"The most remarkable property of the radio-active bodies is their power of spontaneously and continuously radiating energy at a constant rate, without, as far as is known, the action upon them of any external exciting cause. The phenomena at first sight appear to be in direct contradiction to the law of conservation of energy, since no obvious change with time occurs in the radiating material. The phenomena appear still more remarkable when it is considered that the radio active bodies must have been steadily radiating energy since the time of their formation in the earth's crust."