276 
PAUL BIEFELD 
If we write the expression in the form : 
mc“-[-Eo 
we see that the expression me- in the expansion is the energy 
which the point mass possessed before the body took up the 
energy E . 
The General Theory of Relativity. 
' (Theory of Gravitation). 
The restricted theory is valid only in connection with bodies 
or frames of reference in uniform rectilinear motion relative to 
each other. There is no reason thinkable, however, why such 
systems should be preferred for other systems having any kind 
of motion ; for we find nowhere in nature any such distinction 
indicated either in bodies or in the concept of motion. We are 
compelled then to look for the distinction as resting in the ob- 
jective properties of the space-time continuum. We must at- 
tribute to this continuum a “field property” as we do in case of 
the electromagnetic field ; and then find, a way to extend the prin- 
ciple of relativity to frames of reference in non-uniform motion 
relative to each other. 
We find a way of attack by the following consideration. In 
physics the ratio of the masses of two bodies is defined in two 
ways : first, as the inverse ratio of the accelerations imparted to 
them by the same force producing the motion ; second as the 
direct ratio of the forces acting in the same gravitational field. 
In the first relation we call the mass “inertia mass” in the second 
“gravity or weight-mass.” The ratio of these two masses have 
been found by Oetvoes so nearly equal to unity that the deviation 
is of the order of less than 5xl0-\ The examination included 
substances of crystaline structure and of radioactive nature. In- 
ertia and gravity mass are then not only proportional but strictly 
equal. We have, however, no right in face of these results to 
maintain such an equality in thought unless it is reduced to 
an equality of nature of the two concepts. The statement of the 
equality of inertia-mass and gravity-mass is equivalent to the 
statement that the acceleration imparted to a body in a gravita- 
tional field is independent of the nature of the body ; and it is the 
