PRESIDENT’S ADDRESS. 19 
_ milligramme more of ether attached to it than when it has been degraded 
into the non-radio-active forms. Thus if this ether does not increase the 
weight of the radium, the ratio of mass to weight for radium would be 
greater by about one part in 13,000 than for its non-radio-active products. 
I attempted several years ago to find the ratio of mass to weight for 
radium by swinging a little pendulum, the bob of which was made of 
radium. I had only a small quantity of radium, and was not therefore 
able to attain any great accuracy. I found that the difference, if any, 
in the ratio of the mass to weight between radium and other substances 
was not more than one part in 2,000. Lately 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 configura- 
tion of the system, and can be calculated 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 
_ behaviour of the system, and the conception of potential energy is of the 
‘ utmost value in making the knowledge derived from experiment and 
_ observation available for mathematical calculation. It must, however, 
_ I think, be admitted that from the purely philosophical 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 to a 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 discontinuity in the quantity of the 
_ energy, there is in its quality, for we do not recognise 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 
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