Einstein was able to derives the mass-energy equivalency E=mc^2 (without special relativity!) by imagining a thought experiment described by Max Born here:
http://web.ncf.ca/eo200/derivation.html Einstein concludes heat energy E must increase inertial mass by the amount m = E/c^2, or else the apparatus would be able to self-accelerate, i.e. accelerate without an external force in violation of Newtonian mechanics. However, has it ever been confirmed experimentally that the inertia of a body varies inceases with its heat content? Suppose it is not true. While there is abundant evidence that mass can be converted into heat energy, it is not clear, from an experimental standpoint that increasing the heat content of a body necessarily increases the mass of a body. This has some bearing on heat produced during a "cold fusion" phenomena. Instead of seeing the heat as coming from the classic conversion of mass into energy, perhaps the heat comes from the self-acceleration of particular atomic arrangements which functionally resemble the apparatus described at the above link. Harry
