About gradual separation from the usual concepts for the future
In 1881, the founder of atomic physics J.J. Thomson noticed that the inertial mass of а charged particle should increase as it moves, and introduced the concept of electromagnetic mass — that portion of the inertial mass, which is due to the energy of the electrostatic field. A few years later, the understanding has been formed that the electromagnetic mass should increase with increasing particle momentum. By 1907, in the framework his theory of relativity, Einstein introduced the law relating mass and energy. Similar formulas had been already written in the works of Lorentz, Heaviside, Poincaré, et al., but their research belonged to a particular case — to properties of electrically charged objects moving in the alleged aether. Einstein first introduced this relationship as a universal law of dynamics related to all kinds of matter, and not limited to electromagnetism. He had unified all kinds of masses and pointed the inverse relationship: the inertia of any physical object increases with increasing energy.