PHYSICS 



very secure foundation. The gravitational rela- 

 tivity principle is based on the experience that all 

 material bodies are equally accelerated in a gravi- 

 tational field, and I do not feel that the empirical 

 basis for this principle is so secure as that of the 

 restricted form. At any rate, I have for some 

 years been engaged on very delicate experiments 

 in the Wheatstone laboratory to see if small 

 differences in the gravitational acceleration of 

 different kinds of matter cannot be detected. If 

 these experiments lead to a negative result the 

 position of the gravitational theory will be 

 strengthened thereby. If, on the other hand, they 

 should give a positive result, I do not anticipate 

 that it will be beyond the ingenuity of Professor 

 Einstein to frame a theory which will take account 

 of them. 



Many of the most pressing problems of physics 

 at the present time are bound up with what is 

 known as the quantum theory. This theory has 

 arisen as a result of the failure of dynamical prin- 

 ciples to account for a number of important 

 physical phenomena. There is, of course, no 

 convincing a priori reason why dynamics should 

 be adequate to account for all the phenomena of 

 physics, but a belief in such adequacy had, in fact, 

 become widespread. The recalcitrant phenomena 

 were in the main those dealing with heat radiation, 

 the specific heats of bodies at low temperatures, 



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