191 1.] OF ELECTRICITY AND MATTER. 331 



across it, every one of them has given negative results, and has 

 thus shown that the relative parts of the system hear the same rela- 

 tion to each other, no matter zvhat the direction of motion is. 



It is like finding that an object made up of metal and glass had 

 no strains set up in it when heated, a result which could only be 

 attained if the metal and glass had the same coefficient of expansion. 



What are we to conclude in the case before us? There seems 

 to be no alternative. \\'e already have seen that electrons are 

 among the fundamental constituents of all atoms ; we have seen that 

 radiant energy is electromagnetic and that such energy permeates all 

 matter. We have seen that energ}' resembles matter in possessing 

 mass, and that, therefore, to the same degree, matter resembles 

 energ}\ The necessary conclusion seems to be that all physical phe- 

 nomena obex the same general lazes of zehich the knozcn electro- 

 magnetic lazes are as yet the completest expression. 



And now to what have we committed ourselves by this con- 

 clusion as regards changes due to motion? ^Merely this: that all 

 real systems being ultimately " electromagnetic " in the above sense, 

 undergo certain changes when set in motion, but these changes are 

 such as to leave all parts bearing the same relation to each other. 

 Thus since the knowledge of an observer travelling with the system 

 is only relative, he is not able to detect such absolute changes, just 

 as we are not able to detect the motion of the earth. The changes 

 in his svstem would be noticeable to an observer whose instruments 

 did not move, but cannot be detected by moving instruments. 



The kind of change which wq have said is produced in a system 

 by setting it in motion has one property which is important and pro- 

 foundly significant. It, is that the moving observer sees precisely 

 the same change in stationary systems which he is passing as the 

 stationary observer sees in the moving system ; so that not only can 

 the moving observer not detect his motion by means of his instru- 

 ments, but the tzi'o obserz'ers together, if their memory fail, can by 

 no means tell which is moving. There is, in other words, a very 

 complete symmetry with regard to what the two observers can 

 actually find out about their systems, although we called one of them 

 stationary in the beginning. 



Because of this complete symmetry the most conservative among 



