196 



SCIENTIFIC THOUGHT. 



elaboration as admits on the one side the Maxwellian 

 definitions of the propagation of electro-magnetic waves, 

 and on the other the definition of electrons as per- 

 manent but movable states of twist or strain, which 

 form the atoms of electricity, and possibly, in their 

 aggregate, ponderable matter itself. The history of 

 thought is mainly interested in this latest and most 

 comprehensive " theory of the electric and luminiferous 

 medium," because it is almost entirely based upon that 

 great advance in physical theory which we owe to 

 Helmholtz and Lord Kelvin, " the discovery of the 

 types of permanent motion, which could combine and 

 interact with each other without losing their individu- 

 ality, though each of them pervaded the whole field." 

 This has rendered possible an entirely new mode of 

 treatment, 1 and at least made thinkable the reconciliation 

 of the two apparently contradictory notions of modern 

 physics, the continuity and uniformity of the all- 

 pervading ether and the discontinuity of the embedded 

 particles of matter and electricity. The history of 

 thought also takes further note that these latest and 

 yet unfinished theories revert, after the interval of thirty 



originally presented itself . . . in the 

 course of an inquiry into the 

 competence of the aether devised 

 by MacCullagh to serve for elec- 

 trical purposes as well as optical 

 ones" (' JEther and Matter,' p. vi.) 

 "No attempt was made to ascer- 

 tain whether MacCullagh's plenum 

 could, in addition to its vibratory 

 functions, take up such a state of 

 permanent strain as would repre- 

 sent the electrostatic actions be- 

 tween charged conductors, or such 

 state of motion as would represent 



the electro-dynamic action between 

 currents. The first hint on this 

 side of the matter was Fitzgerald's 

 passing remark in 1880 (' Phil. 

 Trans.,' "On the Electro-magnetic 

 Theory of Light"), that Mac- 

 Cullagh's optical equations 'are 

 identical with those of the elec- 

 tro-dynamical theory of optics de- 

 veloped by Maxwell ' " (p. 78). 



1 See Larmor's Address to the 

 British Association at Bradford 

 ('Report,' p. 624). 



