PPT Slide
Max Planck made an in-depth investigation of the energy "confined" within a body (M. Planck, Sitz. der preuss. Akad.Wiss., Physik. Math. Klasse. 13 (June, 1907)). Planck used the momentum of radiation concept of Poincaré.
Planck derived the expression M=E/c2 and stated that "through every absorption or emission of heat the inertial mass of a body alters, and the increment of mass is always equal to the quantity of heat divided by the square of the velocity of light in vacuo.”