178 ADAMS— THE EINSTEIN THEORY. 



real difficulty. In the attempt to account, on this principle, for the 

 forces between circuits carrying electric currents, not one, but an 

 infinite number of laws of force between the elements of the circuits 

 was found to answer. Experiment could not decide which was the 

 law of force because experiments could be made only with complete 

 circuits. An end was soon put to the controversy which raged over 

 this question by the publication of Maxwell's Theory of Electricity 

 and Magnetism. In this theory action at a distance played no part. 

 All the forces between electrically charged bodies, between magnet- 

 ized bodies, the mutual forces between electric circuits and between 

 magnets and electric circuits were ascribed to a system of pressures 

 and tensions in a universal medium which pervaded all bodies and 

 extended throughout all space. And this medium was the same as 

 that which had been previously postulated as the vehicle for the 

 waves of light. The goal in the dynamical explanation of physical 

 phenomena now changed to the attempt to account for them by 

 direct action through a medium instead of by action at a distance. 

 For electric and magnetic effects the idea of action at a distance 

 became unnecessary, but for the commonest force of all — gravita- 

 tion — it could not be dispensed with. Although the elementary law 

 of gravitational attraction is remarkably similar to the elementary 

 laws of electric and magnetic attractions and repulsions, there are 

 sufficient differences between them to place the force of gravity in 

 a different category from the other natural forces. Gravitational 

 force is always attractive ; electric and magnetic forces may be 

 attractive or repulsive ; gravitational force appears to be wholly 

 independent of the medium through which it acts ; electric and 

 magnetic forces are enormously influenced by the medium. These 

 differences led IMaxwell to predict that attempts to account for 

 gravitational force by a system of pressures and tensions in a 

 medium, analogous to those used to account for electric and mag- 

 netic forces, would be doomed to failure. 



An answer to this riddle of gravitation has been given by Ein- 

 stein, and this answer has come through the general theory of 

 relativity. The principle of relativity has arisen through repeated 

 failures to detect any influence upon optical phenomena by experi- 

 ments performed on the earth due to the motion of the earth about 



