16 PRESIDENT’S ADDRESS. 
of matter and ether, not only is even the densest matter excessively 
porous and discontinuous, with vast interspaces in and among the 
atoms, but the constitution of matter is such that there appears to be 
no displacement in the ordinary sense at all; the ether is itself so 
modified as to constitute the matter in some way. Of course that 
portion moves, its inertia is what we observe, and its amount depends 
on the potential energy in its associated electric field, but the motion 
is not like that of a foreign body, it is that of some inherent and 
merely individualised portion of the stuff itself. Certain it is that 
the ether exhibits no trace of viscosity.? 
Matter in motion, Ether under strain, constitute the fundamental 
concrete things we have to do with in physics. The first pair represent 
kinetic energy, the second potential energy; and all the activities of 
the material universe are represented by alternations from one of these 
forms to the other. 
Whenever this transference and transformation of energy occur, 
work is done, and some effect is produced, but the energy is never 
diminished in quantity: it is merely passed on from one body to 
another, always from ether to matter or vice versa,—except in the case 
of radiation, which simulates matter—and from one form to another. 
The forms of energy can be classified as either a translation, a 
rotation, or a vibration, of pieces of matter of different sizes, from stars 
and planets down to atoms and electrons; or else an etherial strain 
which in various different ways is manifested by the behaviour of such 
masses of matter as appeal to our senses.” - 
Some of the facts responsible for the suggestion that energy is 
atomic seem to me to depend on the discontinuous nature of the 
structure of a material atom, and on the high velocity of its constituent 
particles. The apparently discontinuous emission of radiation is, I 
believe, due to features in the real discontinuity of matter. Disturb- 
ances inside an atom appear to be essentially catastrophic; a portion is 
liable to be ejected with violence. There appears to be a critical 
velocity below which ejection does not take place; and, when it does, 
there also occurs a sudden re-arrangement of parts which is presumably 
responsible for some perceptible etherial radiation. Hence it is, I 
suppose. that radiation comes off in gushes or bursts; and hence it 
appears to consist of indivisible units. The occasional phenomenon of 
new stars, as compared with the steady orbital motion of the millions 
of recognised bodies, may be suggested as an astronomical analogue. 
The hypothesis of quanta was devised to reconcile the law that 
* For details of my experiment on this subject see Phil. Trans. Roy. Soc. for 
1893 and 1897; or a very abbreviated reference to it, and to the other matters 
above-mentioned, in my small book The Liher of Space. 
* See, in the Philosophical Magazine for 1879, my article on a Classification 
of the forms of energy, 
