164 THE LIMITATIONS OF SCIENCE 



Each theorist advanced an hypothesis of the most 

 tentative character which all felt to be inadequate. 

 Yet, in spite of the confusion, three points of agree- 

 ment may be noted. In the first place, the ether as a 

 material substance is impossible and even useless; the 

 name, it is true, is left but it expresses now but the 

 shadow of a reality. When the ether is called an 

 electro-magnetic something, or a space differentiated 

 only by the transient presence of energy, the name may 

 signify something but it is hardly intelligible except as 

 a mathematical symbol. Secondly, these theories 

 agree in assuming electricity to be an entity; that is, 

 we are to suppose that it is not due to a state of mat- 

 ter, manifested by a special mechanical force, but a 

 substance like matter, divisible into atomic elements 

 and having inertia. Thirdly, the mass inertia of mat- 

 ter is not an invariable quantity but changes with the 

 velocity of a body; thus it is the measure of matter 

 only while there is no motion. 



Such was the state of physical theory when Professor 

 Einstein announced, in an article published in the 

 Annalen der Physik for the year 1905, a new scientific 

 principle which he believes will reconcile the contra- 

 dictions of our new hypotheses; satisfy the three con- 

 ceptions mentioned; and correct and amplify the New- 

 tonian mechanics so that it will again harmonize with 

 modern science. Whether this Principle of Relativity 

 will accomplish all that its discoverer hopes can be 



