1917] on Recent Developments of Molecular Physics 79 



lead to a nul result." On this basis, supplemented by certain exten- 

 sions and generalisations, it is found possible to construct a definite 

 and consistent system of laws. They of course differ from the 

 dynamical laws based upon the supposed existence of an ether, but 

 they differ only where relative motion is involved, because the rela- 

 tivity theory makes its laws agree with those of ordinary dynamics 

 when there is no relative motion. Wherever the old and the new 

 theories differ, an appeal to experiment has so far invariably decided 

 in favour of the new theory of relativity. The most recent triumph 

 of the new theory is of such great interest that it may perhaps be 

 mentioned in some detail. 



Gravitation has always stood aloof from other physical phenomena. 

 Since Xewton formulated the law of the inverse square of the dis- 

 tance, nothing has been added to the law and nothing taken away. 

 Except that it represents a natural weakening of gravitational effect 

 by spreading out in space, no explanation of the law has ever been 

 given, nor even a plausible conjecture as to the relation between 

 gravitation and other physical agencies. Eecently Einstein has 

 found that the Xewtonian law is inconsistent with the postulates of 

 his general relativity theory. On amending the law so as to con- 

 form to these postulates, it appears that the orbit of a planet about ■ 

 the sun ought no longer to be a simple ellipse, as it was under the 

 Newtonian law, but rather an ellipse slowly rotating in its own 

 plane. For instance, the orbit of Mercury ought to revolve at a rate 

 of al)out 42 • D" per century. Now, for some time one of the out- 

 standing problems of astronomy has been the explanation of the 

 irregularities in the orbit of Mercury. After allowing for all known 

 causes of irregularity, there was found to be outstanding a secular 

 advance of the perihelion, or more simply a slow rotation of the 

 orbit, of amount almost exactly equal to the •42 '9'' per century 

 predicted by the relativity theory of Einstein. 



To sum up, then, it is clear that the first of our two clouds has 

 been dissipated by the theory of relativity. This is not surprising, 

 for the theory was in effect designed for this special purpose. But it 

 is important that the cloud has been removed without another one 

 appearing to replace it. Indeed, other clouds have also been 

 removed in the process, such as that surrounding the orbital motion 

 of Mercury. The relativity theory has succeeded, as we have seen, 

 by relegating the ether to a position of absolute unimportance. 

 Whether the ether exists or not w^e do not and cannot know, but 

 the relativity theory indicates that everthing happens exactly as if 

 the ether did not exist. The new theory is concerned with the 

 discovery and formulation of laws rather than with their causes, and 

 so makes no claim to pronounce on the existence of an ether ; but it 

 is clear from the experimental evidence of the Michelson-Morley 

 experiment, that if it is finally necessary to call upon an ether to 

 interpret phenomena, this ether will be something quite different 



