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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.