194 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
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 gravita- 
tional attraction it 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 sub- 
stance with a smaller amount of potential energy. Thus the weights 
of equal masses of these substances would be different. Now, experi- 
ments with pendulums, as Newton pointed out, enable us to deter- 
mine 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 1830, 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, or brass did not 
differ by as much as one part in 60,000. 
The substances tried by Newton and Bessel did not, however, in- 
clude any of those substances which possess the marvelous power of 
radioactivity; the discovery of these came much later, and is one 
of the most striking achievements of modern physics. 
These radioactive substances are constantly giving out large quanti- 
ties of heat, presumably at the expense of their potential energy; 
thus when these substances reach their final nonradioactive state their 
potential energy must be less than when they were radioactive. 
Professor Rutherford’s measurements show that the energy emitted 
by 1 gram of radium in the course of its degradation to nonradio- 
active forms is equal to the kinetic energy of a mass of one-thirteenth 
of a milligram moving with the velocity of light. 
This energy, according to the rule I have stated, corresponds to 
a mass of one-thirteenth of a milligram of the ether, and thus a gram 
of radium in its radioactive state must have at least one-thirteenth of 
a milligram more of ether attached to it than when it has been 
degraded into the nonradioactive 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 
nonradioactive 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 
