"In the year 1883... Ernst Mach, published a book Mechanics and Its Evolution, which turned out to be in many respects a landmark in our understanding of the laws of motion. Mach presented a critical analysis of Newtonian mechanics and directed the attention of scientists to the fact that the "constancy of mass," if the operational definition \frac{m_2}{m_1} = \frac{a_2}{a_1} [compare two masses by pushing them with the same force and comparing accelerations] is used, is an experimental fact and by no means a "philosophical truth" which can be derived from "intelligible principles." There was a possibility that experiments would show a change of mass caused by external circumstances. ...[T]oward the end of the nineteenth century, J. J. Thomson derived from Maxwell's electromagnetic field theory the result that mass points behave like particles with electric charges. In the twentieth century, the motion of fast electrically charged particles was systematically investigated... in the . If electrostatic forces are acting in the direction of the actual velocity, high speed particles (i.e., with speeds comparable to the speed of light) obtain accelerations which are noticeably smaller than the accelerations of slow particles in the same electrostatic field."