This may have been hashed before, so forgive my indulgence if this post is a logical fallacy. Energy = Mass, OK Now it's stated that by increasing the temperature of a body the mass increases. I would derive this as the energy needed to raise the temp is transferred into the body. now the body with an increase in temp would be radiating this increase and thus also be reducing it's mass over a period of time, as the temp is reduced to pre-energy transference. Am I correct in this assumption? So is a nuclear reaction different? increasing the velocity of the particles to split the atomic structure requires energy and thus an increase in mass. I can't seem to find an agreed equivalence to this. Rest mass shouldn't apply to either case as the work or energy needed to increase the body temp is not at rest right? I'll state that I'm not formally educated in these fields so I'm sure most of the 'scribbly' writing goes over my head, however I can follow the process and interactions.