“The mass of a body is a measure of its energy content.”
Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig? ("Does the inertia of a body depend upon its energy content?")
Annalen der Physik 18, 639-641 (1905). Quoted in Concepts of Mass in Classical and Modern Physics by Max Jammer (1961), p. 177 http://books.google.com/books?id=lYvz0_8aGsMC&lpg=PP1&pg=PA177#v=onepage&q&f=false
1900s
