2 Dr. D. F. Comstock on the 
between the wave-lengths of the spectral lines, make it seem 
probable that if matter is to be considered as an electrical 
system, it must be much more complex than a system com- 
posed entirely of electrous separated by distances great in 
comparison to their size. It becomes therefore of interest to 
see whether any relations can be found between the mass of 
an electric system in general, and any of its other properties. 
It will be found that a general relation does exist, which is not 
only of considerable interest in itself, but also suggests other 
relations. 
3. The straightforward calculation of the mass of an electric 
system possessing any distribution of charge and any internal 
velocities below that of light presents considerable difficulty ; 
for such calculation involves the use of the scalar and vector 
potential, and these are not effective instantaneously at all parts 
of the system. Any expression for the mass of the system 
calculated in this way will therefore involve terms which vary 
in an extremely complicated way with the internal velocities 
when these are not very small. The same is true with respect 
to the velocity of the system as a whole. In the following 
discussion the problem is attacked in an entirely different way, 
which is not open to this objection. 
As the constraints of the system are intimately involved, 
it will be well first to consider them. 
4. The position of internal constraints in general electrical 
theory is a very fundamental one. By " constraints " are meant 
rigid connexions of any kind. These act merely as reactions 
to the electrical forces, and do not contribute to the virtual 
work. If the electrical laws are to hold universally, i. e., for 
minute distances as well as for greater ones, it is obvious that 
no electrical system can exist as such unless there are such 
constraints to balance the electrical forces. Even a single 
electron would dissipate itself through the mutual repulsion 
of its elements, were it not for some form of internal constraint. 
Besides holding the system together, as it were, these con- 
straints also act in another important way. They may become, 
in common with all geometrical constraints, paths of energy 
flow. We are accustomed to think of the Poynting vector as 
representing completely the energy flow in a purely electrical 
system, but of course this is not in general true. 
Take as a simple example the case of a large plane air- 
condenser moving in a direction perpendicular to the plane 
of its plates. If the condenser is charged there is obviously 
a transference of energy at a rate equal to the internal 
energy multiplied by the velocity of movement. The Poynting 
