is small compared with unity, the third of these terms is always small in comparison with the second, which last is alone considered in classical mechanics. The first term _mc_2 does not contain the velocity, and requires no consideration if we are only dealing with the question as to how the energy of a point-mass; depends on the velocity. We shall speak of its essential significance later. The most important result of a general character to which the special theory of relativity has led is concerned with the conception of mass. Before the advent of relativity, physics recognised two conservation laws of fundamental importance, namely, the law of the conservation of energy and the law of the conservation of mass these two fundamental laws appeared to be quite independent of each other. By means of the theory of relativity they have been united into one law. We shall now briefly consider how this unification came about, and what meaning is to be attached to it. The principle of relativity requires that the law of the conservation of energy should hold not only with reference to a co-ordinate system _K_, but also with respect to every co-ordinate system _K′_ which is in a state of uniform motion of translation relative to _K_, or, briefly, relative to every “Galileian” system of co-ordinates. In contrast to classical mechanics; the Lorentz transformation is the deciding factor in the transition from one such system to another. By means of comparatively simple considerations we are led to draw the following conclusion from these premises, in conjunction with the fundamental equations of the electrodynamics of Maxwell: A body moving with the velocity _v_, which absorbs[11] an amount of energy _E_0 in the form of radiation without suffering an alteration in velocity in the process, has, as a consequence, its energy increased by an amount image024 [11] _E_0 is the energy taken up, as judged from a co-ordinate system moving with the body. In consideration of the expression given above for the kinetic energy of the body, the required energy of the body comes out to be image025 Thus the body has the same energy as a body of mass image026 moving with the velocity _v_. Hence we can say: If a body takes up an amount of energy _E_0, then its inertial mass increases by an amount image027 the inertial mass of a body is not a constant but varies according to the change in the energy of the body. The inertial mass of a system of bodies can even be regarded as a measure of its energy. The law of the conservation of the mass of a system becomes identical with the law of the conservation of energy, and is only valid provided that the system neither takes up nor sends out energy. Writing the expression for the energy in the form image028 we see that the term _mc_2, which has hitherto attracted our attention, is nothing else than the energy possessed by the body[12] before it absorbed the energy _E_0.