physical nature of the intervening medium has an influence on the force, and the mathematical formula expressing the magnitude of the force must take account of this. We must therefore introduce into the formula a coefficient, the numerical value of which will not only depend on the system of units chosen, but also on the medium filling the space through which the force acts. We thus arrive at the following mathematical expression—
where and are the two masses, is the distance and is the force, all expressed in any system of units which may be convenient for the particular case in hand. The coefficient will naturally depend on the magnitude of the units chosen; on their nature, that is, whether we deal with gravitational masses, electric charges or magnetism; and on the medium filling the space through which the force acts.
We do not know what electricity is any more than we know what magnetism is; all we know is that they are not of the nature of ponderable masses, and that under certain circumstances they may become the vehicle for the transmission of energy in a
