191 1.] OF ELECTRICITY AND MATTER. 323 



fundamental physical concepts and it is an attitude which we seldom 

 assume, so that these remarks have been necessary to introduce 

 properly the three fundamental realities which modern physical 

 theory now contemplates, namely: the atom, the electron, and that 

 mysterious but perhaps even more fundamental entity known as 

 energy. 



A few years ago I would have mentioned also the ether, but I 

 am a little reluctant to do so now. Xot that there has been a sudden 

 revolution in the realm of thought, resulting in the complete over- 

 throw of the old regime, but rather that development has been such 

 as to render the concept of an ether less and less impressive — one 

 might say — and less and less important. Changes of opinion in 

 such matters are, it seems to me, partly questions of emphasis, and 

 radiant energy in all its nakedness is now usurping much that the 

 ether has long stood for. 



Of course, the loss, or rather the dimness, of the ether concept 

 implies a certain loss of concreteness ; but, as has been said, con- 

 creteness in new concepts, founded as it is on familiarity, is a 

 secondary virtue and is of far less importance than the value of a 

 concept in furthering the great process of induction which leads us 

 to more and more general truths. 



Although the concept of the ether is slowly dimming, the concept 

 of the atom is becoming more and more definite and vivid. The 

 study of the scintillations caused by radium rays and the work of 

 Rutherford in counting the alpha particles give us, for the first time 

 in the history of physics, definite observable results which apparently 

 can only be due to the action of single atoms, and which therefore 

 furnish proof beyond reasonable doubt that the atom and molecule 

 are names of actual realities, and are not merely two prominent 

 words in the statement of a useful hypothesis. 



The kinetic theory of matter, carrying with it the concept of 

 temperature as violence of atomic vibration, has also been strength- 

 ened enormously by the work of Perrin and Einstein on the Brown- 

 ian movement. They find that microscopic particles in solutions 

 have a perpetual motion in close agreement with the kinetic theory. 

 Indeed, they act in all ways like big molecules, obeying the kinetic 

 laws deducible from mechanics. Their observable agitation is part 



