2 6o THE POPULAR SCIENCE MONTHLY 



hold of Eiemann's idea of space as a manifold. As this is wholly extra- 

 physical and not in my realm I shall do no more than mention it. 



Thus the phenomenon of gravitation remains a mystery; for so far 

 every hypothesis made seems to have insurmountable difficulties. I am 

 not sure that any of them shed light enough to even convince us that 

 we are on the right track. It seems to have little or nothing in common 

 with various other things of which we have some knowledge. A few 

 years ago when radio-active substances were discovered, we found 

 phenomena which at first did not seem to agree with well-known 

 chemical and physical laws; but as experimentation progressed con- 

 firmation became more general and to-day many, if not all, the discrep- 

 ancies have disappeared. Not so with gravitation. It still remains 

 one of the least understood properties of matter. Probably if we could 

 learn something of the mechanism of gravitation, that attraction between 

 particles which only manifests itself at very small distances (cohesion) 

 might be better understood. 



Nearly two hundred and fifty years ago one of the greatest intellects 

 connected with science turned his attention to gravitation. In that two 

 hundred and fifty years physical science has made rapid advances. A 

 boy who has completed a year's work in elementary physics could enter- 

 tain Newton in electricity were it possible for the great philosopher to 

 return to earth. After learning of the great progress in electricity, I 

 can imagine him in his eager desire for knowledge turning to the boy 

 and expecting some light on gravitation. Alas, not only the high school 

 boy, but not even the most learned can give any definite information on 

 gravitation. The problem is about where Newton left it. 



