THE IDENTITY OF LIGHT AND ELECTRICITY. 183 



sessing those properties, although, we were not in the hahit of 

 regarding them as electrical phenomena, and gave them the name 

 of light. If Maxwell's electrical theory was rejected, there was 

 no more reason for accepting his views concerning light. In like 

 manner, if it was affirmed that light is a phenomenon of an elas- 

 tic nature, his theory of electricity became impossible. But when 

 his theory was studied without prepossession with the ideas that 

 were current, all the parts could be seen to lend one another a 

 mutual support, like the stones of a vault, and the whole resem- 

 bled a gigantic arch thrown across the unknown, and uniting two 

 known truths. 



The difficulty of the theory did not permit it at first to acquire 

 a large number of partisans. But after its inner sense was dis- 

 cerned it was followed out to its ultimate consequences, and then 

 the value of its fundamental hypotheses was tested. Experiments 

 were at first limited to a few propositions, the accessory parts of 

 the theory. I have compared Maxwell's system to an arch trav- 

 ersing an abyss of the unknown. I might add that it was some 

 time before the abutments could be connected. It was thus put 

 in a position where it could support itself, but the span was too 

 wide to permit any new structure to be built upon it. To accom- 

 plish that object pillars were needed, rising from the ground, to 

 support the middle of the arch. The demonstration of the possi- 

 bility of obtaining electrical or magnetic effects directly from 

 light would constitute one of the pillars and confirm the the- 

 ory; it would have immediately established the electrical part, 

 and indirectly the optical part of it. The completion and sym- 

 metry of the structure demanded the building of both the pillars 

 to which we compare these principles, but one was enough to 

 begin with. The construction of the former pillar has not yet 

 been undertaken ; but after a multitude of researches a solid base 

 has been found for the second, with sufficiently ample founda- 

 tions, on which a part of the pillar has been raised. With the 

 co-operation of many workers it will soon reach the top of the 

 arch and afford support to the weight of the edifice which is to be 

 raised upon it. 



I have had the privilege of taking part in this portion of the 

 work. To this fact I owe it that I am now laying my ideas before 

 you ; and I hope that I may be excused if I try at present to direct 

 all attention to this part of the edifice. I shall unhappily be 

 obliged, for want of time, to omit the labors of a large number of 

 seekers, and shall be unable to show to what extent my experi- 

 ments had been prepared for by my predecessors, and how near 

 some of them had come to a definite result. 



It does not at first seem so difficult to show whether propaga- 

 tion of electrical or magnetic forces is or is not instantaneous ; to 



