64 Prof. Max Planck : New Paths 



heat by every compound of radium ; and the physical 

 principle is that of the Conservation of Energy. The conflict 

 ended finally in the complete victory of that principle, al- 

 though voices were heard endeavouring to throw doubt upon 

 its complete validity. 



A radium salt enclosed in a sufficiently thick envelope of 

 lead constantly evolves heat amounting for 1 gram of radium 

 to 135 calories per hour. It therefore constantly remains 

 warmer than its surroundings, just like a heated stove. The 

 Principle of Conservation of Energy asserts that this heat 

 cannot be evolved from nothing, but must have its cause in 

 some other change representing its equivalent. In the case 

 of the stove, it is the constant process of combustion, but as 

 no chemical process is going on in the case of the Radium 

 compound, we must assume a change in the Radium atom 

 itself ; and this hypothesis, which from the point of view of 

 previous chemical science is bold and unprecedented, has 

 been corroborated in every direction. 



From a purely formal point of view, there is, no doubt, a 

 certain contradiction in the conception of a changeable 

 atom, since originally atoms were defined as the unchange- 

 able constituents of all matter. Accordingly one might feel 

 bound to reserve the term " atom " for the really unchange- 

 able elements, such perhaps as Electrons and Hydrogen. 

 But apart from the fact that we may perhaps never be able 

 to establish the existence of invariable elements in the ab- 

 solute sense, such a change in terminology would produce a 

 wild confusion in literature. Indeed the modern chemical 

 atoms have long ceased to be the atoms of Democritus, they 

 can be numerically and accurately specified by a much sharper 

 definition, it is only they that are meant when we speak of a 

 Transmutation of the Atom, and any misunderstanding in 

 the direction indicated seems clearly excluded. 



A thesis not less self-evident than the Invariability of 

 Atoms was, until recently, the Mutual Independence of 

 Space and Time. The question whether two occurrences 

 taking place at different points are simultaneous or are not, 

 had a definite physical meaning, without the need of any 

 enquiry about the observer who measured the time. To-day, 

 that is altered. For a fact — so far invariably corroborated 

 by the most delicate optical and electrodynamic experi- 

 ments — which is briefly and not quite clearly described as 

 the Relativity of all Motion, has brought that simple 

 conception into conflict with the so-called principle of the 

 Constancy of the Velocity of Light established by the 

 electrodynamics of Maxwell and Lorentz. This principle 



