Russian physicist Nikolay Alekseevich Umov (1846–1915) was the first scientist to indicate interrelation between mass and energy proposing the formula E = kmc^2 as early as in 1873 (with k representing a value from 1.0 to 0.5).

See: https://en.wikipedia.org/wiki/Nikolay_Umov

Einstein’s paper “Does the Inertia of a Body Depend upon its Energy-Content?” (27 Sep 1905) only states:
“If a body gives off the energy (L) in the form of radiation, its mass diminishes by L/c^2.” followed by:
“The mass of a body is a measure of its energy-content; if the energy changes by L, the mass changes in the same sense by L/9 × 10^20, the energy being measured in ergs, and the mass in grammes.” \

See: https://www.fourmilab.ch/etexts/einstein/E_mc2/www/

How this became E=mc^2 is just bizarre.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)