Are the general principles of dynamics—which since Newton have served as the foundation of physical science and which seem unshakable—on the point of being abandoned or at least profoundly changed? Many people have been wondering that for a few years now. According to them, the discovery of radium could lead to overturning scientific dogmas that were thought to be the most solid: both the impossibility of transmutation of metals and also the fundamental postulates of mechanics. Maybe it’s too hasty to consider these discoveries as firmly established and to break our idols from the past; maybe it would be better, before taking sides, to wait for more numerous and more probative experiments. It is no less necessary today to understand the new doctrines and the already very serious arguments on which they rest.