i 1904, inspireredes Boltwood til at beskrive grundstoffernes sammenhænge i de forskellige henfaldsserier.