12 Dr. D. F. Comstock on the 
Combining (19) and (20) we can obtain 
„j{i|E,= + E.) + I 
and remembering that 
we have finally 
T = ^(H/ + H/), 
M =-^ 2 , (22) 
n i+ « 
which, since («?,) is along (#), is identical with (16). Thus 
(16) is verified for the case where the moving system 
possesses no internal motion. 
Perhaps the simplest symmetrical system containing 
magnetic as well as electric energy is that formed by a great 
number of charged spheres moving in straight lines out 
from a common centre, with velocities small enough so that 
the fourth and higher powers may be neglected. They are 
to be at distances from each other great in comparison with 
their size, and at equal distances from the common centre. 
If the system be now given the slow velocity (r^ as a whole 
the total momentum accompanying this motion may be deter- 
mined. Because of limited space the calculation will not be 
here given, but if it be carried out along established lines it 
will be found that the mass is ~ v ., times the sum of the 
electric and magnetic energies, thus verifying equation (18). 
10. We conclude that, if ordinary material mass has an elec- 
tromagnetic basis, such mass for slow velocities is proportional 
to the. total electromagnetic energy-content of the body, and 
the laws of conservation of mass and energy become closely 
related if not identical. In any case the expression given 
represents the electromagnetic part of the total mass whatever 
that may be. 
Considerations suggested by the Foregoing. 
The Atomic Weights. 
11. If the conclusion of the last article is correct a dimi- 
nution in mass should follow a loss of energy in material 
transformations. Calculation shows, however, that in the 
