90 PRESIDENT’S ADDRESS. 
has disappeared with a fuller knowledge of what the heat energy in a 
gas is due to. When we were ignorant of the nature of this energy, 
the transition from kinetic into thermal energy seemed discontinuous ; 
but now we know that this energy is the kinetic energy of the molecules 
of which the gas is composed, so that there is no change in the type of 
energy when the kinetic energy of visible motion is transformed into 
the thermal energy of a gas—it is just the transference of kinetic energy 
from one body to another. 
If we regard potential energy as the kinetic energy of portions of the 
ether attached to the system, then all energy is kinetic energy, due to 
the motion of matter or of portions of ether attached to the matter. I 
showed, many years ago, in my ‘Applications of Dynamics to Physics 
and Chemistry,’ that we could imitate the effects of the potential 
energy of a system by means of the kinetic energy of invisible systems 
connected in an appropriate manner with the main system, and that 
the potential energy of the visible universe may in reality be the kinetic 
energy of an invisible one connected up with it. We naturally suppose 
that this invisible universe is the luminiferous ether, that portions of the 
ether in rapid motion are connected with the visible systems, and that 
their kinetic energy is the potential energy of the systems. 
We may thus regard the ether as a bank in which we may deposit 
energy and withdraw it at our convenience. The mass of the ether 
attached to the system will change as the potential energy changes, and 
thus the mass of a system whose potential energy is changing cannot 
be constant; the fluctuations in mass under ordinary conditions are, 
however, so small that they cannot be detected by any means at present 
at our disposal. Inasmuch as the various forms of potential energy are 
continually being changed into heat energy, which is the kinetic energy 
of the molecules of matter, there is a constant tendency for the mass of 
a system such as the earth or the sun to diminish, and thus as time 
goes on for the mass of ether gripped by the material universe to become 
smaller and smaller; the rate at which it would diminish would, how- 
ever, get slower as time went on, and there is no reason to think that it 
would ever get below a very large value. 
Radiation of light and heat from an incandescent body like the sun 
involves a constant loss of mass by the body. Hach unit of energy 
radiated carries off its quota of mass, but as the mass ejected from the 
sun per year is only one part in 20 billionths (1 in 2 X 10") of the mass 
of the sun, and as this diminution in mass is not necessarily accompanied 
by any decrease in its gravitational attraction, we cannot expect to be 
able to get any evidence of this effect. 
As our knowledge of the properties of light has progressed, we have 
been driven to recognise that the ether, when transmitting light, 
possesses properties which, before the introduction of the electro- 
magnetic theory, would have been thought to be peculiar to an emission 
