PROGRESS IN PHYSICS—THOMSON. 195 
we have been using at the Cavendish Laboratory a pendulum whose 
bob was filled with uranium oxide. We have got good reasons for 
supposing that uranium is a parent of radium, so that the great 
potential energy and large ethereal mass possessed by the radium 
will be also in the uranium; the experiments are not yet completed. 
It is, perhaps, expecting almost too much to hope that the radio- 
active substances may add to the great services they have already 
done to science by furnishing the first case in which there is some 
differentiation in the action of gravity. 
The mass of ether bound by any system is such that if it were to 
move with the velocity of light its kinetic energy would be equal 
to the potential energy of the system. This result suggests a new 
view of the nature of potential energy. Potential energy is usually 
regarded as essentially different from kinetic energy. Potential 
energy depends on the configuration of the system, and can be cal- 
culated from it when we have the requisite data; kinetic energy, on 
the other hand, depends upon the velocity of the system. According 
to the principle of the conservation of energy the one form can 
be converted into the other at a fixed rate of exchange, so that when 
one unit of one kind disappears a unit of the other simultaneously 
appears. 
Now, in many cases, this rule is all that we require to calculate 
the behavior of the system, and the conception of potential energy 
is of the utmost value in making the knowledge derived from experi- 
ment and observation available for mathematical calculation. It 
must, however, I think, be admitted that from the purely philosoph- 
ical point of view it is open to serious objection. It violates, for 
example, the principle of continuity. When a thing changes from a 
state A toa different state B, the principle of continuity requires that 
it must pass through a number of states intermediate between A and 
B, so that the transition is made gradually, and not abruptly. Now, 
when kinetic energy changes into potential, although there is no dis- 
continuity in the quantity of the energy, there is in its quality, for we 
do not recognize any kind of energy intermediate between that due 
to the motion and that due to the position of the system, and some 
portions of energy are supposed to change per saltum from the 
kinetic to the potential form. In the case of the transition of 
kinetic energy into heat energy in a gas, the discontinuity has dis- 
appeared 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 mole- 
cules 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 trans- 
