Das Prinzip von der Erhaltung der Schwerpunktsbewegung und die Trägheit der Energie" (Principle of the Conservation of the centre of mass motion and the inertia of energy).

Leipzig, Johann Ambrosius Barth, 1906). Bound in a fine recent hmorocco. Gilt lettering on spine. ""Annalen der Physik"", Bd. 20, Heft 8, pp. 433-640. The Einstein paper pp. 627-633.

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First edition of this paper in the periodical form in which Einstein shows that the conservation of mass is a special application of his energy principle (E= Mc2) - ""Einstein considers a weightless (horizontal) cylinder filled with electromagnetic radiation and closed to the external world. A certain amount of radiating energy E moves from the left end of the cylinder to the right end, transferring a certain amount of momentum (due to radiation pressure) to the left end which translates the cylinder to the left. When the radiation arrives at the right end, the motion stops. We now imagine, that a practically massless body absorbs this radiation, moves back to the left and deposits it to its original state. Then the body moves back to right end, into its original position. Now a complete cycle has taken place, the system is back in its original state, but the center of mass has moved by a certain amount to the left. This process can be repeated any number of times with the result that a body, all by itself, without any external forces, can change its centre of mass by an arbitrary amount, in contradiction to all physical evidence. This conclusion is avoided if we assume that the energy E is associated with the mass M=E/c2 in which case the centre of mass remains permanently at rest, in agreement with our ecpectations."" Cornelius Lanczos). - Weil: 13.