[12] As judged from a co-ordinate system moving with the body. A direct comparison of this relation with experiment is not possible at the present time (1920; see[Note], p. 48), owing to the fact that the changes in energy _E_0 to which we can subject a system are not large enough to make themselves perceptible as a change in the inertial mass of the system. image027 is too small in comparison with the mass _m_, which was present before the alteration of the energy. It is owing to this circumstance that classical mechanics was able to establish successfully the conservation of mass as a law of independent validity. [Note] The equation E = mc2 has been thoroughly proved time and again since this time. Let me add a final remark of a fundamental nature. The success of the Faraday-Maxwell interpretation of electromagnetic action at a distance resulted in physicists becoming convinced that there are no such things as instantaneous actions at a distance (not involving an intermediary medium) of the type of Newton’s law of gravitation. According to the theory of relativity, action at a distance with the velocity of light always takes the place of instantaneous action at a distance or of action at a distance with an infinite velocity of transmission. This is connected with the fact that the velocity _c_ plays a fundamental role in this theory. In Part II we shall see in what way this result becomes modified in the general theory of relativity. XVI. EXPERIENCE AND THE SPECIAL THEORY OF RELATIVITY