18 PRESIDENT’S ADDRESS. 
density of the ether ; all we have to do is to increase the distance between 
the wires in proportion as we increase the density of the ether. 
Let us now consider how much ether is carried along by ordinary 
matter, and what effects this might be expected to produce. 
The simplest electrical system we know, an electrified sphere, has 
attached to it a mass of ether proportional to its potential energy, and 
such that if the mass were to move with the velocity of light its kinetic 
energy would equal the electrostatic potential energy of the particle. 
This result can be extended to any electrified system, and it can be 
shown that such a system binds a mass of the ether proportional to its 
potential energy. Thus a part of the mass of any system is propor- 
tional to the potential energy of the system. 
The question now arises, Does this part of the mass add anything to 
the weight of the body? If the ether were not subject to gravitational 
attraction x certainly would not ; and even if the ether were ponderable, we 
might expect that as the mass is swimming in a sea of ether it would 
not increase the weight of the body to which it is attached. But if it does 
not, then a body with a large amount of potential energy may have an 
appreciable amount of its mass in a form which does not increase its 
weight, and thus the weight of a given mass of it may be less than that 
of an equal mass of some substance with a smaller amount of potential 
energy. ‘Thus the weights of equal masses of these substances would be 
different. Now, experiments with pendulums, as Newton pointed out, 
enable us to determine with great accuracy the weights of equal masses 
of different substances. Newton himself made experiments of this kind, 
and found that the weights of equal masses were the same for all the 
materials he tried. Bessel, in 1880, made some experiments on this 
subject which are still the most accurate we possess, and he showed 
that the weights of equal masses of lead, silver, iron, brass did not 
differ by as much as one part in 60,000. 
The substances tried by Newton and Bessel did not, however, include 
any of those substances which possess the marvellous power of radio- 
activity ; the discovery of these came much later, and is one of the most 
striking achievements of modern physics. 
These radio-active substances are constantly giving out large 
quantities of heat, presumably at the expense of their potential energy ; 
thus when these substances reach their final non-radio-active state their 
potential energy must be less than when they were radio-active. 
Professor Rutherford’s measurements show that the energy emitted by 
one gramme of radium in the course of its degradation to non-radio- 
active forms is equal to the kinetic energy of a mass of },th of a milli- 
gramme moving with the velocity of light. 
This energy, according to the rule I have stated, corresponds 
to a mass of 4th of a milligramme of the ether, and thus a gramme 
of radium in its radio-active state must have at least ~,th of a 
